AD8045 mystery...

[Lasse Langwadt Christensen]

lørdag den 21. november 2020 kl. 22.48.03 UTC+1 skrev jla...@highlandsniptechnology.com:

On Sat, 21 Nov 2020 13:11:36 -0800 (PST), Lasse Langwadt Christensen
lang...@fonz.dk> wrote:

lørdag den 21. november 2020 kl. 17.05.34 UTC+1 skrev jla...@highlandsniptechnology.com:
On Sat, 21 Nov 2020 07:05:11 -0500, Phil Hobbs
pcdhSpamM...@electrooptical.net> wrote:

On 11/21/20 12:39 AM, jla...@highlandsniptechnology.com wrote:
On Fri, 20 Nov 2020 22:35:52 -0500, Phil Hobbs
pcdhSpamM...@electrooptical.net> wrote:

On 11/19/20 10:44 AM, jla...@highlandsniptechnology.com wrote:
On Thu, 19 Nov 2020 10:30:17 -0500, Phil Hobbs
pcdhSpamM...@electrooptical.net> wrote:

On 11/19/20 8:22 AM, George Herold wrote:
On Wednesday, November 18, 2020 at 2:27:39 PM UTC-5, Phil
Hobbs wrote:
Hi, all.

So I have this SiPM/MPPC front end. It has pop options to
use either an On Semi MicroFC-10010 1-mm SiPM chip or a
packaged Hamamatsu S13362-3050DG 3-mm MPPC with integral
TE cooler, both bootstrapped by a SAV-551+ running at 20
mA. So far, it all works.

(The SAV-551+ is amazingly stable--I\'ve got a shipping
product that runs a very similar bootstrap across a 2-inch
FFC cable. Bandwidth suffers a bit, but it shows no
tendency to oscillate.)

The mystery is in the TIA stage. It\'s a vanilla op amp TIA
made from either an ADA4899 (600 MHz, 300 V/us) or AD8045
(1 GHz, 1300 V/us @ Av=1), which are pin compatible in the
3-mm LFCSP package. Both are voltage feedback amps.

I\'m seeing a 3 dB bandwidth of 220 MHz, together with a
faster rolloff than I expect: -3 dB @ 220 MHz, -9 dB @ 320
MHz. It\'s not slew limiting, because the waveform looks
pretty good on a 3-GHz scope (TDS 694C) and the rolloff
stays the same when I drop the input by 6 dB.

The layout is pretty tight (the whole board is only an
inch square), so getting enough stray capacitance across
R_F to account for it is implausible--it would need about
1.4 pF. DecouplingBypassing is good--

For test, I removed the 0-ohm jumper that connects the
bootstrapped SiPM to the summing junction, and added a 1k
input resistor, forming an inverting amp with a nominal
gain of -0.5.

That\'s connected to the terminated end of an RG-174/U
cable going to a PTS-500 synthesizer. The output goes via
a 10-ohm resistor into a properly-terminated 50-ohm cable
(the TDS 694C is 50-ohm only).

Here I\'m expecting a bandwidth somewhere between the
datasheet\'s 1 GHz @ Av=1 and 400 MHz @ Av=-1, but it\'s way
off. There\'s no visible change when I put the jumper back
in, on account of the swoopy bootstrap.

I was going to suggest looking at the \'speed\' of the light
source. But the above seems to point to something \'in\' the
amp stage... (Is that right?) (And maybe check the light
source rise time anyway?)

George H.

So where do you suppose the missing factor of ~3 in
bandwidth went?


This has the SiPM and bootstrap disconnected (0 ohm jumper
removed) and a 1/20W leaded 1k resistor bodged in to make an
inverting amp with a gain of -0.5.

I\'m looking at the trace capacitance to figure out if that
might be it. There\'s about 3/4 inch of 10-mil trace on the
summing junction, but that ought to produce a high frequency
peak if anything. hard to find 1.4 pF across the feedback
resistor. Once I\'m back in the lab I\'ll measure a bare board
with a Boonton and see.

Cheers

Phil Hobbs

What\'s the board stackup? Not the specified one, but the real
one. I\'ve been burned by what some of the fast-turn proto houses
do.

https://www.dropbox.com/s/p3vpbaofzqurebz/Z462_PCB_Way_2.png?raw=1



The SJ capacitance is 2.4 pF, as measured on a Boonton, about
twice what I expected. That seems to be the issue--in simulation
it produces a pretty big gain peak, which reduces the bandwidth.

Time to Dremel the ground plane.

How many layers?

Only four, but of course ground is L2 and there\'s an L3 ground pour in
that area. It is PCBway, so maybe they did the same thing to me. (A
generally very good outfit in many ways, especially price and delivery.)
Several of the chinese quick-turn houses make 4-layer boards with very
thin (like 4 mil) outer dielectrics. Maybe they roll process the
outers and glue them to a core or something.

I believe the common process for 4 layers is first a core with the inner
layers and then sandwiched with pre-peg and copper to make the outer layers

thin outer dielectrics means traces to things like DDR ram does have to be mile wide

my keyboard must be broken, it should have said \"does not have to be mile wide\"

Thin, actually?

A 50 ohm microstrip on 4 mils thick FR4 dielectric is 6 mils wide. A
75 ohm trace would be 2 mils wide.

yep things DDR3 is supposed to be something like 40-50R

 

[Phil Hobbs]

On 11/21/20 11:05 AM, jlarkin@highlandsniptechnology.com wrote:

On Sat, 21 Nov 2020 07:05:11 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/21/20 12:39 AM, jlarkin@highlandsniptechnology.com wrote:
On Fri, 20 Nov 2020 22:35:52 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/19/20 10:44 AM, jlarkin@highlandsniptechnology.com wrote:
On Thu, 19 Nov 2020 10:30:17 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/19/20 8:22 AM, George Herold wrote:
On Wednesday, November 18, 2020 at 2:27:39 PM UTC-5, Phil
Hobbs wrote:
Hi, all.

So I have this SiPM/MPPC front end. It has pop options to
use either an On Semi MicroFC-10010 1-mm SiPM chip or a
packaged Hamamatsu S13362-3050DG 3-mm MPPC with integral
TE cooler, both bootstrapped by a SAV-551+ running at 20
mA. So far, it all works.

(The SAV-551+ is amazingly stable--I\'ve got a shipping
product that runs a very similar bootstrap across a 2-inch
FFC cable. Bandwidth suffers a bit, but it shows no
tendency to oscillate.)

The mystery is in the TIA stage. It\'s a vanilla op amp TIA
made from either an ADA4899 (600 MHz, 300 V/us) or AD8045
(1 GHz, 1300 V/us @ Av=1), which are pin compatible in the
3-mm LFCSP package. Both are voltage feedback amps.

I\'m seeing a 3 dB bandwidth of 220 MHz, together with a
faster rolloff than I expect: -3 dB @ 220 MHz, -9 dB @ 320
MHz. It\'s not slew limiting, because the waveform looks
pretty good on a 3-GHz scope (TDS 694C) and the rolloff
stays the same when I drop the input by 6 dB.

The layout is pretty tight (the whole board is only an
inch square), so getting enough stray capacitance across
R_F to account for it is implausible--it would need about
1.4 pF. DecouplingBypassing is good--

For test, I removed the 0-ohm jumper that connects the
bootstrapped SiPM to the summing junction, and added a 1k
input resistor, forming an inverting amp with a nominal
gain of -0.5.

That\'s connected to the terminated end of an RG-174/U
cable going to a PTS-500 synthesizer. The output goes via
a 10-ohm resistor into a properly-terminated 50-ohm cable
(the TDS 694C is 50-ohm only).

Here I\'m expecting a bandwidth somewhere between the
datasheet\'s 1 GHz @ Av=1 and 400 MHz @ Av=-1, but it\'s way
off. There\'s no visible change when I put the jumper back
in, on account of the swoopy bootstrap.

I was going to suggest looking at the \'speed\' of the light
source. But the above seems to point to something \'in\' the
amp stage... (Is that right?) (And maybe check the light
source rise time anyway?)

George H.

So where do you suppose the missing factor of ~3 in
bandwidth went?


This has the SiPM and bootstrap disconnected (0 ohm jumper
removed) and a 1/20W leaded 1k resistor bodged in to make an
inverting amp with a gain of -0.5.

I\'m looking at the trace capacitance to figure out if that
might be it. There\'s about 3/4 inch of 10-mil trace on the
summing junction, but that ought to produce a high frequency
peak if anything. hard to find 1.4 pF across the feedback
resistor. Once I\'m back in the lab I\'ll measure a bare board
with a Boonton and see.

Cheers

Phil Hobbs

What\'s the board stackup? Not the specified one, but the real
one. I\'ve been burned by what some of the fast-turn proto houses
do.

https://www.dropbox.com/s/p3vpbaofzqurebz/Z462_PCB_Way_2.png?raw=1



The SJ capacitance is 2.4 pF, as measured on a Boonton, about
twice what I expected. That seems to be the issue--in simulation
it produces a pretty big gain peak, which reduces the bandwidth.

Time to Dremel the ground plane.

How many layers?

Only four, but of course ground is L2 and there\'s an L3 ground pour in
that area. It is PCBway, so maybe they did the same thing to me. (A
generally very good outfit in many ways, especially price and delivery.)

Several of the chinese quick-turn houses make 4-layer boards with very
thin (like 4 mil) outer dielectrics. Maybe they roll process the
outers and glue them to a core or something.



Maybe section the board to see what the stackup actually is.


Monday I\'ll look at it under the good microscope. (About three years
ago, I got a beautiful Mitutoyo FS-110 with 2x-50x long working-distance
objectives for $2k on eBay. Apparently the guy didn\'t know what he had,
because he shipped this massive precision instrument in a cardboard box
with foam peanuts. The box was a mess when it got here, but the scope
survived because it\'s a beast.)

I sheared and sandpapered the example I posted, and shot it with my
super-good microscope

https://www.amazon.com/gp/product/B01IV0TV50/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

I have one a bit like that, but with a better base:
<https://www.ebay.com/itm/Andonstar-500X-2MP-USB-Digital-Microscope-Video-webcam-Magnifier-Camera-Stand/143844969460>

It does need a rubber band to apply a rotational preload to the arm.

Here is a Dremel-optimized SMA connector.


https://www.dropbox.com/s/gc5j45995nqiftu/Isola_Right_Trim.jpg?raw=1

https://www.dropbox.com/s/t0cnfe9do4slvvf/Isola_Trimmed_Right_TDR.jpg?raw=1

Nice.

We\'ve had that problem too--you need some extra pad area for the
solder fillet, but that makes a nasty capacitive discontinuity at the
connector.

The impedance of that SMA edge-launch connector, center pin to the
four ground pins, is about 100 ohms in free air. So the PCB has to be
about 100 in the pin region too. We\'ve worked that out, cutting away
inners and paving over the bottom with ground. We simulated the whole
geometry with ATLC to get the dims right. That was cool.

The case I posted was testing a laminate sample, 20 mils thick.

The $1.50 edge-launches are just as good as the $12 microwave
connectors if the layout is right, at least as far as we can resolve
with 30 ps TDR.

I sometimes cut away layer 2 (or more) under critical circuit nodes.
My triggered Colpitts oscillator has a driven guard patch on layer 5.

You bootstrap photodiodes, so you might bootstrap the PCB too.

I\'ve often done that in high-Z TIAs, like 10M. The bootstrap
effectiveness is limited by the capacitance to ground from the summing
junction.

I wasn\'t expecting it to be a big issue with a 511-ohm TIA, but then I
wasn\'t expecting that much capacitance either. I\'ll have to calculate
whether the SAV-551+ can bootstrap the pour as well as the SiPM.

The usual local feedback tricks such as the PNP wraparound sort of need
fast PNPs, which are no longer made. (The Renesas/Intersil ones are
a sort-of exception.)

We\'re using this board as the front end to a time stretcher(*) for
bathymetric lidar, which consists of an array of T/Hs sampling the same
signal at ~300 ps intervals. The rep rate is only a kilohertz or
thereabouts, so there\'s lots of time to digitize and read out the data.

A 14-bit, 3 GS/s digitizer would be a lot of iron to accomplish the same
thing.

(*) _not_ a pulse stretcher.

Cheers

Phil Hobbs

\"A sadder and a wiser man / he woke the morrow morn.\" (Coleridge)

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[server]

On Sat, 21 Nov 2020 18:48:57 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/21/20 11:05 AM, jlarkin@highlandsniptechnology.com wrote:
On Sat, 21 Nov 2020 07:05:11 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/21/20 12:39 AM, jlarkin@highlandsniptechnology.com wrote:
On Fri, 20 Nov 2020 22:35:52 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/19/20 10:44 AM, jlarkin@highlandsniptechnology.com wrote:
On Thu, 19 Nov 2020 10:30:17 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/19/20 8:22 AM, George Herold wrote:
On Wednesday, November 18, 2020 at 2:27:39 PM UTC-5, Phil
Hobbs wrote:
Hi, all.

So I have this SiPM/MPPC front end. It has pop options to
use either an On Semi MicroFC-10010 1-mm SiPM chip or a
packaged Hamamatsu S13362-3050DG 3-mm MPPC with integral
TE cooler, both bootstrapped by a SAV-551+ running at 20
mA. So far, it all works.

(The SAV-551+ is amazingly stable--I\'ve got a shipping
product that runs a very similar bootstrap across a 2-inch
FFC cable. Bandwidth suffers a bit, but it shows no
tendency to oscillate.)

The mystery is in the TIA stage. It\'s a vanilla op amp TIA
made from either an ADA4899 (600 MHz, 300 V/us) or AD8045
(1 GHz, 1300 V/us @ Av=1), which are pin compatible in the
3-mm LFCSP package. Both are voltage feedback amps.

I\'m seeing a 3 dB bandwidth of 220 MHz, together with a
faster rolloff than I expect: -3 dB @ 220 MHz, -9 dB @ 320
MHz. It\'s not slew limiting, because the waveform looks
pretty good on a 3-GHz scope (TDS 694C) and the rolloff
stays the same when I drop the input by 6 dB.

The layout is pretty tight (the whole board is only an
inch square), so getting enough stray capacitance across
R_F to account for it is implausible--it would need about
1.4 pF. DecouplingBypassing is good--

For test, I removed the 0-ohm jumper that connects the
bootstrapped SiPM to the summing junction, and added a 1k
input resistor, forming an inverting amp with a nominal
gain of -0.5.

That\'s connected to the terminated end of an RG-174/U
cable going to a PTS-500 synthesizer. The output goes via
a 10-ohm resistor into a properly-terminated 50-ohm cable
(the TDS 694C is 50-ohm only).

Here I\'m expecting a bandwidth somewhere between the
datasheet\'s 1 GHz @ Av=1 and 400 MHz @ Av=-1, but it\'s way
off. There\'s no visible change when I put the jumper back
in, on account of the swoopy bootstrap.

I was going to suggest looking at the \'speed\' of the light
source. But the above seems to point to something \'in\' the
amp stage... (Is that right?) (And maybe check the light
source rise time anyway?)

George H.

So where do you suppose the missing factor of ~3 in
bandwidth went?


This has the SiPM and bootstrap disconnected (0 ohm jumper
removed) and a 1/20W leaded 1k resistor bodged in to make an
inverting amp with a gain of -0.5.

I\'m looking at the trace capacitance to figure out if that
might be it. There\'s about 3/4 inch of 10-mil trace on the
summing junction, but that ought to produce a high frequency
peak if anything. hard to find 1.4 pF across the feedback
resistor. Once I\'m back in the lab I\'ll measure a bare board
with a Boonton and see.

Cheers

Phil Hobbs

What\'s the board stackup? Not the specified one, but the real
one. I\'ve been burned by what some of the fast-turn proto houses
do.

https://www.dropbox.com/s/p3vpbaofzqurebz/Z462_PCB_Way_2.png?raw=1



The SJ capacitance is 2.4 pF, as measured on a Boonton, about
twice what I expected. That seems to be the issue--in simulation
it produces a pretty big gain peak, which reduces the bandwidth.

Time to Dremel the ground plane.

How many layers?

Only four, but of course ground is L2 and there\'s an L3 ground pour in
that area. It is PCBway, so maybe they did the same thing to me. (A
generally very good outfit in many ways, especially price and delivery.)

Several of the chinese quick-turn houses make 4-layer boards with very
thin (like 4 mil) outer dielectrics. Maybe they roll process the
outers and glue them to a core or something.



Maybe section the board to see what the stackup actually is.


Monday I\'ll look at it under the good microscope. (About three years
ago, I got a beautiful Mitutoyo FS-110 with 2x-50x long working-distance
objectives for $2k on eBay. Apparently the guy didn\'t know what he had,
because he shipped this massive precision instrument in a cardboard box
with foam peanuts. The box was a mess when it got here, but the scope
survived because it\'s a beast.)

I sheared and sandpapered the example I posted, and shot it with my
super-good microscope

https://www.amazon.com/gp/product/B01IV0TV50/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

I have one a bit like that, but with a better base:
https://www.ebay.com/itm/Andonstar-500X-2MP-USB-Digital-Microscope-Video-webcam-Magnifier-Camera-Stand/143844969460

It does need a rubber band to apply a rotational preload to the arm.

Here is a Dremel-optimized SMA connector.


https://www.dropbox.com/s/gc5j45995nqiftu/Isola_Right_Trim.jpg?raw=1

https://www.dropbox.com/s/t0cnfe9do4slvvf/Isola_Trimmed_Right_TDR.jpg?raw=1

Nice.

We\'ve had that problem too--you need some extra pad area for the
solder fillet, but that makes a nasty capacitive discontinuity at the
connector.

The impedance of that SMA edge-launch connector, center pin to the
four ground pins, is about 100 ohms in free air. So the PCB has to be
about 100 in the pin region too. We\'ve worked that out, cutting away
inners and paving over the bottom with ground. We simulated the whole
geometry with ATLC to get the dims right. That was cool.

The case I posted was testing a laminate sample, 20 mils thick.

The $1.50 edge-launches are just as good as the $12 microwave
connectors if the layout is right, at least as far as we can resolve
with 30 ps TDR.

I sometimes cut away layer 2 (or more) under critical circuit nodes.
My triggered Colpitts oscillator has a driven guard patch on layer 5.

You bootstrap photodiodes, so you might bootstrap the PCB too.

I\'ve often done that in high-Z TIAs, like 10M. The bootstrap
effectiveness is limited by the capacitance to ground from the summing
junction.

I wasn\'t expecting it to be a big issue with a 511-ohm TIA, but then I
wasn\'t expecting that much capacitance either. I\'ll have to calculate
whether the SAV-551+ can bootstrap the pour as well as the SiPM.

I was think of a follower driving a follower, so the guard doesn\'t
load the first one. Gain would be lower on the second bootstrap, but
still a lot better than none.

The usual local feedback tricks such as the PNP wraparound sort of need
fast PNPs, which are no longer made. (The Renesas/Intersil ones are
a sort-of exception.)

We\'re using this board as the front end to a time stretcher(*) for
bathymetric lidar, which consists of an array of T/Hs sampling the same
signal at ~300 ps intervals. The rep rate is only a kilohertz or
thereabouts, so there\'s lots of time to digitize and read out the data.

A 14-bit, 3 GS/s digitizer would be a lot of iron to accomplish the same
thing.

(*) _not_ a pulse stretcher.

Cheers

Phil Hobbs

\"A sadder and a wiser man / he woke the morrow morn.\" (Coleridge)

--

John Larkin Highland Technology, Inc

The best designs are necessarily accidental.

 

[Phil Hobbs]

On 11/21/20 10:17 PM, jlarkin@highlandsniptechnology.com wrote:

On Sat, 21 Nov 2020 18:48:57 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/21/20 11:05 AM, jlarkin@highlandsniptechnology.com wrote:
On Sat, 21 Nov 2020 07:05:11 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/21/20 12:39 AM, jlarkin@highlandsniptechnology.com wrote:
On Fri, 20 Nov 2020 22:35:52 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/19/20 10:44 AM, jlarkin@highlandsniptechnology.com wrote:
On Thu, 19 Nov 2020 10:30:17 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/19/20 8:22 AM, George Herold wrote:
On Wednesday, November 18, 2020 at 2:27:39 PM UTC-5, Phil
Hobbs wrote:
Hi, all.

So I have this SiPM/MPPC front end. It has pop options to
use either an On Semi MicroFC-10010 1-mm SiPM chip or a
packaged Hamamatsu S13362-3050DG 3-mm MPPC with integral
TE cooler, both bootstrapped by a SAV-551+ running at 20
mA. So far, it all works.

(The SAV-551+ is amazingly stable--I\'ve got a shipping
product that runs a very similar bootstrap across a 2-inch
FFC cable. Bandwidth suffers a bit, but it shows no
tendency to oscillate.)

The mystery is in the TIA stage. It\'s a vanilla op amp TIA
made from either an ADA4899 (600 MHz, 300 V/us) or AD8045
(1 GHz, 1300 V/us @ Av=1), which are pin compatible in the
3-mm LFCSP package. Both are voltage feedback amps.

I\'m seeing a 3 dB bandwidth of 220 MHz, together with a
faster rolloff than I expect: -3 dB @ 220 MHz, -9 dB @ 320
MHz. It\'s not slew limiting, because the waveform looks
pretty good on a 3-GHz scope (TDS 694C) and the rolloff
stays the same when I drop the input by 6 dB.

The layout is pretty tight (the whole board is only an
inch square), so getting enough stray capacitance across
R_F to account for it is implausible--it would need about
1.4 pF. DecouplingBypassing is good--

For test, I removed the 0-ohm jumper that connects the
bootstrapped SiPM to the summing junction, and added a 1k
input resistor, forming an inverting amp with a nominal
gain of -0.5.

That\'s connected to the terminated end of an RG-174/U
cable going to a PTS-500 synthesizer. The output goes via
a 10-ohm resistor into a properly-terminated 50-ohm cable
(the TDS 694C is 50-ohm only).

Here I\'m expecting a bandwidth somewhere between the
datasheet\'s 1 GHz @ Av=1 and 400 MHz @ Av=-1, but it\'s way
off. There\'s no visible change when I put the jumper back
in, on account of the swoopy bootstrap.

I was going to suggest looking at the \'speed\' of the light
source. But the above seems to point to something \'in\' the
amp stage... (Is that right?) (And maybe check the light
source rise time anyway?)

George H.

So where do you suppose the missing factor of ~3 in
bandwidth went?


This has the SiPM and bootstrap disconnected (0 ohm jumper
removed) and a 1/20W leaded 1k resistor bodged in to make an
inverting amp with a gain of -0.5.

I\'m looking at the trace capacitance to figure out if that
might be it. There\'s about 3/4 inch of 10-mil trace on the
summing junction, but that ought to produce a high frequency
peak if anything. hard to find 1.4 pF across the feedback
resistor. Once I\'m back in the lab I\'ll measure a bare board
with a Boonton and see.

Cheers

Phil Hobbs

What\'s the board stackup? Not the specified one, but the real
one. I\'ve been burned by what some of the fast-turn proto houses
do.

https://www.dropbox.com/s/p3vpbaofzqurebz/Z462_PCB_Way_2.png?raw=1



The SJ capacitance is 2.4 pF, as measured on a Boonton, about
twice what I expected. That seems to be the issue--in simulation
it produces a pretty big gain peak, which reduces the bandwidth.

Time to Dremel the ground plane.

How many layers?

Only four, but of course ground is L2 and there\'s an L3 ground pour in
that area. It is PCBway, so maybe they did the same thing to me. (A
generally very good outfit in many ways, especially price and delivery.)

Several of the chinese quick-turn houses make 4-layer boards with very
thin (like 4 mil) outer dielectrics. Maybe they roll process the
outers and glue them to a core or something.



Maybe section the board to see what the stackup actually is.


Monday I\'ll look at it under the good microscope. (About three years
ago, I got a beautiful Mitutoyo FS-110 with 2x-50x long working-distance
objectives for $2k on eBay. Apparently the guy didn\'t know what he had,
because he shipped this massive precision instrument in a cardboard box
with foam peanuts. The box was a mess when it got here, but the scope
survived because it\'s a beast.)

I sheared and sandpapered the example I posted, and shot it with my
super-good microscope

https://www.amazon.com/gp/product/B01IV0TV50/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

I have one a bit like that, but with a better base:
https://www.ebay.com/itm/Andonstar-500X-2MP-USB-Digital-Microscope-Video-webcam-Magnifier-Camera-Stand/143844969460

It does need a rubber band to apply a rotational preload to the arm.

Here is a Dremel-optimized SMA connector.


https://www.dropbox.com/s/gc5j45995nqiftu/Isola_Right_Trim.jpg?raw=1

https://www.dropbox.com/s/t0cnfe9do4slvvf/Isola_Trimmed_Right_TDR.jpg?raw=1

Nice.

We\'ve had that problem too--you need some extra pad area for the
solder fillet, but that makes a nasty capacitive discontinuity at the
connector.

The impedance of that SMA edge-launch connector, center pin to the
four ground pins, is about 100 ohms in free air. So the PCB has to be
about 100 in the pin region too. We\'ve worked that out, cutting away
inners and paving over the bottom with ground. We simulated the whole
geometry with ATLC to get the dims right. That was cool.

The case I posted was testing a laminate sample, 20 mils thick.

The $1.50 edge-launches are just as good as the $12 microwave
connectors if the layout is right, at least as far as we can resolve
with 30 ps TDR.

I sometimes cut away layer 2 (or more) under critical circuit nodes.
My triggered Colpitts oscillator has a driven guard patch on layer 5.

You bootstrap photodiodes, so you might bootstrap the PCB too.

I\'ve often done that in high-Z TIAs, like 10M. The bootstrap
effectiveness is limited by the capacitance to ground from the summing
junction.

I wasn\'t expecting it to be a big issue with a 511-ohm TIA, but then I
wasn\'t expecting that much capacitance either. I\'ll have to calculate
whether the SAV-551+ can bootstrap the pour as well as the SiPM.

I was think of a follower driving a follower, so the guard doesn\'t
load the first one. Gain would be lower on the second bootstrap, but
still a lot better than none.

Agreed, assuming the additional phase shift doesn\'t bite me in the
backside. Might be a good application for a BFP650F.

I sometimes just bias a JFET bootstrap at its zero-TC current and use
its output directly to drive the noninverting input of an op amp. You
do get some residual DC offset, but in a high-Z TIA that\'s par for the
course anyway, due to photodiode leakage current, so nobody minds.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Phil Hobbs]

On 11/21/20 6:48 PM, Phil Hobbs wrote:

On 11/21/20 11:05 AM, jlarkin@highlandsniptechnology.com wrote:
On Sat, 21 Nov 2020 07:05:11 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/21/20 12:39 AM, jlarkin@highlandsniptechnology.com wrote:
On Fri, 20 Nov 2020 22:35:52 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/19/20 10:44 AM, jlarkin@highlandsniptechnology.com
wrote:
On Thu, 19 Nov 2020 10:30:17 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/19/20 8:22 AM, George Herold wrote:
On Wednesday, November 18, 2020 at 2:27:39 PM UTC-5,
Phil Hobbs wrote:
Hi, all.

So I have this SiPM/MPPC front end. It has pop
options to use either an On Semi MicroFC-10010 1-mm
SiPM chip or a packaged Hamamatsu S13362-3050DG 3-mm
MPPC with integral TE cooler, both bootstrapped by a
SAV-551+ running at 20 mA. So far, it all works.

(The SAV-551+ is amazingly stable--I\'ve got a
shipping product that runs a very similar bootstrap
across a 2-inch FFC cable. Bandwidth suffers a bit,
but it shows no tendency to oscillate.)

The mystery is in the TIA stage. It\'s a vanilla op
amp TIA made from either an ADA4899 (600 MHz, 300
V/us) or AD8045 (1 GHz, 1300 V/us @ Av=1), which are
pin compatible in the 3-mm LFCSP package. Both are
voltage feedback amps.

I\'m seeing a 3 dB bandwidth of 220 MHz, together with
a faster rolloff than I expect: -3 dB @ 220 MHz, -9
dB @ 320 MHz. It\'s not slew limiting, because the
waveform looks pretty good on a 3-GHz scope (TDS
694C) and the rolloff stays the same when I drop the
input by 6 dB.

The layout is pretty tight (the whole board is only
an inch square), so getting enough stray capacitance
across R_F to account for it is implausible--it would
need about 1.4 pF. DecouplingBypassing is good--

For test, I removed the 0-ohm jumper that connects
the bootstrapped SiPM to the summing junction, and
added a 1k input resistor, forming an inverting amp
with a nominal gain of -0.5.

That\'s connected to the terminated end of an
RG-174/U cable going to a PTS-500 synthesizer. The
output goes via a 10-ohm resistor into a
properly-terminated 50-ohm cable (the TDS 694C is
50-ohm only).

Here I\'m expecting a bandwidth somewhere between the
datasheet\'s 1 GHz @ Av=1 and 400 MHz @ Av=-1, but
it\'s way off. There\'s no visible change when I put
the jumper back in, on account of the swoopy
bootstrap.

I was going to suggest looking at the \'speed\' of the
light source. But the above seems to point to something
\'in\' the amp stage... (Is that right?) (And maybe check
the light source rise time anyway?)

George H.

So where do you suppose the missing factor of ~3 in
bandwidth went?


This has the SiPM and bootstrap disconnected (0 ohm
jumper removed) and a 1/20W leaded 1k resistor bodged in
to make an inverting amp with a gain of -0.5.

I\'m looking at the trace capacitance to figure out if
that might be it. There\'s about 3/4 inch of 10-mil trace
on the summing junction, but that ought to produce a high
frequency peak if anything. hard to find 1.4 pF across
the feedback resistor. Once I\'m back in the lab I\'ll
measure a bare board with a Boonton and see.

Cheers

Phil Hobbs

What\'s the board stackup? Not the specified one, but the
real one. I\'ve been burned by what some of the fast-turn
proto houses do.

https://www.dropbox.com/s/p3vpbaofzqurebz/Z462_PCB_Way_2.png?raw=1





The SJ capacitance is 2.4 pF, as measured on a Boonton, about
twice what I expected. That seems to be the issue--in
simulation it produces a pretty big gain peak, which reduces
the bandwidth.

Time to Dremel the ground plane.

How many layers?

Only four, but of course ground is L2 and there\'s an L3 ground
pour in that area. It is PCBway, so maybe they did the same
thing to me. (A generally very good outfit in many ways,
especially price and delivery.)

Several of the chinese quick-turn houses make 4-layer boards with
very thin (like 4 mil) outer dielectrics. Maybe they roll process
the outers and glue them to a core or something.



Maybe section the board to see what the stackup actually is.


Monday I\'ll look at it under the good microscope. (About three
years ago, I got a beautiful Mitutoyo FS-110 with 2x-50x long
working-distance objectives for $2k on eBay. Apparently the guy
didn\'t know what he had, because he shipped this massive
precision instrument in a cardboard box with foam peanuts. The
box was a mess when it got here, but the scope survived because
it\'s a beast.)

I sheared and sandpapered the example I posted, and shot it with
my super-good microscope

https://www.amazon.com/gp/product/B01IV0TV50/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1



I have one a bit like that, but with a better base:
https://www.ebay.com/itm/Andonstar-500X-2MP-USB-Digital-Microscope-Video-webcam-Magnifier-Camera-Stand/143844969460



It does need a rubber band to apply a rotational preload to the arm.

Here is a Dremel-optimized SMA connector.


https://www.dropbox.com/s/gc5j45995nqiftu/Isola_Right_Trim.jpg?raw=1



https://www.dropbox.com/s/t0cnfe9do4slvvf/Isola_Trimmed_Right_TDR.jpg?raw=1


Nice.

We\'ve had that problem too--you need some extra pad area for the
solder fillet, but that makes a nasty capacitive discontinuity at
the connector.

The impedance of that SMA edge-launch connector, center pin to the
four ground pins, is about 100 ohms in free air. So the PCB has to
be about 100 in the pin region too. We\'ve worked that out, cutting
away inners and paving over the bottom with ground. We simulated
the whole geometry with ATLC to get the dims right. That was cool.

The case I posted was testing a laminate sample, 20 mils thick.

The $1.50 edge-launches are just as good as the $12 microwave
connectors if the layout is right, at least as far as we can
resolve with 30 ps TDR.

I sometimes cut away layer 2 (or more) under critical circuit
nodes. My triggered Colpitts oscillator has a driven guard patch on
layer 5.

You bootstrap photodiodes, so you might bootstrap the PCB too.

I\'ve often done that in high-Z TIAs, like 10M. The bootstrap
effectiveness is limited by the capacitance to ground from the
summing junction.

I wasn\'t expecting it to be a big issue with a 511-ohm TIA, but then
I wasn\'t expecting that much capacitance either. I\'ll have to
calculate whether the SAV-551+ can bootstrap the pour as well as the
SiPM.

The usual local feedback tricks such as the PNP wraparound sort of
need fast PNPs, which are no longer made. (The Renesas/Intersil
ones are a sort-of exception.)

Well, I took out the Dremel, with one of JL\'s fave dental burrs on it,
and got the SJ capacitance down to 1.2 pF. It was actually pretty easy,
and the results are sort of interesting-looking.

<https://electrooptical.net/www/sed/SiPMdremelled2.png>

Top-to-bottom dimension is 18 mm.

Cheers

Phil Hobbs

Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[server]

On Mon, 23 Nov 2020 13:58:26 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/21/20 6:48 PM, Phil Hobbs wrote:
On 11/21/20 11:05 AM, jlarkin@highlandsniptechnology.com wrote:
On Sat, 21 Nov 2020 07:05:11 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/21/20 12:39 AM, jlarkin@highlandsniptechnology.com wrote:
On Fri, 20 Nov 2020 22:35:52 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/19/20 10:44 AM, jlarkin@highlandsniptechnology.com
wrote:
On Thu, 19 Nov 2020 10:30:17 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/19/20 8:22 AM, George Herold wrote:
On Wednesday, November 18, 2020 at 2:27:39 PM UTC-5,
Phil Hobbs wrote:
Hi, all.

So I have this SiPM/MPPC front end. It has pop
options to use either an On Semi MicroFC-10010 1-mm
SiPM chip or a packaged Hamamatsu S13362-3050DG 3-mm
MPPC with integral TE cooler, both bootstrapped by a
SAV-551+ running at 20 mA. So far, it all works.

(The SAV-551+ is amazingly stable--I\'ve got a
shipping product that runs a very similar bootstrap
across a 2-inch FFC cable. Bandwidth suffers a bit,
but it shows no tendency to oscillate.)

The mystery is in the TIA stage. It\'s a vanilla op
amp TIA made from either an ADA4899 (600 MHz, 300
V/us) or AD8045 (1 GHz, 1300 V/us @ Av=1), which are
pin compatible in the 3-mm LFCSP package. Both are
voltage feedback amps.

I\'m seeing a 3 dB bandwidth of 220 MHz, together with
a faster rolloff than I expect: -3 dB @ 220 MHz, -9
dB @ 320 MHz. It\'s not slew limiting, because the
waveform looks pretty good on a 3-GHz scope (TDS
694C) and the rolloff stays the same when I drop the
input by 6 dB.

The layout is pretty tight (the whole board is only
an inch square), so getting enough stray capacitance
across R_F to account for it is implausible--it would
need about 1.4 pF. DecouplingBypassing is good--

For test, I removed the 0-ohm jumper that connects
the bootstrapped SiPM to the summing junction, and
added a 1k input resistor, forming an inverting amp
with a nominal gain of -0.5.

That\'s connected to the terminated end of an
RG-174/U cable going to a PTS-500 synthesizer. The
output goes via a 10-ohm resistor into a
properly-terminated 50-ohm cable (the TDS 694C is
50-ohm only).

Here I\'m expecting a bandwidth somewhere between the
datasheet\'s 1 GHz @ Av=1 and 400 MHz @ Av=-1, but
it\'s way off. There\'s no visible change when I put
the jumper back in, on account of the swoopy
bootstrap.

I was going to suggest looking at the \'speed\' of the
light source. But the above seems to point to something
\'in\' the amp stage... (Is that right?) (And maybe check
the light source rise time anyway?)

George H.

So where do you suppose the missing factor of ~3 in
bandwidth went?


This has the SiPM and bootstrap disconnected (0 ohm
jumper removed) and a 1/20W leaded 1k resistor bodged in
to make an inverting amp with a gain of -0.5.

I\'m looking at the trace capacitance to figure out if
that might be it. There\'s about 3/4 inch of 10-mil trace
on the summing junction, but that ought to produce a high
frequency peak if anything. hard to find 1.4 pF across
the feedback resistor. Once I\'m back in the lab I\'ll
measure a bare board with a Boonton and see.

Cheers

Phil Hobbs

What\'s the board stackup? Not the specified one, but the
real one. I\'ve been burned by what some of the fast-turn
proto houses do.

https://www.dropbox.com/s/p3vpbaofzqurebz/Z462_PCB_Way_2.png?raw=1





The SJ capacitance is 2.4 pF, as measured on a Boonton, about
twice what I expected. That seems to be the issue--in
simulation it produces a pretty big gain peak, which reduces
the bandwidth.

Time to Dremel the ground plane.

How many layers?

Only four, but of course ground is L2 and there\'s an L3 ground
pour in that area. It is PCBway, so maybe they did the same
thing to me. (A generally very good outfit in many ways,
especially price and delivery.)

Several of the chinese quick-turn houses make 4-layer boards with
very thin (like 4 mil) outer dielectrics. Maybe they roll process
the outers and glue them to a core or something.



Maybe section the board to see what the stackup actually is.


Monday I\'ll look at it under the good microscope. (About three
years ago, I got a beautiful Mitutoyo FS-110 with 2x-50x long
working-distance objectives for $2k on eBay. Apparently the guy
didn\'t know what he had, because he shipped this massive
precision instrument in a cardboard box with foam peanuts. The
box was a mess when it got here, but the scope survived because
it\'s a beast.)

I sheared and sandpapered the example I posted, and shot it with
my super-good microscope

https://www.amazon.com/gp/product/B01IV0TV50/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1



I have one a bit like that, but with a better base:
https://www.ebay.com/itm/Andonstar-500X-2MP-USB-Digital-Microscope-Video-webcam-Magnifier-Camera-Stand/143844969460



It does need a rubber band to apply a rotational preload to the arm.

Here is a Dremel-optimized SMA connector.


https://www.dropbox.com/s/gc5j45995nqiftu/Isola_Right_Trim.jpg?raw=1



https://www.dropbox.com/s/t0cnfe9do4slvvf/Isola_Trimmed_Right_TDR.jpg?raw=1


Nice.

We\'ve had that problem too--you need some extra pad area for the
solder fillet, but that makes a nasty capacitive discontinuity at
the connector.

The impedance of that SMA edge-launch connector, center pin to the
four ground pins, is about 100 ohms in free air. So the PCB has to
be about 100 in the pin region too. We\'ve worked that out, cutting
away inners and paving over the bottom with ground. We simulated
the whole geometry with ATLC to get the dims right. That was cool.

The case I posted was testing a laminate sample, 20 mils thick.

The $1.50 edge-launches are just as good as the $12 microwave
connectors if the layout is right, at least as far as we can
resolve with 30 ps TDR.

I sometimes cut away layer 2 (or more) under critical circuit
nodes. My triggered Colpitts oscillator has a driven guard patch on
layer 5.

You bootstrap photodiodes, so you might bootstrap the PCB too.

I\'ve often done that in high-Z TIAs, like 10M. The bootstrap
effectiveness is limited by the capacitance to ground from the
summing junction.

I wasn\'t expecting it to be a big issue with a 511-ohm TIA, but then
I wasn\'t expecting that much capacitance either. I\'ll have to
calculate whether the SAV-551+ can bootstrap the pour as well as the
SiPM.

The usual local feedback tricks such as the PNP wraparound sort of
need fast PNPs, which are no longer made. (The Renesas/Intersil
ones are a sort-of exception.)

Well, I took out the Dremel, with one of JL\'s fave dental burrs on it,
and got the SJ capacitance down to 1.2 pF. It was actually pretty easy,
and the results are sort of interesting-looking.

https://electrooptical.net/www/sed/SiPMdremelled2.png

Top-to-bottom dimension is 18 mm.

Cheers

Phil Hobbs

Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

Is that a 2-layer board? They are usually about 15 pf per square inch,
with a lot of fringing.

If we get our pcb laser blaster going, we\'ll mostly make 2-sided
boards with all ground plane on the back. It will also be a huge time
saver if we leave a lot of copper on the top, namely blast away just
enough copper to make the insulated features. That will make a lot of
capacitance, and lots of coplanar waveguide.

I\'m envisioning secondary Dremel operations, or maybe a lot of
lasering.





--

John Larkin Highland Technology, Inc

The best designs are necessarily accidental.

 

[Phil Hobbs]

On 11/23/20 2:43 PM, jlarkin@highlandsniptechnology.com wrote:

On Mon, 23 Nov 2020 13:58:26 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/21/20 6:48 PM, Phil Hobbs wrote:
On 11/21/20 11:05 AM, jlarkin@highlandsniptechnology.com wrote:
On Sat, 21 Nov 2020 07:05:11 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/21/20 12:39 AM, jlarkin@highlandsniptechnology.com wrote:
On Fri, 20 Nov 2020 22:35:52 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/19/20 10:44 AM, jlarkin@highlandsniptechnology.com
wrote:
On Thu, 19 Nov 2020 10:30:17 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/19/20 8:22 AM, George Herold wrote:
On Wednesday, November 18, 2020 at 2:27:39 PM UTC-5,
Phil Hobbs wrote:
Hi, all.

So I have this SiPM/MPPC front end. It has pop
options to use either an On Semi MicroFC-10010 1-mm
SiPM chip or a packaged Hamamatsu S13362-3050DG 3-mm
MPPC with integral TE cooler, both bootstrapped by a
SAV-551+ running at 20 mA. So far, it all works.

(The SAV-551+ is amazingly stable--I\'ve got a
shipping product that runs a very similar bootstrap
across a 2-inch FFC cable. Bandwidth suffers a bit,
but it shows no tendency to oscillate.)

The mystery is in the TIA stage. It\'s a vanilla op
amp TIA made from either an ADA4899 (600 MHz, 300
V/us) or AD8045 (1 GHz, 1300 V/us @ Av=1), which are
pin compatible in the 3-mm LFCSP package. Both are
voltage feedback amps.

I\'m seeing a 3 dB bandwidth of 220 MHz, together with
a faster rolloff than I expect: -3 dB @ 220 MHz, -9
dB @ 320 MHz. It\'s not slew limiting, because the
waveform looks pretty good on a 3-GHz scope (TDS
694C) and the rolloff stays the same when I drop the
input by 6 dB.

The layout is pretty tight (the whole board is only
an inch square), so getting enough stray capacitance
across R_F to account for it is implausible--it would
need about 1.4 pF. DecouplingBypassing is good--

For test, I removed the 0-ohm jumper that connects
the bootstrapped SiPM to the summing junction, and
added a 1k input resistor, forming an inverting amp
with a nominal gain of -0.5.

That\'s connected to the terminated end of an
RG-174/U cable going to a PTS-500 synthesizer. The
output goes via a 10-ohm resistor into a
properly-terminated 50-ohm cable (the TDS 694C is
50-ohm only).

Here I\'m expecting a bandwidth somewhere between the
datasheet\'s 1 GHz @ Av=1 and 400 MHz @ Av=-1, but
it\'s way off. There\'s no visible change when I put
the jumper back in, on account of the swoopy
bootstrap.

I was going to suggest looking at the \'speed\' of the
light source. But the above seems to point to something
\'in\' the amp stage... (Is that right?) (And maybe check
the light source rise time anyway?)

George H.

So where do you suppose the missing factor of ~3 in
bandwidth went?


This has the SiPM and bootstrap disconnected (0 ohm
jumper removed) and a 1/20W leaded 1k resistor bodged in
to make an inverting amp with a gain of -0.5.

I\'m looking at the trace capacitance to figure out if
that might be it. There\'s about 3/4 inch of 10-mil trace
on the summing junction, but that ought to produce a high
frequency peak if anything. hard to find 1.4 pF across
the feedback resistor. Once I\'m back in the lab I\'ll
measure a bare board with a Boonton and see.

Cheers

Phil Hobbs

What\'s the board stackup? Not the specified one, but the
real one. I\'ve been burned by what some of the fast-turn
proto houses do.

https://www.dropbox.com/s/p3vpbaofzqurebz/Z462_PCB_Way_2.png?raw=1





The SJ capacitance is 2.4 pF, as measured on a Boonton, about
twice what I expected. That seems to be the issue--in
simulation it produces a pretty big gain peak, which reduces
the bandwidth.

Time to Dremel the ground plane.

How many layers?

Only four, but of course ground is L2 and there\'s an L3 ground
pour in that area. It is PCBway, so maybe they did the same
thing to me. (A generally very good outfit in many ways,
especially price and delivery.)

Several of the chinese quick-turn houses make 4-layer boards with
very thin (like 4 mil) outer dielectrics. Maybe they roll process
the outers and glue them to a core or something.



Maybe section the board to see what the stackup actually is.


Monday I\'ll look at it under the good microscope. (About three
years ago, I got a beautiful Mitutoyo FS-110 with 2x-50x long
working-distance objectives for $2k on eBay. Apparently the guy
didn\'t know what he had, because he shipped this massive
precision instrument in a cardboard box with foam peanuts. The
box was a mess when it got here, but the scope survived because
it\'s a beast.)

I sheared and sandpapered the example I posted, and shot it with
my super-good microscope

https://www.amazon.com/gp/product/B01IV0TV50/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1



I have one a bit like that, but with a better base:
https://www.ebay.com/itm/Andonstar-500X-2MP-USB-Digital-Microscope-Video-webcam-Magnifier-Camera-Stand/143844969460



It does need a rubber band to apply a rotational preload to the arm.

Here is a Dremel-optimized SMA connector.


https://www.dropbox.com/s/gc5j45995nqiftu/Isola_Right_Trim.jpg?raw=1



https://www.dropbox.com/s/t0cnfe9do4slvvf/Isola_Trimmed_Right_TDR.jpg?raw=1


Nice.

We\'ve had that problem too--you need some extra pad area for the
solder fillet, but that makes a nasty capacitive discontinuity at
the connector.

The impedance of that SMA edge-launch connector, center pin to the
four ground pins, is about 100 ohms in free air. So the PCB has to
be about 100 in the pin region too. We\'ve worked that out, cutting
away inners and paving over the bottom with ground. We simulated
the whole geometry with ATLC to get the dims right. That was cool.

The case I posted was testing a laminate sample, 20 mils thick.

The $1.50 edge-launches are just as good as the $12 microwave
connectors if the layout is right, at least as far as we can
resolve with 30 ps TDR.

I sometimes cut away layer 2 (or more) under critical circuit
nodes. My triggered Colpitts oscillator has a driven guard patch on
layer 5.

You bootstrap photodiodes, so you might bootstrap the PCB too.

I\'ve often done that in high-Z TIAs, like 10M. The bootstrap
effectiveness is limited by the capacitance to ground from the
summing junction.

I wasn\'t expecting it to be a big issue with a 511-ohm TIA, but then
I wasn\'t expecting that much capacitance either. I\'ll have to
calculate whether the SAV-551+ can bootstrap the pour as well as the
SiPM.

The usual local feedback tricks such as the PNP wraparound sort of
need fast PNPs, which are no longer made. (The Renesas/Intersil
ones are a sort-of exception.)

Well, I took out the Dremel, with one of JL\'s fave dental burrs on it,
and got the SJ capacitance down to 1.2 pF. It was actually pretty easy,
and the results are sort of interesting-looking.

https://electrooptical.net/www/sed/SiPMdremelled2.png

Top-to-bottom dimension is 18 mm.


Is that a 2-layer board? They are usually about 15 pf per square inch,
with a lot of fringing.

No, it\'s 4 layers, with ground on L2. The #40 wire going down the
middle of the canyon there replaces an L3 trace.

If we get our pcb laser blaster going, we\'ll mostly make 2-sided
boards with all ground plane on the back. It will also be a huge time
saver if we leave a lot of copper on the top, namely blast away just
enough copper to make the insulated features. That will make a lot of
capacitance, and lots of coplanar waveguide.

I\'m envisioning secondary Dremel operations, or maybe a lot of
lasering.

Fun. BTW PCBway recently cut their assembly prices by a lot.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Lasse Langwadt Christensen]

mandag den 23. november 2020 kl. 21.11.24 UTC+1 skrev Phil Hobbs:

On 11/23/20 2:43 PM, jla...@highlandsniptechnology.com wrote:
On Mon, 23 Nov 2020 13:58:26 -0500, Phil Hobbs
pcdhSpamM...@electrooptical.net> wrote:

On 11/21/20 6:48 PM, Phil Hobbs wrote:
On 11/21/20 11:05 AM, jla...@highlandsniptechnology.com wrote:
On Sat, 21 Nov 2020 07:05:11 -0500, Phil Hobbs
pcdhSpamM...@electrooptical.net> wrote:

On 11/21/20 12:39 AM, jla...@highlandsniptechnology.com wrote:
On Fri, 20 Nov 2020 22:35:52 -0500, Phil Hobbs
pcdhSpamM...@electrooptical.net> wrote:

On 11/19/20 10:44 AM, jla...@highlandsniptechnology.com
wrote:
On Thu, 19 Nov 2020 10:30:17 -0500, Phil Hobbs
pcdhSpamM...@electrooptical.net> wrote:

On 11/19/20 8:22 AM, George Herold wrote:
On Wednesday, November 18, 2020 at 2:27:39 PM UTC-5,
Phil Hobbs wrote:
Hi, all.

So I have this SiPM/MPPC front end. It has pop
options to use either an On Semi MicroFC-10010 1-mm
SiPM chip or a packaged Hamamatsu S13362-3050DG 3-mm
MPPC with integral TE cooler, both bootstrapped by a
SAV-551+ running at 20 mA. So far, it all works.

(The SAV-551+ is amazingly stable--I\'ve got a
shipping product that runs a very similar bootstrap
across a 2-inch FFC cable. Bandwidth suffers a bit,
but it shows no tendency to oscillate.)

The mystery is in the TIA stage. It\'s a vanilla op
amp TIA made from either an ADA4899 (600 MHz, 300
V/us) or AD8045 (1 GHz, 1300 V/us @ Av=1), which are
pin compatible in the 3-mm LFCSP package. Both are
voltage feedback amps.

I\'m seeing a 3 dB bandwidth of 220 MHz, together with
a faster rolloff than I expect: -3 dB @ 220 MHz, -9
dB @ 320 MHz. It\'s not slew limiting, because the
waveform looks pretty good on a 3-GHz scope (TDS
694C) and the rolloff stays the same when I drop the
input by 6 dB.

The layout is pretty tight (the whole board is only
an inch square), so getting enough stray capacitance
across R_F to account for it is implausible--it would
need about 1.4 pF. DecouplingBypassing is good--

For test, I removed the 0-ohm jumper that connects
the bootstrapped SiPM to the summing junction, and
added a 1k input resistor, forming an inverting amp
with a nominal gain of -0.5.

That\'s connected to the terminated end of an
RG-174/U cable going to a PTS-500 synthesizer. The
output goes via a 10-ohm resistor into a
properly-terminated 50-ohm cable (the TDS 694C is
50-ohm only).

Here I\'m expecting a bandwidth somewhere between the
datasheet\'s 1 GHz @ Av=1 and 400 MHz @ Av=-1, but
it\'s way off. There\'s no visible change when I put
the jumper back in, on account of the swoopy
bootstrap.

I was going to suggest looking at the \'speed\' of the
light source. But the above seems to point to something
\'in\' the amp stage... (Is that right?) (And maybe check
the light source rise time anyway?)

George H.

So where do you suppose the missing factor of ~3 in
bandwidth went?


This has the SiPM and bootstrap disconnected (0 ohm
jumper removed) and a 1/20W leaded 1k resistor bodged in
to make an inverting amp with a gain of -0.5.

I\'m looking at the trace capacitance to figure out if
that might be it. There\'s about 3/4 inch of 10-mil trace
on the summing junction, but that ought to produce a high
frequency peak if anything. hard to find 1.4 pF across
the feedback resistor. Once I\'m back in the lab I\'ll
measure a bare board with a Boonton and see.

Cheers

Phil Hobbs

What\'s the board stackup? Not the specified one, but the
real one. I\'ve been burned by what some of the fast-turn
proto houses do.

https://www.dropbox.com/s/p3vpbaofzqurebz/Z462_PCB_Way_2.png?raw=1





The SJ capacitance is 2.4 pF, as measured on a Boonton, about
twice what I expected. That seems to be the issue--in
simulation it produces a pretty big gain peak, which reduces
the bandwidth.

Time to Dremel the ground plane.

How many layers?

Only four, but of course ground is L2 and there\'s an L3 ground
pour in that area. It is PCBway, so maybe they did the same
thing to me. (A generally very good outfit in many ways,
especially price and delivery.)

Several of the chinese quick-turn houses make 4-layer boards with
very thin (like 4 mil) outer dielectrics. Maybe they roll process
the outers and glue them to a core or something.



Maybe section the board to see what the stackup actually is.


Monday I\'ll look at it under the good microscope. (About three
years ago, I got a beautiful Mitutoyo FS-110 with 2x-50x long
working-distance objectives for $2k on eBay. Apparently the guy
didn\'t know what he had, because he shipped this massive
precision instrument in a cardboard box with foam peanuts. The
box was a mess when it got here, but the scope survived because
it\'s a beast.)

I sheared and sandpapered the example I posted, and shot it with
my super-good microscope

https://www.amazon.com/gp/product/B01IV0TV50/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1



I have one a bit like that, but with a better base:
https://www.ebay.com/itm/Andonstar-500X-2MP-USB-Digital-Microscope-Video-webcam-Magnifier-Camera-Stand/143844969460



It does need a rubber band to apply a rotational preload to the arm.

Here is a Dremel-optimized SMA connector.


https://www.dropbox.com/s/gc5j45995nqiftu/Isola_Right_Trim.jpg?raw=1



https://www.dropbox.com/s/t0cnfe9do4slvvf/Isola_Trimmed_Right_TDR.jpg?raw=1


Nice.

We\'ve had that problem too--you need some extra pad area for the
solder fillet, but that makes a nasty capacitive discontinuity at
the connector.

The impedance of that SMA edge-launch connector, center pin to the
four ground pins, is about 100 ohms in free air. So the PCB has to
be about 100 in the pin region too. We\'ve worked that out, cutting
away inners and paving over the bottom with ground. We simulated
the whole geometry with ATLC to get the dims right. That was cool.

The case I posted was testing a laminate sample, 20 mils thick.

The $1.50 edge-launches are just as good as the $12 microwave
connectors if the layout is right, at least as far as we can
resolve with 30 ps TDR.

I sometimes cut away layer 2 (or more) under critical circuit
nodes. My triggered Colpitts oscillator has a driven guard patch on
layer 5.

You bootstrap photodiodes, so you might bootstrap the PCB too.

I\'ve often done that in high-Z TIAs, like 10M. The bootstrap
effectiveness is limited by the capacitance to ground from the
summing junction.

I wasn\'t expecting it to be a big issue with a 511-ohm TIA, but then
I wasn\'t expecting that much capacitance either. I\'ll have to
calculate whether the SAV-551+ can bootstrap the pour as well as the
SiPM.

The usual local feedback tricks such as the PNP wraparound sort of
need fast PNPs, which are no longer made. (The Renesas/Intersil
ones are a sort-of exception.)

Well, I took out the Dremel, with one of JL\'s fave dental burrs on it,
and got the SJ capacitance down to 1.2 pF. It was actually pretty easy,
and the results are sort of interesting-looking.

https://electrooptical.net/www/sed/SiPMdremelled2.png

Top-to-bottom dimension is 18 mm.


Is that a 2-layer board? They are usually about 15 pf per square inch,
with a lot of fringing.
No, it\'s 4 layers, with ground on L2. The #40 wire going down the
middle of the canyon there replaces an L3 trace.
If we get our pcb laser blaster going, we\'ll mostly make 2-sided
boards with all ground plane on the back. It will also be a huge time
saver if we leave a lot of copper on the top, namely blast away just
enough copper to make the insulated features. That will make a lot of
capacitance, and lots of coplanar waveguide.

I\'m envisioning secondary Dremel operations, or maybe a lot of
lasering.
Fun. BTW PCBway recently cut their assembly prices by a lot.

4layer boards come with free assemble from jlcpcb until xmas
but they are limited to own their parts

 

[server]

On Mon, 23 Nov 2020 15:11:05 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/23/20 2:43 PM, jlarkin@highlandsniptechnology.com wrote:
On Mon, 23 Nov 2020 13:58:26 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/21/20 6:48 PM, Phil Hobbs wrote:
On 11/21/20 11:05 AM, jlarkin@highlandsniptechnology.com wrote:
On Sat, 21 Nov 2020 07:05:11 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/21/20 12:39 AM, jlarkin@highlandsniptechnology.com wrote:
On Fri, 20 Nov 2020 22:35:52 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/19/20 10:44 AM, jlarkin@highlandsniptechnology.com
wrote:
On Thu, 19 Nov 2020 10:30:17 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/19/20 8:22 AM, George Herold wrote:
On Wednesday, November 18, 2020 at 2:27:39 PM UTC-5,
Phil Hobbs wrote:
Hi, all.

So I have this SiPM/MPPC front end. It has pop
options to use either an On Semi MicroFC-10010 1-mm
SiPM chip or a packaged Hamamatsu S13362-3050DG 3-mm
MPPC with integral TE cooler, both bootstrapped by a
SAV-551+ running at 20 mA. So far, it all works.

(The SAV-551+ is amazingly stable--I\'ve got a
shipping product that runs a very similar bootstrap
across a 2-inch FFC cable. Bandwidth suffers a bit,
but it shows no tendency to oscillate.)

The mystery is in the TIA stage. It\'s a vanilla op
amp TIA made from either an ADA4899 (600 MHz, 300
V/us) or AD8045 (1 GHz, 1300 V/us @ Av=1), which are
pin compatible in the 3-mm LFCSP package. Both are
voltage feedback amps.

I\'m seeing a 3 dB bandwidth of 220 MHz, together with
a faster rolloff than I expect: -3 dB @ 220 MHz, -9
dB @ 320 MHz. It\'s not slew limiting, because the
waveform looks pretty good on a 3-GHz scope (TDS
694C) and the rolloff stays the same when I drop the
input by 6 dB.

The layout is pretty tight (the whole board is only
an inch square), so getting enough stray capacitance
across R_F to account for it is implausible--it would
need about 1.4 pF. DecouplingBypassing is good--

For test, I removed the 0-ohm jumper that connects
the bootstrapped SiPM to the summing junction, and
added a 1k input resistor, forming an inverting amp
with a nominal gain of -0.5.

That\'s connected to the terminated end of an
RG-174/U cable going to a PTS-500 synthesizer. The
output goes via a 10-ohm resistor into a
properly-terminated 50-ohm cable (the TDS 694C is
50-ohm only).

Here I\'m expecting a bandwidth somewhere between the
datasheet\'s 1 GHz @ Av=1 and 400 MHz @ Av=-1, but
it\'s way off. There\'s no visible change when I put
the jumper back in, on account of the swoopy
bootstrap.

I was going to suggest looking at the \'speed\' of the
light source. But the above seems to point to something
\'in\' the amp stage... (Is that right?) (And maybe check
the light source rise time anyway?)

George H.

So where do you suppose the missing factor of ~3 in
bandwidth went?


This has the SiPM and bootstrap disconnected (0 ohm
jumper removed) and a 1/20W leaded 1k resistor bodged in
to make an inverting amp with a gain of -0.5.

I\'m looking at the trace capacitance to figure out if
that might be it. There\'s about 3/4 inch of 10-mil trace
on the summing junction, but that ought to produce a high
frequency peak if anything. hard to find 1.4 pF across
the feedback resistor. Once I\'m back in the lab I\'ll
measure a bare board with a Boonton and see.

Cheers

Phil Hobbs

What\'s the board stackup? Not the specified one, but the
real one. I\'ve been burned by what some of the fast-turn
proto houses do.

https://www.dropbox.com/s/p3vpbaofzqurebz/Z462_PCB_Way_2.png?raw=1





The SJ capacitance is 2.4 pF, as measured on a Boonton, about
twice what I expected. That seems to be the issue--in
simulation it produces a pretty big gain peak, which reduces
the bandwidth.

Time to Dremel the ground plane.

How many layers?

Only four, but of course ground is L2 and there\'s an L3 ground
pour in that area. It is PCBway, so maybe they did the same
thing to me. (A generally very good outfit in many ways,
especially price and delivery.)

Several of the chinese quick-turn houses make 4-layer boards with
very thin (like 4 mil) outer dielectrics. Maybe they roll process
the outers and glue them to a core or something.



Maybe section the board to see what the stackup actually is.


Monday I\'ll look at it under the good microscope. (About three
years ago, I got a beautiful Mitutoyo FS-110 with 2x-50x long
working-distance objectives for $2k on eBay. Apparently the guy
didn\'t know what he had, because he shipped this massive
precision instrument in a cardboard box with foam peanuts. The
box was a mess when it got here, but the scope survived because
it\'s a beast.)

I sheared and sandpapered the example I posted, and shot it with
my super-good microscope

https://www.amazon.com/gp/product/B01IV0TV50/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1



I have one a bit like that, but with a better base:
https://www.ebay.com/itm/Andonstar-500X-2MP-USB-Digital-Microscope-Video-webcam-Magnifier-Camera-Stand/143844969460



It does need a rubber band to apply a rotational preload to the arm.

Here is a Dremel-optimized SMA connector.


https://www.dropbox.com/s/gc5j45995nqiftu/Isola_Right_Trim.jpg?raw=1



https://www.dropbox.com/s/t0cnfe9do4slvvf/Isola_Trimmed_Right_TDR.jpg?raw=1


Nice.

We\'ve had that problem too--you need some extra pad area for the
solder fillet, but that makes a nasty capacitive discontinuity at
the connector.

The impedance of that SMA edge-launch connector, center pin to the
four ground pins, is about 100 ohms in free air. So the PCB has to
be about 100 in the pin region too. We\'ve worked that out, cutting
away inners and paving over the bottom with ground. We simulated
the whole geometry with ATLC to get the dims right. That was cool.

The case I posted was testing a laminate sample, 20 mils thick.

The $1.50 edge-launches are just as good as the $12 microwave
connectors if the layout is right, at least as far as we can
resolve with 30 ps TDR.

I sometimes cut away layer 2 (or more) under critical circuit
nodes. My triggered Colpitts oscillator has a driven guard patch on
layer 5.

You bootstrap photodiodes, so you might bootstrap the PCB too.

I\'ve often done that in high-Z TIAs, like 10M. The bootstrap
effectiveness is limited by the capacitance to ground from the
summing junction.

I wasn\'t expecting it to be a big issue with a 511-ohm TIA, but then
I wasn\'t expecting that much capacitance either. I\'ll have to
calculate whether the SAV-551+ can bootstrap the pour as well as the
SiPM.

The usual local feedback tricks such as the PNP wraparound sort of
need fast PNPs, which are no longer made. (The Renesas/Intersil
ones are a sort-of exception.)

Well, I took out the Dremel, with one of JL\'s fave dental burrs on it,
and got the SJ capacitance down to 1.2 pF. It was actually pretty easy,
and the results are sort of interesting-looking.

https://electrooptical.net/www/sed/SiPMdremelled2.png

Top-to-bottom dimension is 18 mm.


Is that a 2-layer board? They are usually about 15 pf per square inch,
with a lot of fringing.

No, it\'s 4 layers, with ground on L2. The #40 wire going down the
middle of the canyon there replaces an L3 trace.

If we get our pcb laser blaster going, we\'ll mostly make 2-sided
boards with all ground plane on the back. It will also be a huge time
saver if we leave a lot of copper on the top, namely blast away just
enough copper to make the insulated features. That will make a lot of
capacitance, and lots of coplanar waveguide.

I\'m envisioning secondary Dremel operations, or maybe a lot of
lasering.

Fun. BTW PCBway recently cut their assembly prices by a lot.

Cheers

Phil Hobbs

Did you ever section that board? Maybe the L2 ground plane is very
close to the layer 1 traces. Seems like a lot of c for some tiny
features like that.

We got our own p+p line because kitting and logistics were a hassle
for using outside assemblers.



--

John Larkin Highland Technology, Inc

The best designs are necessarily accidental.

 

[Chris Jones]

On 24/11/2020 06:43, jlarkin@highlandsniptechnology.com wrote:

On Mon, 23 Nov 2020 13:58:26 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/21/20 6:48 PM, Phil Hobbs wrote:
On 11/21/20 11:05 AM, jlarkin@highlandsniptechnology.com wrote:
On Sat, 21 Nov 2020 07:05:11 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/21/20 12:39 AM, jlarkin@highlandsniptechnology.com wrote:
On Fri, 20 Nov 2020 22:35:52 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/19/20 10:44 AM, jlarkin@highlandsniptechnology.com
wrote:
On Thu, 19 Nov 2020 10:30:17 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/19/20 8:22 AM, George Herold wrote:
On Wednesday, November 18, 2020 at 2:27:39 PM UTC-5,
Phil Hobbs wrote:
Hi, all.

So I have this SiPM/MPPC front end. It has pop
options to use either an On Semi MicroFC-10010 1-mm
SiPM chip or a packaged Hamamatsu S13362-3050DG 3-mm
MPPC with integral TE cooler, both bootstrapped by a
SAV-551+ running at 20 mA. So far, it all works.

(The SAV-551+ is amazingly stable--I\'ve got a
shipping product that runs a very similar bootstrap
across a 2-inch FFC cable. Bandwidth suffers a bit,
but it shows no tendency to oscillate.)

The mystery is in the TIA stage. It\'s a vanilla op
amp TIA made from either an ADA4899 (600 MHz, 300
V/us) or AD8045 (1 GHz, 1300 V/us @ Av=1), which are
pin compatible in the 3-mm LFCSP package. Both are
voltage feedback amps.

I\'m seeing a 3 dB bandwidth of 220 MHz, together with
a faster rolloff than I expect: -3 dB @ 220 MHz, -9
dB @ 320 MHz. It\'s not slew limiting, because the
waveform looks pretty good on a 3-GHz scope (TDS
694C) and the rolloff stays the same when I drop the
input by 6 dB.

The layout is pretty tight (the whole board is only
an inch square), so getting enough stray capacitance
across R_F to account for it is implausible--it would
need about 1.4 pF. DecouplingBypassing is good--

For test, I removed the 0-ohm jumper that connects
the bootstrapped SiPM to the summing junction, and
added a 1k input resistor, forming an inverting amp
with a nominal gain of -0.5.

That\'s connected to the terminated end of an
RG-174/U cable going to a PTS-500 synthesizer. The
output goes via a 10-ohm resistor into a
properly-terminated 50-ohm cable (the TDS 694C is
50-ohm only).

Here I\'m expecting a bandwidth somewhere between the
datasheet\'s 1 GHz @ Av=1 and 400 MHz @ Av=-1, but
it\'s way off. There\'s no visible change when I put
the jumper back in, on account of the swoopy
bootstrap.

I was going to suggest looking at the \'speed\' of the
light source. But the above seems to point to something
\'in\' the amp stage... (Is that right?) (And maybe check
the light source rise time anyway?)

George H.

So where do you suppose the missing factor of ~3 in
bandwidth went?


This has the SiPM and bootstrap disconnected (0 ohm
jumper removed) and a 1/20W leaded 1k resistor bodged in
to make an inverting amp with a gain of -0.5.

I\'m looking at the trace capacitance to figure out if
that might be it. There\'s about 3/4 inch of 10-mil trace
on the summing junction, but that ought to produce a high
frequency peak if anything. hard to find 1.4 pF across
the feedback resistor. Once I\'m back in the lab I\'ll
measure a bare board with a Boonton and see.

Cheers

Phil Hobbs

What\'s the board stackup? Not the specified one, but the
real one. I\'ve been burned by what some of the fast-turn
proto houses do.

https://www.dropbox.com/s/p3vpbaofzqurebz/Z462_PCB_Way_2.png?raw=1





The SJ capacitance is 2.4 pF, as measured on a Boonton, about
twice what I expected. That seems to be the issue--in
simulation it produces a pretty big gain peak, which reduces
the bandwidth.

Time to Dremel the ground plane.

How many layers?

Only four, but of course ground is L2 and there\'s an L3 ground
pour in that area. It is PCBway, so maybe they did the same
thing to me. (A generally very good outfit in many ways,
especially price and delivery.)

Several of the chinese quick-turn houses make 4-layer boards with
very thin (like 4 mil) outer dielectrics. Maybe they roll process
the outers and glue them to a core or something.



Maybe section the board to see what the stackup actually is.


Monday I\'ll look at it under the good microscope. (About three
years ago, I got a beautiful Mitutoyo FS-110 with 2x-50x long
working-distance objectives for $2k on eBay. Apparently the guy
didn\'t know what he had, because he shipped this massive
precision instrument in a cardboard box with foam peanuts. The
box was a mess when it got here, but the scope survived because
it\'s a beast.)

I sheared and sandpapered the example I posted, and shot it with
my super-good microscope

https://www.amazon.com/gp/product/B01IV0TV50/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1



I have one a bit like that, but with a better base:
https://www.ebay.com/itm/Andonstar-500X-2MP-USB-Digital-Microscope-Video-webcam-Magnifier-Camera-Stand/143844969460



It does need a rubber band to apply a rotational preload to the arm.

Here is a Dremel-optimized SMA connector.


https://www.dropbox.com/s/gc5j45995nqiftu/Isola_Right_Trim.jpg?raw=1



https://www.dropbox.com/s/t0cnfe9do4slvvf/Isola_Trimmed_Right_TDR.jpg?raw=1


Nice.

We\'ve had that problem too--you need some extra pad area for the
solder fillet, but that makes a nasty capacitive discontinuity at
the connector.

The impedance of that SMA edge-launch connector, center pin to the
four ground pins, is about 100 ohms in free air. So the PCB has to
be about 100 in the pin region too. We\'ve worked that out, cutting
away inners and paving over the bottom with ground. We simulated
the whole geometry with ATLC to get the dims right. That was cool.

The case I posted was testing a laminate sample, 20 mils thick.

The $1.50 edge-launches are just as good as the $12 microwave
connectors if the layout is right, at least as far as we can
resolve with 30 ps TDR.

I sometimes cut away layer 2 (or more) under critical circuit
nodes. My triggered Colpitts oscillator has a driven guard patch on
layer 5.

You bootstrap photodiodes, so you might bootstrap the PCB too.

I\'ve often done that in high-Z TIAs, like 10M. The bootstrap
effectiveness is limited by the capacitance to ground from the
summing junction.

I wasn\'t expecting it to be a big issue with a 511-ohm TIA, but then
I wasn\'t expecting that much capacitance either. I\'ll have to
calculate whether the SAV-551+ can bootstrap the pour as well as the
SiPM.

The usual local feedback tricks such as the PNP wraparound sort of
need fast PNPs, which are no longer made. (The Renesas/Intersil
ones are a sort-of exception.)

Well, I took out the Dremel, with one of JL\'s fave dental burrs on it,
and got the SJ capacitance down to 1.2 pF. It was actually pretty easy,
and the results are sort of interesting-looking.

https://electrooptical.net/www/sed/SiPMdremelled2.png

Top-to-bottom dimension is 18 mm.

Cheers

Phil Hobbs

Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com


Is that a 2-layer board? They are usually about 15 pf per square inch,
with a lot of fringing.

If we get our pcb laser blaster going, we\'ll mostly make 2-sided
boards with all ground plane on the back. It will also be a huge time
saver if we leave a lot of copper on the top, namely blast away just
enough copper to make the insulated features. That will make a lot of
capacitance, and lots of coplanar waveguide.

I\'m envisioning secondary Dremel operations, or maybe a lot of
lasering.

If I need to remove medium-size regions of copper, maybe up to 3/4 inch
wide, from a prototype, then I peel it off as follows: Cut around the
region I want to peel off, using a scalpel, saw, dremel or whatever.
Then stick a knife, scalpel, etc under one corner to lift the copper off
the epoxy just enough to grab hold of, then put a big hot metcal tip
asross the line where I need the epoxy to peel off, e.g. SMTC-161, which
softens the epoxy, and I grab the corner that I lifted with some good
needle-nose pliers. Then I just pull gently upwards with the pliers as I
slide the soldering iron so that it is always heating the part where it
needs to peel off. Sometimes a little solder helps to get the heat from
the iron into the copper foil. Peeling it this way is quick and doesn\'t
create dust or copper swarf to get between pins etc.

 

[server]

On Tue, 24 Nov 2020 10:58:05 +1100, Chris Jones
<lugnut808@spam.yahoo.com> wrote:

On 24/11/2020 06:43, jlarkin@highlandsniptechnology.com wrote:
On Mon, 23 Nov 2020 13:58:26 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/21/20 6:48 PM, Phil Hobbs wrote:
On 11/21/20 11:05 AM, jlarkin@highlandsniptechnology.com wrote:
On Sat, 21 Nov 2020 07:05:11 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/21/20 12:39 AM, jlarkin@highlandsniptechnology.com wrote:
On Fri, 20 Nov 2020 22:35:52 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/19/20 10:44 AM, jlarkin@highlandsniptechnology.com
wrote:
On Thu, 19 Nov 2020 10:30:17 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/19/20 8:22 AM, George Herold wrote:
On Wednesday, November 18, 2020 at 2:27:39 PM UTC-5,
Phil Hobbs wrote:
Hi, all.

So I have this SiPM/MPPC front end. It has pop
options to use either an On Semi MicroFC-10010 1-mm
SiPM chip or a packaged Hamamatsu S13362-3050DG 3-mm
MPPC with integral TE cooler, both bootstrapped by a
SAV-551+ running at 20 mA. So far, it all works.

(The SAV-551+ is amazingly stable--I\'ve got a
shipping product that runs a very similar bootstrap
across a 2-inch FFC cable. Bandwidth suffers a bit,
but it shows no tendency to oscillate.)

The mystery is in the TIA stage. It\'s a vanilla op
amp TIA made from either an ADA4899 (600 MHz, 300
V/us) or AD8045 (1 GHz, 1300 V/us @ Av=1), which are
pin compatible in the 3-mm LFCSP package. Both are
voltage feedback amps.

I\'m seeing a 3 dB bandwidth of 220 MHz, together with
a faster rolloff than I expect: -3 dB @ 220 MHz, -9
dB @ 320 MHz. It\'s not slew limiting, because the
waveform looks pretty good on a 3-GHz scope (TDS
694C) and the rolloff stays the same when I drop the
input by 6 dB.

The layout is pretty tight (the whole board is only
an inch square), so getting enough stray capacitance
across R_F to account for it is implausible--it would
need about 1.4 pF. DecouplingBypassing is good--

For test, I removed the 0-ohm jumper that connects
the bootstrapped SiPM to the summing junction, and
added a 1k input resistor, forming an inverting amp
with a nominal gain of -0.5.

That\'s connected to the terminated end of an
RG-174/U cable going to a PTS-500 synthesizer. The
output goes via a 10-ohm resistor into a
properly-terminated 50-ohm cable (the TDS 694C is
50-ohm only).

Here I\'m expecting a bandwidth somewhere between the
datasheet\'s 1 GHz @ Av=1 and 400 MHz @ Av=-1, but
it\'s way off. There\'s no visible change when I put
the jumper back in, on account of the swoopy
bootstrap.

I was going to suggest looking at the \'speed\' of the
light source. But the above seems to point to something
\'in\' the amp stage... (Is that right?) (And maybe check
the light source rise time anyway?)

George H.

So where do you suppose the missing factor of ~3 in
bandwidth went?


This has the SiPM and bootstrap disconnected (0 ohm
jumper removed) and a 1/20W leaded 1k resistor bodged in
to make an inverting amp with a gain of -0.5.

I\'m looking at the trace capacitance to figure out if
that might be it. There\'s about 3/4 inch of 10-mil trace
on the summing junction, but that ought to produce a high
frequency peak if anything. hard to find 1.4 pF across
the feedback resistor. Once I\'m back in the lab I\'ll
measure a bare board with a Boonton and see.

Cheers

Phil Hobbs

What\'s the board stackup? Not the specified one, but the
real one. I\'ve been burned by what some of the fast-turn
proto houses do.

https://www.dropbox.com/s/p3vpbaofzqurebz/Z462_PCB_Way_2.png?raw=1





The SJ capacitance is 2.4 pF, as measured on a Boonton, about
twice what I expected. That seems to be the issue--in
simulation it produces a pretty big gain peak, which reduces
the bandwidth.

Time to Dremel the ground plane.

How many layers?

Only four, but of course ground is L2 and there\'s an L3 ground
pour in that area. It is PCBway, so maybe they did the same
thing to me. (A generally very good outfit in many ways,
especially price and delivery.)

Several of the chinese quick-turn houses make 4-layer boards with
very thin (like 4 mil) outer dielectrics. Maybe they roll process
the outers and glue them to a core or something.



Maybe section the board to see what the stackup actually is.


Monday I\'ll look at it under the good microscope. (About three
years ago, I got a beautiful Mitutoyo FS-110 with 2x-50x long
working-distance objectives for $2k on eBay. Apparently the guy
didn\'t know what he had, because he shipped this massive
precision instrument in a cardboard box with foam peanuts. The
box was a mess when it got here, but the scope survived because
it\'s a beast.)

I sheared and sandpapered the example I posted, and shot it with
my super-good microscope

https://www.amazon.com/gp/product/B01IV0TV50/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1



I have one a bit like that, but with a better base:
https://www.ebay.com/itm/Andonstar-500X-2MP-USB-Digital-Microscope-Video-webcam-Magnifier-Camera-Stand/143844969460



It does need a rubber band to apply a rotational preload to the arm.

Here is a Dremel-optimized SMA connector.


https://www.dropbox.com/s/gc5j45995nqiftu/Isola_Right_Trim.jpg?raw=1



https://www.dropbox.com/s/t0cnfe9do4slvvf/Isola_Trimmed_Right_TDR.jpg?raw=1


Nice.

We\'ve had that problem too--you need some extra pad area for the
solder fillet, but that makes a nasty capacitive discontinuity at
the connector.

The impedance of that SMA edge-launch connector, center pin to the
four ground pins, is about 100 ohms in free air. So the PCB has to
be about 100 in the pin region too. We\'ve worked that out, cutting
away inners and paving over the bottom with ground. We simulated
the whole geometry with ATLC to get the dims right. That was cool.

The case I posted was testing a laminate sample, 20 mils thick.

The $1.50 edge-launches are just as good as the $12 microwave
connectors if the layout is right, at least as far as we can
resolve with 30 ps TDR.

I sometimes cut away layer 2 (or more) under critical circuit
nodes. My triggered Colpitts oscillator has a driven guard patch on
layer 5.

You bootstrap photodiodes, so you might bootstrap the PCB too.

I\'ve often done that in high-Z TIAs, like 10M. The bootstrap
effectiveness is limited by the capacitance to ground from the
summing junction.

I wasn\'t expecting it to be a big issue with a 511-ohm TIA, but then
I wasn\'t expecting that much capacitance either. I\'ll have to
calculate whether the SAV-551+ can bootstrap the pour as well as the
SiPM.

The usual local feedback tricks such as the PNP wraparound sort of
need fast PNPs, which are no longer made. (The Renesas/Intersil
ones are a sort-of exception.)

Well, I took out the Dremel, with one of JL\'s fave dental burrs on it,
and got the SJ capacitance down to 1.2 pF. It was actually pretty easy,
and the results are sort of interesting-looking.

https://electrooptical.net/www/sed/SiPMdremelled2.png

Top-to-bottom dimension is 18 mm.

Cheers

Phil Hobbs

Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com


Is that a 2-layer board? They are usually about 15 pf per square inch,
with a lot of fringing.

If we get our pcb laser blaster going, we\'ll mostly make 2-sided
boards with all ground plane on the back. It will also be a huge time
saver if we leave a lot of copper on the top, namely blast away just
enough copper to make the insulated features. That will make a lot of
capacitance, and lots of coplanar waveguide.

I\'m envisioning secondary Dremel operations, or maybe a lot of
lasering.


If I need to remove medium-size regions of copper, maybe up to 3/4 inch
wide, from a prototype, then I peel it off as follows: Cut around the
region I want to peel off, using a scalpel, saw, dremel or whatever.
Then stick a knife, scalpel, etc under one corner to lift the copper off
the epoxy just enough to grab hold of, then put a big hot metcal tip
asross the line where I need the epoxy to peel off, e.g. SMTC-161, which
softens the epoxy, and I grab the corner that I lifted with some good
needle-nose pliers. Then I just pull gently upwards with the pliers as I
slide the soldering iron so that it is always heating the part where it
needs to peel off. Sometimes a little solder helps to get the heat from
the iron into the copper foil. Peeling it this way is quick and doesn\'t
create dust or copper swarf to get between pins etc.

I guess we could do that, around the capacitance-critical nodes.



--

John Larkin Highland Technology, Inc

The best designs are necessarily accidental.

 

[Phil Hobbs]

On 11/23/20 6:54 PM, jlarkin@highlandsniptechnology.com wrote:

On Mon, 23 Nov 2020 15:11:05 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/23/20 2:43 PM, jlarkin@highlandsniptechnology.com wrote:
On Mon, 23 Nov 2020 13:58:26 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/21/20 6:48 PM, Phil Hobbs wrote:
On 11/21/20 11:05 AM, jlarkin@highlandsniptechnology.com wrote:
On Sat, 21 Nov 2020 07:05:11 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/21/20 12:39 AM, jlarkin@highlandsniptechnology.com wrote:
On Fri, 20 Nov 2020 22:35:52 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/19/20 10:44 AM, jlarkin@highlandsniptechnology.com
wrote:
On Thu, 19 Nov 2020 10:30:17 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/19/20 8:22 AM, George Herold wrote:
On Wednesday, November 18, 2020 at 2:27:39 PM UTC-5,
Phil Hobbs wrote:
Hi, all.

So I have this SiPM/MPPC front end. It has pop
options to use either an On Semi MicroFC-10010 1-mm
SiPM chip or a packaged Hamamatsu S13362-3050DG 3-mm
MPPC with integral TE cooler, both bootstrapped by a
SAV-551+ running at 20 mA. So far, it all works.

(The SAV-551+ is amazingly stable--I\'ve got a
shipping product that runs a very similar bootstrap
across a 2-inch FFC cable. Bandwidth suffers a bit,
but it shows no tendency to oscillate.)

The mystery is in the TIA stage. It\'s a vanilla op
amp TIA made from either an ADA4899 (600 MHz, 300
V/us) or AD8045 (1 GHz, 1300 V/us @ Av=1), which are
pin compatible in the 3-mm LFCSP package. Both are
voltage feedback amps.

I\'m seeing a 3 dB bandwidth of 220 MHz, together with
a faster rolloff than I expect: -3 dB @ 220 MHz, -9
dB @ 320 MHz. It\'s not slew limiting, because the
waveform looks pretty good on a 3-GHz scope (TDS
694C) and the rolloff stays the same when I drop the
input by 6 dB.

The layout is pretty tight (the whole board is only
an inch square), so getting enough stray capacitance
across R_F to account for it is implausible--it would
need about 1.4 pF. DecouplingBypassing is good--

For test, I removed the 0-ohm jumper that connects
the bootstrapped SiPM to the summing junction, and
added a 1k input resistor, forming an inverting amp
with a nominal gain of -0.5.

That\'s connected to the terminated end of an
RG-174/U cable going to a PTS-500 synthesizer. The
output goes via a 10-ohm resistor into a
properly-terminated 50-ohm cable (the TDS 694C is
50-ohm only).

Here I\'m expecting a bandwidth somewhere between the
datasheet\'s 1 GHz @ Av=1 and 400 MHz @ Av=-1, but
it\'s way off. There\'s no visible change when I put
the jumper back in, on account of the swoopy
bootstrap.

I was going to suggest looking at the \'speed\' of the
light source. But the above seems to point to something
\'in\' the amp stage... (Is that right?) (And maybe check
the light source rise time anyway?)

George H.

So where do you suppose the missing factor of ~3 in
bandwidth went?


This has the SiPM and bootstrap disconnected (0 ohm
jumper removed) and a 1/20W leaded 1k resistor bodged in
to make an inverting amp with a gain of -0.5.

I\'m looking at the trace capacitance to figure out if
that might be it. There\'s about 3/4 inch of 10-mil trace
on the summing junction, but that ought to produce a high
frequency peak if anything. hard to find 1.4 pF across
the feedback resistor. Once I\'m back in the lab I\'ll
measure a bare board with a Boonton and see.

Cheers

Phil Hobbs

What\'s the board stackup? Not the specified one, but the
real one. I\'ve been burned by what some of the fast-turn
proto houses do.

https://www.dropbox.com/s/p3vpbaofzqurebz/Z462_PCB_Way_2.png?raw=1





The SJ capacitance is 2.4 pF, as measured on a Boonton, about
twice what I expected. That seems to be the issue--in
simulation it produces a pretty big gain peak, which reduces
the bandwidth.

Time to Dremel the ground plane.

How many layers?

Only four, but of course ground is L2 and there\'s an L3 ground
pour in that area. It is PCBway, so maybe they did the same
thing to me. (A generally very good outfit in many ways,
especially price and delivery.)

Several of the chinese quick-turn houses make 4-layer boards with
very thin (like 4 mil) outer dielectrics. Maybe they roll process
the outers and glue them to a core or something.



Maybe section the board to see what the stackup actually is.


Monday I\'ll look at it under the good microscope. (About three
years ago, I got a beautiful Mitutoyo FS-110 with 2x-50x long
working-distance objectives for $2k on eBay. Apparently the guy
didn\'t know what he had, because he shipped this massive
precision instrument in a cardboard box with foam peanuts. The
box was a mess when it got here, but the scope survived because
it\'s a beast.)

I sheared and sandpapered the example I posted, and shot it with
my super-good microscope

https://www.amazon.com/gp/product/B01IV0TV50/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1



I have one a bit like that, but with a better base:
https://www.ebay.com/itm/Andonstar-500X-2MP-USB-Digital-Microscope-Video-webcam-Magnifier-Camera-Stand/143844969460



It does need a rubber band to apply a rotational preload to the arm.

Here is a Dremel-optimized SMA connector.


https://www.dropbox.com/s/gc5j45995nqiftu/Isola_Right_Trim.jpg?raw=1



https://www.dropbox.com/s/t0cnfe9do4slvvf/Isola_Trimmed_Right_TDR.jpg?raw=1


Nice.

We\'ve had that problem too--you need some extra pad area for the
solder fillet, but that makes a nasty capacitive discontinuity at
the connector.

The impedance of that SMA edge-launch connector, center pin to the
four ground pins, is about 100 ohms in free air. So the PCB has to
be about 100 in the pin region too. We\'ve worked that out, cutting
away inners and paving over the bottom with ground. We simulated
the whole geometry with ATLC to get the dims right. That was cool.

The case I posted was testing a laminate sample, 20 mils thick.

The $1.50 edge-launches are just as good as the $12 microwave
connectors if the layout is right, at least as far as we can
resolve with 30 ps TDR.

I sometimes cut away layer 2 (or more) under critical circuit
nodes. My triggered Colpitts oscillator has a driven guard patch on
layer 5.

You bootstrap photodiodes, so you might bootstrap the PCB too.

I\'ve often done that in high-Z TIAs, like 10M. The bootstrap
effectiveness is limited by the capacitance to ground from the
summing junction.

I wasn\'t expecting it to be a big issue with a 511-ohm TIA, but then
I wasn\'t expecting that much capacitance either. I\'ll have to
calculate whether the SAV-551+ can bootstrap the pour as well as the
SiPM.

The usual local feedback tricks such as the PNP wraparound sort of
need fast PNPs, which are no longer made. (The Renesas/Intersil
ones are a sort-of exception.)

Well, I took out the Dremel, with one of JL\'s fave dental burrs on it,
and got the SJ capacitance down to 1.2 pF. It was actually pretty easy,
and the results are sort of interesting-looking.

https://electrooptical.net/www/sed/SiPMdremelled2.png

Top-to-bottom dimension is 18 mm.


Is that a 2-layer board? They are usually about 15 pf per square inch,
with a lot of fringing.

No, it\'s 4 layers, with ground on L2. The #40 wire going down the
middle of the canyon there replaces an L3 trace.

If we get our pcb laser blaster going, we\'ll mostly make 2-sided
boards with all ground plane on the back. It will also be a huge time
saver if we leave a lot of copper on the top, namely blast away just
enough copper to make the insulated features. That will make a lot of
capacitance, and lots of coplanar waveguide.

I\'m envisioning secondary Dremel operations, or maybe a lot of
lasering.

Fun. BTW PCBway recently cut their assembly prices by a lot.

Cheers

Phil Hobbs


Did you ever section that board? Maybe the L2 ground plane is very
close to the layer 1 traces. Seems like a lot of c for some tiny
features like that.

We got our own p+p line because kitting and logistics were a hassle
for using outside assemblers.

Yeah, I cut one with snips and sanded it. The prepreg was a bit skinny,
but not 0.1 mm. Mostly I got snookered by the AD8045 datasheet\'s
specsmanship--their headline 1-GHz bandwidth number turns out to be a
result of the details of the frequency compensation, and only applies in
the noninverting configuration. Otherwise it\'s basically a pretty nice
600-MHz GBW op amp, but not something for sub-nanosecond work.

The next version will connect the SAV-551+ bootstrap directly to the
inverting input of an EL5166 CFA, and add a well-filtered chopamp to
take out the resulting DC offset. The pHEMT\'s source won\'t even notice
the current noise of the CFA, whereas if it were connected to the PD it
would dominate the noise.

A nuisance, but quite a preventable one--my least favourite kind.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[server]

On Tue, 24 Nov 2020 17:58:44 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/23/20 6:54 PM, jlarkin@highlandsniptechnology.com wrote:
On Mon, 23 Nov 2020 15:11:05 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/23/20 2:43 PM, jlarkin@highlandsniptechnology.com wrote:
On Mon, 23 Nov 2020 13:58:26 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/21/20 6:48 PM, Phil Hobbs wrote:
On 11/21/20 11:05 AM, jlarkin@highlandsniptechnology.com wrote:
On Sat, 21 Nov 2020 07:05:11 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/21/20 12:39 AM, jlarkin@highlandsniptechnology.com wrote:
On Fri, 20 Nov 2020 22:35:52 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/19/20 10:44 AM, jlarkin@highlandsniptechnology.com
wrote:
On Thu, 19 Nov 2020 10:30:17 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/19/20 8:22 AM, George Herold wrote:
On Wednesday, November 18, 2020 at 2:27:39 PM UTC-5,
Phil Hobbs wrote:
Hi, all.

So I have this SiPM/MPPC front end. It has pop
options to use either an On Semi MicroFC-10010 1-mm
SiPM chip or a packaged Hamamatsu S13362-3050DG 3-mm
MPPC with integral TE cooler, both bootstrapped by a
SAV-551+ running at 20 mA. So far, it all works.

(The SAV-551+ is amazingly stable--I\'ve got a
shipping product that runs a very similar bootstrap
across a 2-inch FFC cable. Bandwidth suffers a bit,
but it shows no tendency to oscillate.)

The mystery is in the TIA stage. It\'s a vanilla op
amp TIA made from either an ADA4899 (600 MHz, 300
V/us) or AD8045 (1 GHz, 1300 V/us @ Av=1), which are
pin compatible in the 3-mm LFCSP package. Both are
voltage feedback amps.

I\'m seeing a 3 dB bandwidth of 220 MHz, together with
a faster rolloff than I expect: -3 dB @ 220 MHz, -9
dB @ 320 MHz. It\'s not slew limiting, because the
waveform looks pretty good on a 3-GHz scope (TDS
694C) and the rolloff stays the same when I drop the
input by 6 dB.

The layout is pretty tight (the whole board is only
an inch square), so getting enough stray capacitance
across R_F to account for it is implausible--it would
need about 1.4 pF. DecouplingBypassing is good--

For test, I removed the 0-ohm jumper that connects
the bootstrapped SiPM to the summing junction, and
added a 1k input resistor, forming an inverting amp
with a nominal gain of -0.5.

That\'s connected to the terminated end of an
RG-174/U cable going to a PTS-500 synthesizer. The
output goes via a 10-ohm resistor into a
properly-terminated 50-ohm cable (the TDS 694C is
50-ohm only).

Here I\'m expecting a bandwidth somewhere between the
datasheet\'s 1 GHz @ Av=1 and 400 MHz @ Av=-1, but
it\'s way off. There\'s no visible change when I put
the jumper back in, on account of the swoopy
bootstrap.

I was going to suggest looking at the \'speed\' of the
light source. But the above seems to point to something
\'in\' the amp stage... (Is that right?) (And maybe check
the light source rise time anyway?)

George H.

So where do you suppose the missing factor of ~3 in
bandwidth went?


This has the SiPM and bootstrap disconnected (0 ohm
jumper removed) and a 1/20W leaded 1k resistor bodged in
to make an inverting amp with a gain of -0.5.

I\'m looking at the trace capacitance to figure out if
that might be it. There\'s about 3/4 inch of 10-mil trace
on the summing junction, but that ought to produce a high
frequency peak if anything. hard to find 1.4 pF across
the feedback resistor. Once I\'m back in the lab I\'ll
measure a bare board with a Boonton and see.

Cheers

Phil Hobbs

What\'s the board stackup? Not the specified one, but the
real one. I\'ve been burned by what some of the fast-turn
proto houses do.

https://www.dropbox.com/s/p3vpbaofzqurebz/Z462_PCB_Way_2.png?raw=1





The SJ capacitance is 2.4 pF, as measured on a Boonton, about
twice what I expected. That seems to be the issue--in
simulation it produces a pretty big gain peak, which reduces
the bandwidth.

Time to Dremel the ground plane.

How many layers?

Only four, but of course ground is L2 and there\'s an L3 ground
pour in that area. It is PCBway, so maybe they did the same
thing to me. (A generally very good outfit in many ways,
especially price and delivery.)

Several of the chinese quick-turn houses make 4-layer boards with
very thin (like 4 mil) outer dielectrics. Maybe they roll process
the outers and glue them to a core or something.



Maybe section the board to see what the stackup actually is.


Monday I\'ll look at it under the good microscope. (About three
years ago, I got a beautiful Mitutoyo FS-110 with 2x-50x long
working-distance objectives for $2k on eBay. Apparently the guy
didn\'t know what he had, because he shipped this massive
precision instrument in a cardboard box with foam peanuts. The
box was a mess when it got here, but the scope survived because
it\'s a beast.)

I sheared and sandpapered the example I posted, and shot it with
my super-good microscope

https://www.amazon.com/gp/product/B01IV0TV50/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1



I have one a bit like that, but with a better base:
https://www.ebay.com/itm/Andonstar-500X-2MP-USB-Digital-Microscope-Video-webcam-Magnifier-Camera-Stand/143844969460



It does need a rubber band to apply a rotational preload to the arm.

Here is a Dremel-optimized SMA connector.


https://www.dropbox.com/s/gc5j45995nqiftu/Isola_Right_Trim.jpg?raw=1



https://www.dropbox.com/s/t0cnfe9do4slvvf/Isola_Trimmed_Right_TDR.jpg?raw=1


Nice.

We\'ve had that problem too--you need some extra pad area for the
solder fillet, but that makes a nasty capacitive discontinuity at
the connector.

The impedance of that SMA edge-launch connector, center pin to the
four ground pins, is about 100 ohms in free air. So the PCB has to
be about 100 in the pin region too. We\'ve worked that out, cutting
away inners and paving over the bottom with ground. We simulated
the whole geometry with ATLC to get the dims right. That was cool.

The case I posted was testing a laminate sample, 20 mils thick.

The $1.50 edge-launches are just as good as the $12 microwave
connectors if the layout is right, at least as far as we can
resolve with 30 ps TDR.

I sometimes cut away layer 2 (or more) under critical circuit
nodes. My triggered Colpitts oscillator has a driven guard patch on
layer 5.

You bootstrap photodiodes, so you might bootstrap the PCB too.

I\'ve often done that in high-Z TIAs, like 10M. The bootstrap
effectiveness is limited by the capacitance to ground from the
summing junction.

I wasn\'t expecting it to be a big issue with a 511-ohm TIA, but then
I wasn\'t expecting that much capacitance either. I\'ll have to
calculate whether the SAV-551+ can bootstrap the pour as well as the
SiPM.

The usual local feedback tricks such as the PNP wraparound sort of
need fast PNPs, which are no longer made. (The Renesas/Intersil
ones are a sort-of exception.)

Well, I took out the Dremel, with one of JL\'s fave dental burrs on it,
and got the SJ capacitance down to 1.2 pF. It was actually pretty easy,
and the results are sort of interesting-looking.

https://electrooptical.net/www/sed/SiPMdremelled2.png

Top-to-bottom dimension is 18 mm.


Is that a 2-layer board? They are usually about 15 pf per square inch,
with a lot of fringing.

No, it\'s 4 layers, with ground on L2. The #40 wire going down the
middle of the canyon there replaces an L3 trace.

If we get our pcb laser blaster going, we\'ll mostly make 2-sided
boards with all ground plane on the back. It will also be a huge time
saver if we leave a lot of copper on the top, namely blast away just
enough copper to make the insulated features. That will make a lot of
capacitance, and lots of coplanar waveguide.

I\'m envisioning secondary Dremel operations, or maybe a lot of
lasering.

Fun. BTW PCBway recently cut their assembly prices by a lot.

Cheers

Phil Hobbs


Did you ever section that board? Maybe the L2 ground plane is very
close to the layer 1 traces. Seems like a lot of c for some tiny
features like that.

We got our own p+p line because kitting and logistics were a hassle
for using outside assemblers.

Yeah, I cut one with snips and sanded it. The prepreg was a bit skinny,
but not 0.1 mm. Mostly I got snookered by the AD8045 datasheet\'s
specsmanship--their headline 1-GHz bandwidth number turns out to be a
result of the details of the frequency compensation, and only applies in
the noninverting configuration. Otherwise it\'s basically a pretty nice
600-MHz GBW op amp, but not something for sub-nanosecond work.

The next version will connect the SAV-551+ bootstrap directly to the
inverting input of an EL5166 CFA, and add a well-filtered chopamp to
take out the resulting DC offset. The pHEMT\'s source won\'t even notice
the current noise of the CFA, whereas if it were connected to the PD it
would dominate the noise.

A nuisance, but quite a preventable one--my least favourite kind.

Cheers

Phil Hobbs

Bootstrap into opamp inverting input? I don\'t understand that.

Do you know about THS4303? It\'s a roughly current-mode opamp with an
internal g=+10 feedback network. 1.8 GHz net bw, and noise is pretty
good.



--

John Larkin Highland Technology, Inc

The best designs are necessarily accidental.

 

[Phil Hobbs]

On 11/24/20 10:33 PM, jlarkin@highlandsniptechnology.com wrote:

On Tue, 24 Nov 2020 17:58:44 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/23/20 6:54 PM, jlarkin@highlandsniptechnology.com wrote:
On Mon, 23 Nov 2020 15:11:05 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/23/20 2:43 PM, jlarkin@highlandsniptechnology.com wrote:
On Mon, 23 Nov 2020 13:58:26 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/21/20 6:48 PM, Phil Hobbs wrote:
On 11/21/20 11:05 AM, jlarkin@highlandsniptechnology.com wrote:
On Sat, 21 Nov 2020 07:05:11 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/21/20 12:39 AM, jlarkin@highlandsniptechnology.com wrote:
On Fri, 20 Nov 2020 22:35:52 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/19/20 10:44 AM, jlarkin@highlandsniptechnology.com
wrote:
On Thu, 19 Nov 2020 10:30:17 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/19/20 8:22 AM, George Herold wrote:
On Wednesday, November 18, 2020 at 2:27:39 PM UTC-5,
Phil Hobbs wrote:
Hi, all.

So I have this SiPM/MPPC front end. It has pop
options to use either an On Semi MicroFC-10010 1-mm
SiPM chip or a packaged Hamamatsu S13362-3050DG 3-mm
MPPC with integral TE cooler, both bootstrapped by a
SAV-551+ running at 20 mA. So far, it all works.

(The SAV-551+ is amazingly stable--I\'ve got a
shipping product that runs a very similar bootstrap
across a 2-inch FFC cable. Bandwidth suffers a bit,
but it shows no tendency to oscillate.)

The mystery is in the TIA stage. It\'s a vanilla op
amp TIA made from either an ADA4899 (600 MHz, 300
V/us) or AD8045 (1 GHz, 1300 V/us @ Av=1), which are
pin compatible in the 3-mm LFCSP package. Both are
voltage feedback amps.

I\'m seeing a 3 dB bandwidth of 220 MHz, together with
a faster rolloff than I expect: -3 dB @ 220 MHz, -9
dB @ 320 MHz. It\'s not slew limiting, because the
waveform looks pretty good on a 3-GHz scope (TDS
694C) and the rolloff stays the same when I drop the
input by 6 dB.

The layout is pretty tight (the whole board is only
an inch square), so getting enough stray capacitance
across R_F to account for it is implausible--it would
need about 1.4 pF. DecouplingBypassing is good--

For test, I removed the 0-ohm jumper that connects
the bootstrapped SiPM to the summing junction, and
added a 1k input resistor, forming an inverting amp
with a nominal gain of -0.5.

That\'s connected to the terminated end of an
RG-174/U cable going to a PTS-500 synthesizer. The
output goes via a 10-ohm resistor into a
properly-terminated 50-ohm cable (the TDS 694C is
50-ohm only).

Here I\'m expecting a bandwidth somewhere between the
datasheet\'s 1 GHz @ Av=1 and 400 MHz @ Av=-1, but
it\'s way off. There\'s no visible change when I put
the jumper back in, on account of the swoopy
bootstrap.

I was going to suggest looking at the \'speed\' of the
light source. But the above seems to point to something
\'in\' the amp stage... (Is that right?) (And maybe check
the light source rise time anyway?)

George H.

So where do you suppose the missing factor of ~3 in
bandwidth went?


This has the SiPM and bootstrap disconnected (0 ohm
jumper removed) and a 1/20W leaded 1k resistor bodged in
to make an inverting amp with a gain of -0.5.

I\'m looking at the trace capacitance to figure out if
that might be it. There\'s about 3/4 inch of 10-mil trace
on the summing junction, but that ought to produce a high
frequency peak if anything. hard to find 1.4 pF across
the feedback resistor. Once I\'m back in the lab I\'ll
measure a bare board with a Boonton and see.

Cheers

Phil Hobbs

What\'s the board stackup? Not the specified one, but the
real one. I\'ve been burned by what some of the fast-turn
proto houses do.

https://www.dropbox.com/s/p3vpbaofzqurebz/Z462_PCB_Way_2.png?raw=1





The SJ capacitance is 2.4 pF, as measured on a Boonton, about
twice what I expected. That seems to be the issue--in
simulation it produces a pretty big gain peak, which reduces
the bandwidth.

Time to Dremel the ground plane.

How many layers?

Only four, but of course ground is L2 and there\'s an L3 ground
pour in that area. It is PCBway, so maybe they did the same
thing to me. (A generally very good outfit in many ways,
especially price and delivery.)

Several of the chinese quick-turn houses make 4-layer boards with
very thin (like 4 mil) outer dielectrics. Maybe they roll process
the outers and glue them to a core or something.



Maybe section the board to see what the stackup actually is.


Monday I\'ll look at it under the good microscope. (About three
years ago, I got a beautiful Mitutoyo FS-110 with 2x-50x long
working-distance objectives for $2k on eBay. Apparently the guy
didn\'t know what he had, because he shipped this massive
precision instrument in a cardboard box with foam peanuts. The
box was a mess when it got here, but the scope survived because
it\'s a beast.)

I sheared and sandpapered the example I posted, and shot it with
my super-good microscope

https://www.amazon.com/gp/product/B01IV0TV50/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1



I have one a bit like that, but with a better base:
https://www.ebay.com/itm/Andonstar-500X-2MP-USB-Digital-Microscope-Video-webcam-Magnifier-Camera-Stand/143844969460



It does need a rubber band to apply a rotational preload to the arm.

Here is a Dremel-optimized SMA connector.


https://www.dropbox.com/s/gc5j45995nqiftu/Isola_Right_Trim.jpg?raw=1



https://www.dropbox.com/s/t0cnfe9do4slvvf/Isola_Trimmed_Right_TDR.jpg?raw=1


Nice.

We\'ve had that problem too--you need some extra pad area for the
solder fillet, but that makes a nasty capacitive discontinuity at
the connector.

The impedance of that SMA edge-launch connector, center pin to the
four ground pins, is about 100 ohms in free air. So the PCB has to
be about 100 in the pin region too. We\'ve worked that out, cutting
away inners and paving over the bottom with ground. We simulated
the whole geometry with ATLC to get the dims right. That was cool.

The case I posted was testing a laminate sample, 20 mils thick.

The $1.50 edge-launches are just as good as the $12 microwave
connectors if the layout is right, at least as far as we can
resolve with 30 ps TDR.

I sometimes cut away layer 2 (or more) under critical circuit
nodes. My triggered Colpitts oscillator has a driven guard patch on
layer 5.

You bootstrap photodiodes, so you might bootstrap the PCB too.

I\'ve often done that in high-Z TIAs, like 10M. The bootstrap
effectiveness is limited by the capacitance to ground from the
summing junction.

I wasn\'t expecting it to be a big issue with a 511-ohm TIA, but then
I wasn\'t expecting that much capacitance either. I\'ll have to
calculate whether the SAV-551+ can bootstrap the pour as well as the
SiPM.

The usual local feedback tricks such as the PNP wraparound sort of
need fast PNPs, which are no longer made. (The Renesas/Intersil
ones are a sort-of exception.)

Well, I took out the Dremel, with one of JL\'s fave dental burrs on it,
and got the SJ capacitance down to 1.2 pF. It was actually pretty easy,
and the results are sort of interesting-looking.

https://electrooptical.net/www/sed/SiPMdremelled2.png

Top-to-bottom dimension is 18 mm.


Is that a 2-layer board? They are usually about 15 pf per square inch,
with a lot of fringing.

No, it\'s 4 layers, with ground on L2. The #40 wire going down the
middle of the canyon there replaces an L3 trace.

If we get our pcb laser blaster going, we\'ll mostly make 2-sided
boards with all ground plane on the back. It will also be a huge time
saver if we leave a lot of copper on the top, namely blast away just
enough copper to make the insulated features. That will make a lot of
capacitance, and lots of coplanar waveguide.

I\'m envisioning secondary Dremel operations, or maybe a lot of
lasering.

Fun. BTW PCBway recently cut their assembly prices by a lot.

Cheers

Phil Hobbs


Did you ever section that board? Maybe the L2 ground plane is very
close to the layer 1 traces. Seems like a lot of c for some tiny
features like that.

We got our own p+p line because kitting and logistics were a hassle
for using outside assemblers.

Yeah, I cut one with snips and sanded it. The prepreg was a bit skinny,
but not 0.1 mm. Mostly I got snookered by the AD8045 datasheet\'s
specsmanship--their headline 1-GHz bandwidth number turns out to be a
result of the details of the frequency compensation, and only applies in
the noninverting configuration. Otherwise it\'s basically a pretty nice
600-MHz GBW op amp, but not something for sub-nanosecond work.

The next version will connect the SAV-551+ bootstrap directly to the
inverting input of an EL5166 CFA, and add a well-filtered chopamp to
take out the resulting DC offset. The pHEMT\'s source won\'t even notice
the current noise of the CFA, whereas if it were connected to the PD it
would dominate the noise.

A nuisance, but quite a preventable one--my least favourite kind.

Bootstrap into opamp inverting input? I don\'t understand that.

Do you know about THS4303? It\'s a roughly current-mode opamp with an
internal g=+10 feedback network. 1.8 GHz net bw, and noise is pretty
good.

If you hang a pHEMT bootstrap on a photodiode, the gate and source go up
and down together to decent accuracy.

The classical bootstrap architecture is off to the side, so that the TIA
connects directly to the cathode of the PD. That makes the DC and
low-frequency behaviour of the bootstrap device essentially irrelevant,
which is nice. The down side is that the current noise of the TIA stage
gets summed with the photocurrent, which can be a problem in low light.

Alternatively, though, you can connect the op amp to the source of the
bootstrap device. There are op amps such as the LM6171A, which is
basically a CFA with a follower driving its inverting input--this
notion just uses the follower to bootstrap the PD as well as driving the
inverting input. (Our QL01 nanowatt photoreceiver does that.)

Since the bootstrap\'s source follows its gate, this is pretty much
equivalent except that (on the plus side) you can use much gnarlier op
amps, and (on the minus side) the bootstrap has to have decent DC and
low-frequency behaviour.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Uwe Bonnes]

jlarkin@highlandsniptechnology.com wrote:

On Tue, 24 Nov 2020 17:58:44 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/23/20 6:54 PM, jlarkin@highlandsniptechnology.com wrote:
On 11/19/20 8:22 AM, George Herold wrote:
On Wednesday, November 18, 2020 at 2:27:39 PM UTC-5,
Phil Hobbs wrote:
Hi, all.

Why do people spoil such an interesting discussion by such excessive
quoting? If one later searches the net for this discussion, one will
drown in quotes...
--
Uwe Bonnes bon@elektron.ikp.physik.tu-darmstadt.de

Institut fuer Kernphysik Schlossgartenstrasse 9 64289 Darmstadt
--------- Tel. 06151 1623569 ------- Fax. 06151 1623305 ---------

 

[Phil Hobbs]

On 11/25/20 4:59 AM, Uwe Bonnes wrote:

jlarkin@highlandsniptechnology.com wrote:
On Tue, 24 Nov 2020 17:58:44 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/23/20 6:54 PM, jlarkin@highlandsniptechnology.com wrote:
On 11/19/20 8:22 AM, George Herold wrote:
On Wednesday, November 18, 2020 at 2:27:39 PM UTC-5,
Phil Hobbs wrote:
Hi, all.
Why do people spoil such an interesting discussion by such excessive
quoting? If one later searches the net for this discussion, one will
drown in quotes...

There\'s this really useful button for that! It\'s labelled \"PgDn\"!

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[server]

On Tue, 24 Nov 2020 23:07:57 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/24/20 10:33 PM, jlarkin@highlandsniptechnology.com wrote:
On Tue, 24 Nov 2020 17:58:44 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/23/20 6:54 PM, jlarkin@highlandsniptechnology.com wrote:
On Mon, 23 Nov 2020 15:11:05 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/23/20 2:43 PM, jlarkin@highlandsniptechnology.com wrote:
On Mon, 23 Nov 2020 13:58:26 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/21/20 6:48 PM, Phil Hobbs wrote:
On 11/21/20 11:05 AM, jlarkin@highlandsniptechnology.com wrote:
On Sat, 21 Nov 2020 07:05:11 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/21/20 12:39 AM, jlarkin@highlandsniptechnology.com wrote:
On Fri, 20 Nov 2020 22:35:52 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/19/20 10:44 AM, jlarkin@highlandsniptechnology.com
wrote:
On Thu, 19 Nov 2020 10:30:17 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/19/20 8:22 AM, George Herold wrote:
On Wednesday, November 18, 2020 at 2:27:39 PM UTC-5,
Phil Hobbs wrote:
Hi, all.

So I have this SiPM/MPPC front end. It has pop
options to use either an On Semi MicroFC-10010 1-mm
SiPM chip or a packaged Hamamatsu S13362-3050DG 3-mm
MPPC with integral TE cooler, both bootstrapped by a
SAV-551+ running at 20 mA. So far, it all works.

(The SAV-551+ is amazingly stable--I\'ve got a
shipping product that runs a very similar bootstrap
across a 2-inch FFC cable. Bandwidth suffers a bit,
but it shows no tendency to oscillate.)

The mystery is in the TIA stage. It\'s a vanilla op
amp TIA made from either an ADA4899 (600 MHz, 300
V/us) or AD8045 (1 GHz, 1300 V/us @ Av=1), which are
pin compatible in the 3-mm LFCSP package. Both are
voltage feedback amps.

I\'m seeing a 3 dB bandwidth of 220 MHz, together with
a faster rolloff than I expect: -3 dB @ 220 MHz, -9
dB @ 320 MHz. It\'s not slew limiting, because the
waveform looks pretty good on a 3-GHz scope (TDS
694C) and the rolloff stays the same when I drop the
input by 6 dB.

The layout is pretty tight (the whole board is only
an inch square), so getting enough stray capacitance
across R_F to account for it is implausible--it would
need about 1.4 pF. DecouplingBypassing is good--

For test, I removed the 0-ohm jumper that connects
the bootstrapped SiPM to the summing junction, and
added a 1k input resistor, forming an inverting amp
with a nominal gain of -0.5.

That\'s connected to the terminated end of an
RG-174/U cable going to a PTS-500 synthesizer. The
output goes via a 10-ohm resistor into a
properly-terminated 50-ohm cable (the TDS 694C is
50-ohm only).

Here I\'m expecting a bandwidth somewhere between the
datasheet\'s 1 GHz @ Av=1 and 400 MHz @ Av=-1, but
it\'s way off. There\'s no visible change when I put
the jumper back in, on account of the swoopy
bootstrap.

I was going to suggest looking at the \'speed\' of the
light source. But the above seems to point to something
\'in\' the amp stage... (Is that right?) (And maybe check
the light source rise time anyway?)

George H.

So where do you suppose the missing factor of ~3 in
bandwidth went?


This has the SiPM and bootstrap disconnected (0 ohm
jumper removed) and a 1/20W leaded 1k resistor bodged in
to make an inverting amp with a gain of -0.5.

I\'m looking at the trace capacitance to figure out if
that might be it. There\'s about 3/4 inch of 10-mil trace
on the summing junction, but that ought to produce a high
frequency peak if anything. hard to find 1.4 pF across
the feedback resistor. Once I\'m back in the lab I\'ll
measure a bare board with a Boonton and see.

Cheers

Phil Hobbs

What\'s the board stackup? Not the specified one, but the
real one. I\'ve been burned by what some of the fast-turn
proto houses do.

https://www.dropbox.com/s/p3vpbaofzqurebz/Z462_PCB_Way_2.png?raw=1





The SJ capacitance is 2.4 pF, as measured on a Boonton, about
twice what I expected. That seems to be the issue--in
simulation it produces a pretty big gain peak, which reduces
the bandwidth.

Time to Dremel the ground plane.

How many layers?

Only four, but of course ground is L2 and there\'s an L3 ground
pour in that area. It is PCBway, so maybe they did the same
thing to me. (A generally very good outfit in many ways,
especially price and delivery.)

Several of the chinese quick-turn houses make 4-layer boards with
very thin (like 4 mil) outer dielectrics. Maybe they roll process
the outers and glue them to a core or something.



Maybe section the board to see what the stackup actually is.


Monday I\'ll look at it under the good microscope. (About three
years ago, I got a beautiful Mitutoyo FS-110 with 2x-50x long
working-distance objectives for $2k on eBay. Apparently the guy
didn\'t know what he had, because he shipped this massive
precision instrument in a cardboard box with foam peanuts. The
box was a mess when it got here, but the scope survived because
it\'s a beast.)

I sheared and sandpapered the example I posted, and shot it with
my super-good microscope

https://www.amazon.com/gp/product/B01IV0TV50/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1



I have one a bit like that, but with a better base:
https://www.ebay.com/itm/Andonstar-500X-2MP-USB-Digital-Microscope-Video-webcam-Magnifier-Camera-Stand/143844969460



It does need a rubber band to apply a rotational preload to the arm.

Here is a Dremel-optimized SMA connector.


https://www.dropbox.com/s/gc5j45995nqiftu/Isola_Right_Trim.jpg?raw=1



https://www.dropbox.com/s/t0cnfe9do4slvvf/Isola_Trimmed_Right_TDR.jpg?raw=1


Nice.

We\'ve had that problem too--you need some extra pad area for the
solder fillet, but that makes a nasty capacitive discontinuity at
the connector.

The impedance of that SMA edge-launch connector, center pin to the
four ground pins, is about 100 ohms in free air. So the PCB has to
be about 100 in the pin region too. We\'ve worked that out, cutting
away inners and paving over the bottom with ground. We simulated
the whole geometry with ATLC to get the dims right. That was cool.

The case I posted was testing a laminate sample, 20 mils thick.

The $1.50 edge-launches are just as good as the $12 microwave
connectors if the layout is right, at least as far as we can
resolve with 30 ps TDR.

I sometimes cut away layer 2 (or more) under critical circuit
nodes. My triggered Colpitts oscillator has a driven guard patch on
layer 5.

You bootstrap photodiodes, so you might bootstrap the PCB too.

I\'ve often done that in high-Z TIAs, like 10M. The bootstrap
effectiveness is limited by the capacitance to ground from the
summing junction.

I wasn\'t expecting it to be a big issue with a 511-ohm TIA, but then
I wasn\'t expecting that much capacitance either. I\'ll have to
calculate whether the SAV-551+ can bootstrap the pour as well as the
SiPM.

The usual local feedback tricks such as the PNP wraparound sort of
need fast PNPs, which are no longer made. (The Renesas/Intersil
ones are a sort-of exception.)

Well, I took out the Dremel, with one of JL\'s fave dental burrs on it,
and got the SJ capacitance down to 1.2 pF. It was actually pretty easy,
and the results are sort of interesting-looking.

https://electrooptical.net/www/sed/SiPMdremelled2.png

Top-to-bottom dimension is 18 mm.


Is that a 2-layer board? They are usually about 15 pf per square inch,
with a lot of fringing.

No, it\'s 4 layers, with ground on L2. The #40 wire going down the
middle of the canyon there replaces an L3 trace.

If we get our pcb laser blaster going, we\'ll mostly make 2-sided
boards with all ground plane on the back. It will also be a huge time
saver if we leave a lot of copper on the top, namely blast away just
enough copper to make the insulated features. That will make a lot of
capacitance, and lots of coplanar waveguide.

I\'m envisioning secondary Dremel operations, or maybe a lot of
lasering.

Fun. BTW PCBway recently cut their assembly prices by a lot.

Cheers

Phil Hobbs


Did you ever section that board? Maybe the L2 ground plane is very
close to the layer 1 traces. Seems like a lot of c for some tiny
features like that.

We got our own p+p line because kitting and logistics were a hassle
for using outside assemblers.

Yeah, I cut one with snips and sanded it. The prepreg was a bit skinny,
but not 0.1 mm. Mostly I got snookered by the AD8045 datasheet\'s
specsmanship--their headline 1-GHz bandwidth number turns out to be a
result of the details of the frequency compensation, and only applies in
the noninverting configuration. Otherwise it\'s basically a pretty nice
600-MHz GBW op amp, but not something for sub-nanosecond work.

The next version will connect the SAV-551+ bootstrap directly to the
inverting input of an EL5166 CFA, and add a well-filtered chopamp to
take out the resulting DC offset. The pHEMT\'s source won\'t even notice
the current noise of the CFA, whereas if it were connected to the PD it
would dominate the noise.

A nuisance, but quite a preventable one--my least favourite kind.

Bootstrap into opamp inverting input? I don\'t understand that.

Do you know about THS4303? It\'s a roughly current-mode opamp with an
internal g=+10 feedback network. 1.8 GHz net bw, and noise is pretty
good.

If you hang a pHEMT bootstrap on a photodiode, the gate and source go up
and down together to decent accuracy.

The classical bootstrap architecture is off to the side, so that the TIA
connects directly to the cathode of the PD. That makes the DC and
low-frequency behaviour of the bootstrap device essentially irrelevant,
which is nice. The down side is that the current noise of the TIA stage
gets summed with the photocurrent, which can be a problem in low light.

Alternatively, though, you can connect the op amp to the source of the
bootstrap device. There are op amps such as the LM6171A, which is
basically a CFA with a follower driving its inverting input--this
notion just uses the follower to bootstrap the PD as well as driving the
inverting input. (Our QL01 nanowatt photoreceiver does that.)

Since the bootstrap\'s source follows its gate, this is pretty much
equivalent except that (on the plus side) you can use much gnarlier op
amps, and (on the minus side) the bootstrap has to have decent DC and
low-frequency behaviour.

Cheers

Phil Hobbs

I still don\'t understand. If the bootstrap output drives the opamp,
what is the input to the bootstrap?

Got a sketch?



--

John Larkin Highland Technology, Inc

The best designs are necessarily accidental.

 

[Phil Hobbs]

On 11/25/20 10:14 AM, jlarkin@highlandsniptechnology.com wrote:

On Tue, 24 Nov 2020 23:07:57 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/24/20 10:33 PM, jlarkin@highlandsniptechnology.com wrote:
On Tue, 24 Nov 2020 17:58:44 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/23/20 6:54 PM, jlarkin@highlandsniptechnology.com wrote:
On Mon, 23 Nov 2020 15:11:05 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/23/20 2:43 PM, jlarkin@highlandsniptechnology.com wrote:
On Mon, 23 Nov 2020 13:58:26 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/21/20 6:48 PM, Phil Hobbs wrote:
On 11/21/20 11:05 AM, jlarkin@highlandsniptechnology.com wrote:
On Sat, 21 Nov 2020 07:05:11 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/21/20 12:39 AM, jlarkin@highlandsniptechnology.com wrote:
On Fri, 20 Nov 2020 22:35:52 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/19/20 10:44 AM, jlarkin@highlandsniptechnology.com
wrote:
On Thu, 19 Nov 2020 10:30:17 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/19/20 8:22 AM, George Herold wrote:
On Wednesday, November 18, 2020 at 2:27:39 PM UTC-5,
Phil Hobbs wrote:
Hi, all.

So I have this SiPM/MPPC front end. It has pop
options to use either an On Semi MicroFC-10010 1-mm
SiPM chip or a packaged Hamamatsu S13362-3050DG 3-mm
MPPC with integral TE cooler, both bootstrapped by a
SAV-551+ running at 20 mA. So far, it all works.

(The SAV-551+ is amazingly stable--I\'ve got a
shipping product that runs a very similar bootstrap
across a 2-inch FFC cable. Bandwidth suffers a bit,
but it shows no tendency to oscillate.)

The mystery is in the TIA stage. It\'s a vanilla op
amp TIA made from either an ADA4899 (600 MHz, 300
V/us) or AD8045 (1 GHz, 1300 V/us @ Av=1), which are
pin compatible in the 3-mm LFCSP package. Both are
voltage feedback amps.

I\'m seeing a 3 dB bandwidth of 220 MHz, together with
a faster rolloff than I expect: -3 dB @ 220 MHz, -9
dB @ 320 MHz. It\'s not slew limiting, because the
waveform looks pretty good on a 3-GHz scope (TDS
694C) and the rolloff stays the same when I drop the
input by 6 dB.

The layout is pretty tight (the whole board is only
an inch square), so getting enough stray capacitance
across R_F to account for it is implausible--it would
need about 1.4 pF. DecouplingBypassing is good--

For test, I removed the 0-ohm jumper that connects
the bootstrapped SiPM to the summing junction, and
added a 1k input resistor, forming an inverting amp
with a nominal gain of -0.5.

That\'s connected to the terminated end of an
RG-174/U cable going to a PTS-500 synthesizer. The
output goes via a 10-ohm resistor into a
properly-terminated 50-ohm cable (the TDS 694C is
50-ohm only).

Here I\'m expecting a bandwidth somewhere between the
datasheet\'s 1 GHz @ Av=1 and 400 MHz @ Av=-1, but
it\'s way off. There\'s no visible change when I put
the jumper back in, on account of the swoopy
bootstrap.

I was going to suggest looking at the \'speed\' of the
light source. But the above seems to point to something
\'in\' the amp stage... (Is that right?) (And maybe check
the light source rise time anyway?)

George H.

So where do you suppose the missing factor of ~3 in
bandwidth went?


This has the SiPM and bootstrap disconnected (0 ohm
jumper removed) and a 1/20W leaded 1k resistor bodged in
to make an inverting amp with a gain of -0.5.

I\'m looking at the trace capacitance to figure out if
that might be it. There\'s about 3/4 inch of 10-mil trace
on the summing junction, but that ought to produce a high
frequency peak if anything. hard to find 1.4 pF across
the feedback resistor. Once I\'m back in the lab I\'ll
measure a bare board with a Boonton and see.

Cheers

Phil Hobbs

What\'s the board stackup? Not the specified one, but the
real one. I\'ve been burned by what some of the fast-turn
proto houses do.

https://www.dropbox.com/s/p3vpbaofzqurebz/Z462_PCB_Way_2.png?raw=1





The SJ capacitance is 2.4 pF, as measured on a Boonton, about
twice what I expected. That seems to be the issue--in
simulation it produces a pretty big gain peak, which reduces
the bandwidth.

Time to Dremel the ground plane.

How many layers?

Only four, but of course ground is L2 and there\'s an L3 ground
pour in that area. It is PCBway, so maybe they did the same
thing to me. (A generally very good outfit in many ways,
especially price and delivery.)

Several of the chinese quick-turn houses make 4-layer boards with
very thin (like 4 mil) outer dielectrics. Maybe they roll process
the outers and glue them to a core or something.



Maybe section the board to see what the stackup actually is.


Monday I\'ll look at it under the good microscope. (About three
years ago, I got a beautiful Mitutoyo FS-110 with 2x-50x long
working-distance objectives for $2k on eBay. Apparently the guy
didn\'t know what he had, because he shipped this massive
precision instrument in a cardboard box with foam peanuts. The
box was a mess when it got here, but the scope survived because
it\'s a beast.)

I sheared and sandpapered the example I posted, and shot it with
my super-good microscope

https://www.amazon.com/gp/product/B01IV0TV50/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1



I have one a bit like that, but with a better base:
https://www.ebay.com/itm/Andonstar-500X-2MP-USB-Digital-Microscope-Video-webcam-Magnifier-Camera-Stand/143844969460



It does need a rubber band to apply a rotational preload to the arm.

Here is a Dremel-optimized SMA connector.


https://www.dropbox.com/s/gc5j45995nqiftu/Isola_Right_Trim.jpg?raw=1



https://www.dropbox.com/s/t0cnfe9do4slvvf/Isola_Trimmed_Right_TDR.jpg?raw=1


Nice.

We\'ve had that problem too--you need some extra pad area for the
solder fillet, but that makes a nasty capacitive discontinuity at
the connector.

The impedance of that SMA edge-launch connector, center pin to the
four ground pins, is about 100 ohms in free air. So the PCB has to
be about 100 in the pin region too. We\'ve worked that out, cutting
away inners and paving over the bottom with ground. We simulated
the whole geometry with ATLC to get the dims right. That was cool.

The case I posted was testing a laminate sample, 20 mils thick.

The $1.50 edge-launches are just as good as the $12 microwave
connectors if the layout is right, at least as far as we can
resolve with 30 ps TDR.

I sometimes cut away layer 2 (or more) under critical circuit
nodes. My triggered Colpitts oscillator has a driven guard patch on
layer 5.

You bootstrap photodiodes, so you might bootstrap the PCB too.

I\'ve often done that in high-Z TIAs, like 10M. The bootstrap
effectiveness is limited by the capacitance to ground from the
summing junction.

I wasn\'t expecting it to be a big issue with a 511-ohm TIA, but then
I wasn\'t expecting that much capacitance either. I\'ll have to
calculate whether the SAV-551+ can bootstrap the pour as well as the
SiPM.

The usual local feedback tricks such as the PNP wraparound sort of
need fast PNPs, which are no longer made. (The Renesas/Intersil
ones are a sort-of exception.)

Well, I took out the Dremel, with one of JL\'s fave dental burrs on it,
and got the SJ capacitance down to 1.2 pF. It was actually pretty easy,
and the results are sort of interesting-looking.

https://electrooptical.net/www/sed/SiPMdremelled2.png

Top-to-bottom dimension is 18 mm.


Is that a 2-layer board? They are usually about 15 pf per square inch,
with a lot of fringing.

No, it\'s 4 layers, with ground on L2. The #40 wire going down the
middle of the canyon there replaces an L3 trace.

If we get our pcb laser blaster going, we\'ll mostly make 2-sided
boards with all ground plane on the back. It will also be a huge time
saver if we leave a lot of copper on the top, namely blast away just
enough copper to make the insulated features. That will make a lot of
capacitance, and lots of coplanar waveguide.

I\'m envisioning secondary Dremel operations, or maybe a lot of
lasering.

Fun. BTW PCBway recently cut their assembly prices by a lot.

Cheers

Phil Hobbs


Did you ever section that board? Maybe the L2 ground plane is very
close to the layer 1 traces. Seems like a lot of c for some tiny
features like that.

We got our own p+p line because kitting and logistics were a hassle
for using outside assemblers.

Yeah, I cut one with snips and sanded it. The prepreg was a bit skinny,
but not 0.1 mm. Mostly I got snookered by the AD8045 datasheet\'s
specsmanship--their headline 1-GHz bandwidth number turns out to be a
result of the details of the frequency compensation, and only applies in
the noninverting configuration. Otherwise it\'s basically a pretty nice
600-MHz GBW op amp, but not something for sub-nanosecond work.

The next version will connect the SAV-551+ bootstrap directly to the
inverting input of an EL5166 CFA, and add a well-filtered chopamp to
take out the resulting DC offset. The pHEMT\'s source won\'t even notice
the current noise of the CFA, whereas if it were connected to the PD it
would dominate the noise.

A nuisance, but quite a preventable one--my least favourite kind.

Bootstrap into opamp inverting input? I don\'t understand that.

Do you know about THS4303? It\'s a roughly current-mode opamp with an
internal g=+10 feedback network. 1.8 GHz net bw, and noise is pretty
good.

If you hang a pHEMT bootstrap on a photodiode, the gate and source go up
and down together to decent accuracy.

The classical bootstrap architecture is off to the side, so that the TIA
connects directly to the cathode of the PD. That makes the DC and
low-frequency behaviour of the bootstrap device essentially irrelevant,
which is nice. The down side is that the current noise of the TIA stage
gets summed with the photocurrent, which can be a problem in low light.

Alternatively, though, you can connect the op amp to the source of the
bootstrap device. There are op amps such as the LM6171A, which is
basically a CFA with a follower driving its inverting input--this
notion just uses the follower to bootstrap the PD as well as driving the
inverting input. (Our QL01 nanowatt photoreceiver does that.)

Since the bootstrap\'s source follows its gate, this is pretty much
equivalent except that (on the plus side) you can use much gnarlier op
amps, and (on the minus side) the bootstrap has to have decent DC and
low-frequency behaviour.

Cheers

Phil Hobbs

I still don\'t understand. If the bootstrap output drives the opamp,
what is the input to the bootstrap?

Got a sketch?



https://electrooptical.net/www/sed/VFandCFbootstraps.pdf

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Uwe Bonnes]

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/25/20 4:59 AM, Uwe Bonnes wrote:
jlarkin@highlandsniptechnology.com wrote:
On Tue, 24 Nov 2020 17:58:44 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/23/20 6:54 PM, jlarkin@highlandsniptechnology.com wrote:
On 11/19/20 8:22 AM, George Herold wrote:
On Wednesday, November 18, 2020 at 2:27:39 PM UTC-5,
Phil Hobbs wrote:
Hi, all.
Why do people spoil such an interesting discussion by such excessive
quoting? If one later searches the net for this discussion, one will
drown in quotes...

There\'s this really useful button for that! It\'s labelled \"PgDn\"!

Guess what: I know and use that button. But I don\'t understand to what
purpose people force me to use it multiple times.

Bye
--
Uwe Bonnes bon@elektron.ikp.physik.tu-darmstadt.de

Institut fuer Kernphysik Schlossgartenstrasse 9 64289 Darmstadt
--------- Tel. 06151 1623569 ------- Fax. 06151 1623305 ---------

 

[Phil Hobbs]

On 11/25/20 2:52 PM, Uwe Bonnes wrote:

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
On 11/25/20 4:59 AM, Uwe Bonnes wrote:
jlarkin@highlandsniptechnology.com wrote:
On Tue, 24 Nov 2020 17:58:44 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 11/23/20 6:54 PM, jlarkin@highlandsniptechnology.com wrote:
On 11/19/20 8:22 AM, George Herold wrote:
On Wednesday, November 18, 2020 at 2:27:39 PM UTC-5,
Phil Hobbs wrote:
Hi, all.
Why do people spoil such an interesting discussion by such excessive
quoting? If one later searches the net for this discussion, one will
drown in quotes...

There\'s this really useful button for that! It\'s labelled \"PgDn\"!

Guess what: I know and use that button. But I don\'t understand to what
purpose people force me to use it multiple times.

Bye

It\'s just duelling conveniences. Sitting on PgDn for a couple of
seconds is a lot faster than snipping intelligently, so IMO it\'s okay to
expect people to do that.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com