Driven switch body. (what about greater than one bootstrap)

On Sat, 21 Sep 2019 20:42:33 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 9/21/19 5:05 PM, jlarkin@highlandsniptechnology.com wrote:
On Fri, 20 Sep 2019 19:21:13 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 9/18/19 7:50 PM, John Larkin wrote:
On Wed, 18 Sep 2019 15:56:21 -0700 (PDT), tabbypurr@gmail.com wrote:

On Wednesday, 18 September 2019 20:35:19 UTC+1, George Herold wrote:

I keep bashing my head into this 5pF of switch C.

could always reduce it with a knife switch

What's the c of a coaxial relay?


NT

We use a lot of little Fujitsu FTR-B3GA relays. Capacitance is tiny
and they work nicely to about 3 GHz.

George could use them to do the actual switching.

https://www.dropbox.com/s/se162xpw86hpmzs/DSC06884.JPG?raw=1



Typically about 0.2 pF contact-to-contact, and a bit more from contact
to coil.

Cheers

Phil Hobbs

Being DPDT, tricks can be played in a TIA topology.

I guess one could booststrap unused contacts, or even the coil. But
I'm working at 400 Hz and hundreds of watts lately.


Yup, I did both in a switchable 1G/50G TIA for a scanning surface
voltage tool for a now-defunct company called Qcept. Plus I had to
short out the 1G resistor when on the 50G range, because otherwise its
Johnson noise across the 0.2 pF of the open contacts would have
dominated the noise floor.

That's one of those times that not doing a careful error budget will
bite you in the posterior.

Since the company went mammaries-topmost, here's part of the schematic:
https://electrooptical.net/www/sed/QceptTransimpedanceAmpRev1.pdf


Cheers

Phil Hobbs

Nice. Take no prisoners over-the-top everywhere.

 

[Phil Hobbs]

On 9/21/19 9:53 PM, Winfield Hill wrote:

Phil Hobbs wrote...

Yup, I did both in a switchable 1G/50G TIA for a scanning surface
voltage tool for a now-defunct company called Qcept. Plus I had to
short out the 1G resistor when on the 50G range, because otherwise its
Johnson noise across the 0.2 pF of the open contacts would have
dominated the noise floor.

That's one of those times that not doing a careful error budget will
bite you in the posterior.

Since the company went mammaries-topmost, here's part of the schematic:
https://electrooptical.net/www/sed/QceptTransimpedanceAmpRev1.pdf

I don't understand the drawing. Where does the Q9 BF862
JFET's source current go?

This is a version they sent me to check--Q6 is the tail current source
for Q9, but it's drawn upside down. (My output is paper schematics plus
BOMs, so their layout guy did the schematic capture.)

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

 

[Winfield Hill]

Phil Hobbs wrote...

Yup, I did both in a switchable 1G/50G TIA for a scanning surface
voltage tool for a now-defunct company called Qcept. Plus I had to
short out the 1G resistor when on the 50G range, because otherwise its
Johnson noise across the 0.2 pF of the open contacts would have
dominated the noise floor.

That's one of those times that not doing a careful error budget will
bite you in the posterior.

Since the company went mammaries-topmost, here's part of the schematic:
https://electrooptical.net/www/sed/QceptTransimpedanceAmpRev1.pdf

I don't understand the drawing. Where does the Q9 BF862
JFET's source current go?


--
Thanks,
- Win

 

[Phil Hobbs]

On 9/21/19 9:04 PM, jlarkin@highlandsniptechnology.com wrote:

On Sat, 21 Sep 2019 20:42:33 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 9/21/19 5:05 PM, jlarkin@highlandsniptechnology.com wrote:
On Fri, 20 Sep 2019 19:21:13 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 9/18/19 7:50 PM, John Larkin wrote:
On Wed, 18 Sep 2019 15:56:21 -0700 (PDT), tabbypurr@gmail.com wrote:

On Wednesday, 18 September 2019 20:35:19 UTC+1, George Herold wrote:

I keep bashing my head into this 5pF of switch C.

could always reduce it with a knife switch

What's the c of a coaxial relay?


NT

We use a lot of little Fujitsu FTR-B3GA relays. Capacitance is tiny
and they work nicely to about 3 GHz.

George could use them to do the actual switching.

https://www.dropbox.com/s/se162xpw86hpmzs/DSC06884.JPG?raw=1



Typically about 0.2 pF contact-to-contact, and a bit more from contact
to coil.

Cheers

Phil Hobbs

Being DPDT, tricks can be played in a TIA topology.

I guess one could booststrap unused contacts, or even the coil. But
I'm working at 400 Hz and hundreds of watts lately.


Yup, I did both in a switchable 1G/50G TIA for a scanning surface
voltage tool for a now-defunct company called Qcept. Plus I had to
short out the 1G resistor when on the 50G range, because otherwise its
Johnson noise across the 0.2 pF of the open contacts would have
dominated the noise floor.

That's one of those times that not doing a careful error budget will
bite you in the posterior.

Since the company went mammaries-topmost, here's part of the schematic:
https://electrooptical.net/www/sed/QceptTransimpedanceAmpRev1.pdf


Cheers

Phil Hobbs

Nice. Take no prisoners over-the-top everywhere.

I had to change K1 to a 9-V coil part, because it didn't pull in
reliably with a 12 V coil. Otherwise it worked fine. I still have one
someplace.

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

 

[Winfield Hill]

Phil Hobbs wrote...

On 9/21/19 9:53 PM, Winfield Hill wrote:
Phil Hobbs wrote...

Yup, I did both in a switchable 1G/50G TIA for a scanning surface
voltage tool for a now-defunct company called Qcept. Plus I had to
short out the 1G resistor when on the 50G range, because otherwise its
Johnson noise across the 0.2 pF of the open contacts would have
dominated the noise floor.

That's one of those times that not doing a careful error budget will
bite you in the posterior.

Since the company went mammaries-topmost, here's part of the schematic:
https://electrooptical.net/www/sed/QceptTransimpedanceAmpRev1.pdf

I don't understand the drawing. Where does the Q9 BF862
JFET's source current go?

This is a version they sent me to check--Q6 is the tail current source
for Q9, but it's drawn upside down. (My output is paper schematics plus
BOMs, so their layout guy did the schematic capture.)

Normally a TIA's output voltage is relative to the summing-junction
return, FGND in this case, but here it's relative to FGND - Vgs of
the JFET, where Vgs is a poorly-defined value. But you added a 2nd
BF862 to match, trying to bias to Vgs=0 (for Idss = 10 to 25mA). Was
this a design where you only needed to know relative output changes?


--
Thanks,
- Win

 

Normally a TIA's output voltage is relative to the summing-junction
 return, FGND in this case, but here it's relative to FGND - Vgs of
 the JFET, where Vgs is a poorly-defined value.  But you added a 2nd
 BF862 to match, trying to bias to Vgs=0 (for Idss = 10 to 25mA).  Was
 this a design where you only needed to know relative output changes?

FGND is "floating ground". The probe tip has to have a bias voltage (AC and/or DC), and the simplest approach was to float the whole front end.

Cheers

Phil Hobbs