Transformer-coupled dpot...

[Phil Hobbs]

So I\'m doing a spin of my swoopy nanoamp photoreceiver board, to make it
smaller and more versatile.

The original QL01 has two pots, one for offset and one for HF boost.
The HF boost is a lead-lag network that attenuates the voltage the FB
resistor sees, reducing the effect of its parallel capacitance.

The first one can easily be a dpot, but the second one is a bit more of
a challenge. It\'s not too hard to level-shift unidirectional I2c up to
the op amp\'s output--a PNP CB stage with the base grounded, followed by
an NPN CB stage with the base about 3V above the negative supply rail,
as in the LTspice files below (i2cls.lib and I2Cshifter.asc. The dpot
can be an AD5273BRJZ1, whose 6 MHz bandwidth will work fine.

Problem is, at high frequency most of the signal swing appears across
the dpot, so if I do it this way I\'ll have to reduce the supplies.

I\'m considering two things: first, leaving the resistors fixed and
switching the caps in and out with a high-voltage mux, or else (more
interestingly) transformer-coupling the dpot. The first

It\'ll need a fair amount of inductance to look like 1k ohms at 200 kHz
and above, probably a couple of mH, and a turns ratio of 2:1 or so to
reduce the voltage swing.

Anybody here done something like 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

 

[server]

On Mon, 17 Aug 2020 17:30:47 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

So I\'m doing a spin of my swoopy nanoamp photoreceiver board, to make it
smaller and more versatile.

The original QL01 has two pots, one for offset and one for HF boost.
The HF boost is a lead-lag network that attenuates the voltage the FB
resistor sees, reducing the effect of its parallel capacitance.

The first one can easily be a dpot, but the second one is a bit more of
a challenge. It\'s not too hard to level-shift unidirectional I2c up to
the op amp\'s output--a PNP CB stage with the base grounded, followed by
an NPN CB stage with the base about 3V above the negative supply rail,
as in the LTspice files below (i2cls.lib and I2Cshifter.asc. The dpot
can be an AD5273BRJZ1, whose 6 MHz bandwidth will work fine.

Problem is, at high frequency most of the signal swing appears across
the dpot, so if I do it this way I\'ll have to reduce the supplies.

I\'m considering two things: first, leaving the resistors fixed and
switching the caps in and out with a high-voltage mux, or else (more
interestingly) transformer-coupling the dpot. The first

It\'ll need a fair amount of inductance to look like 1k ohms at 200 kHz
and above, probably a couple of mH, and a turns ratio of 2:1 or so to
reduce the voltage swing.

Anybody here done something like that?

Cheers

Phil Hobbs

I guess the object of the level shifter is to interface with the
floating dpot. The sim can\'t find the TLV431 model, which I guess is
just a sort of floating power supply.

A transformer is an interesting way to isolate a variable part, like a
pot or a varicap or something.

We use

COILCRAFT PWB1010LB

which is a 780 uH 1:1 transformer spec\'d for 3 KHz to 125 MHz.

but we use it for signal coupling, not \"impedance isolation.\"

Why not a varicap to ground, inside the feedback loop? Sorta in place
of C3.






--

John Larkin Highland Technology, Inc

Science teaches us to doubt.

Claude Bernard

 

[server]

On Mon, 17 Aug 2020 17:30:47 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

So I\'m doing a spin of my swoopy nanoamp photoreceiver board, to make it
smaller and more versatile.

The original QL01 has two pots, one for offset and one for HF boost.
The HF boost is a lead-lag network that attenuates the voltage the FB
resistor sees, reducing the effect of its parallel capacitance.

The first one can easily be a dpot, but the second one is a bit more of
a challenge. It\'s not too hard to level-shift unidirectional I2c up to
the op amp\'s output--a PNP CB stage with the base grounded, followed by
an NPN CB stage with the base about 3V above the negative supply rail,
as in the LTspice files below (i2cls.lib and I2Cshifter.asc. The dpot
can be an AD5273BRJZ1, whose 6 MHz bandwidth will work fine.

Problem is, at high frequency most of the signal swing appears across
the dpot, so if I do it this way I\'ll have to reduce the supplies.

I\'m considering two things: first, leaving the resistors fixed and
switching the caps in and out with a high-voltage mux, or else (more
interestingly) transformer-coupling the dpot. The first

It\'ll need a fair amount of inductance to look like 1k ohms at 200 kHz
and above, probably a couple of mH, and a turns ratio of 2:1 or so to
reduce the voltage swing.

Anybody here done something like that?

Cheers

Phil Hobbs

You might use a digital capacitor to tweak HF response.

https://www.mouser.com/Passive-Components/Capacitors/Trimmer-Variable-Capacitors/_/N-5g7r?keyword=digital




--

John Larkin Highland Technology, Inc

Science teaches us to doubt.

Claude Bernard

 

[Phil Hobbs]

On 2020-08-18 10:19, jlarkin@highlandsniptechnology.com wrote:

On Mon, 17 Aug 2020 17:30:47 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

So I\'m doing a spin of my swoopy nanoamp photoreceiver board, to
make it smaller and more versatile.

The original QL01 has two pots, one for offset and one for HF
boost. The HF boost is a lead-lag network that attenuates the
voltage the FB resistor sees, reducing the effect of its parallel
capacitance.

The first one can easily be a dpot, but the second one is a bit
more of a challenge. It\'s not too hard to level-shift
unidirectional I2c up to the op amp\'s output--a PNP CB stage with
the base grounded, followed by an NPN CB stage with the base about
3V above the negative supply rail, as in the LTspice files below
(i2cls.lib and I2Cshifter.asc. The dpot can be an AD5273BRJZ1,
whose 6 MHz bandwidth will work fine.

Problem is, at high frequency most of the signal swing appears
across the dpot, so if I do it this way I\'ll have to reduce the
supplies.

I\'m considering two things: first, leaving the resistors fixed and
switching the caps in and out with a high-voltage mux, or else
(more interestingly) transformer-coupling the dpot. The first

It\'ll need a fair amount of inductance to look like 1k ohms at 200
kHz and above, probably a couple of mH, and a turns ratio of 2:1 or
so to reduce the voltage swing.

Anybody here done something like that?

Cheers

Phil Hobbs

You might use a digital capacitor to tweak HF response.

https://www.mouser.com/Passive-Components/Capacitors/Trimmer-Variable-Capacitors/_/N-5g7r?keyword=digital

Those are very pretty. I sort of need at least a 10V swing, though.

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, 18 Aug 2020 10:58:30 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-08-18 10:19, jlarkin@highlandsniptechnology.com wrote:
On Mon, 17 Aug 2020 17:30:47 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

So I\'m doing a spin of my swoopy nanoamp photoreceiver board, to
make it smaller and more versatile.

The original QL01 has two pots, one for offset and one for HF
boost. The HF boost is a lead-lag network that attenuates the
voltage the FB resistor sees, reducing the effect of its parallel
capacitance.

The first one can easily be a dpot, but the second one is a bit
more of a challenge. It\'s not too hard to level-shift
unidirectional I2c up to the op amp\'s output--a PNP CB stage with
the base grounded, followed by an NPN CB stage with the base about
3V above the negative supply rail, as in the LTspice files below
(i2cls.lib and I2Cshifter.asc. The dpot can be an AD5273BRJZ1,
whose 6 MHz bandwidth will work fine.

Problem is, at high frequency most of the signal swing appears
across the dpot, so if I do it this way I\'ll have to reduce the
supplies.

I\'m considering two things: first, leaving the resistors fixed and
switching the caps in and out with a high-voltage mux, or else
(more interestingly) transformer-coupling the dpot. The first

It\'ll need a fair amount of inductance to look like 1k ohms at 200
kHz and above, probably a couple of mH, and a turns ratio of 2:1 or
so to reduce the voltage swing.

Anybody here done something like that?

Cheers

Phil Hobbs

You might use a digital capacitor to tweak HF response.

https://www.mouser.com/Passive-Components/Capacitors/Trimmer-Variable-Capacitors/_/N-5g7r?keyword=digital

Those are very pretty. I sort of need at least a 10V swing, though.

Cheers

Phil Hobbs

Slide it a little closer to the summing point maybe?



--

John Larkin Highland Technology, Inc

Science teaches us to doubt.

Claude Bernard

 

[Phil Hobbs]

On 2020-08-18 11:05, jlarkin@highlandsniptechnology.com wrote:

On Tue, 18 Aug 2020 10:58:30 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-08-18 10:19, jlarkin@highlandsniptechnology.com wrote:
On Mon, 17 Aug 2020 17:30:47 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

So I\'m doing a spin of my swoopy nanoamp photoreceiver board, to
make it smaller and more versatile.

The original QL01 has two pots, one for offset and one for HF
boost. The HF boost is a lead-lag network that attenuates the
voltage the FB resistor sees, reducing the effect of its parallel
capacitance.

The first one can easily be a dpot, but the second one is a bit
more of a challenge. It\'s not too hard to level-shift
unidirectional I2c up to the op amp\'s output--a PNP CB stage with
the base grounded, followed by an NPN CB stage with the base about
3V above the negative supply rail, as in the LTspice files below
(i2cls.lib and I2Cshifter.asc. The dpot can be an AD5273BRJZ1,
whose 6 MHz bandwidth will work fine.

Problem is, at high frequency most of the signal swing appears
across the dpot, so if I do it this way I\'ll have to reduce the
supplies.

I\'m considering two things: first, leaving the resistors fixed and
switching the caps in and out with a high-voltage mux, or else
(more interestingly) transformer-coupling the dpot. The first

It\'ll need a fair amount of inductance to look like 1k ohms at 200
kHz and above, probably a couple of mH, and a turns ratio of 2:1 or
so to reduce the voltage swing.

Anybody here done something like that?

Cheers

Phil Hobbs

You might use a digital capacitor to tweak HF response.

https://www.mouser.com/Passive-Components/Capacitors/Trimmer-Variable-Capacitors/_/N-5g7r?keyword=digital

Those are very pretty. I sort of need at least a 10V swing, though.

Cheers

Phil Hobbs

Slide it a little closer to the summing point maybe?

Hard to do without destroying the SNR, because the larger Johnson noise
of the small segment of the feedback resistor can flow through the
lead-lag network. The same thing happens if you try splitting the
resistor and bootstrapping the midpoint via a lower-resistance
string--the SNR is the same as if you just used the first resistor by
itself.

If transformer coupling a dpot doesn\'t work, I\'ll probably wind up
switching capacitors to ground with a DG409 or something like that. The
caps would be in the hundreds of pF, in series with 120 ohms or so,
which wouldn\'t tax the HV mux\'s capabilities too much.

I\'d really like to use a dpot though, because that can be programmed at
test time whereas using a mux would mean I\'d need a micro on a hitherto
all-analogue board.

Mini-Circuits sells transformers that work down to 4 kHz at 50 ohms, so
ISTM they should work at 40 kHz at 500 ohms. We\'ll see.

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 2020-08-18 11:27, Phil Hobbs wrote:

On 2020-08-18 11:05, jlarkin@highlandsniptechnology.com wrote:
On Tue, 18 Aug 2020 10:58:30 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-08-18 10:19, jlarkin@highlandsniptechnology.com wrote:
On Mon, 17 Aug 2020 17:30:47 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

So I\'m doing a spin of my swoopy nanoamp photoreceiver board, to
make it smaller and more versatile.

The original QL01 has two pots, one for offset and one for HF
boost. The HF boost is a lead-lag network that attenuates the
voltage the FB resistor sees, reducing the effect of its parallel
capacitance.

The first one can easily be a dpot, but the second one is a bit
more of a challenge.  It\'s not too hard to level-shift
unidirectional I2c up to the op amp\'s output--a PNP CB stage with
the base grounded, followed by an NPN CB stage with the base about
3V above the negative supply rail, as in the LTspice files below
(i2cls.lib and I2Cshifter.asc.  The dpot can be an AD5273BRJZ1,
whose 6 MHz bandwidth will work fine.

Problem is, at high frequency most of the signal swing appears
across the dpot, so if I do it this way I\'ll have to reduce the
supplies.

I\'m considering two things: first, leaving the resistors fixed and
switching the caps in and out with a high-voltage mux, or else
(more interestingly) transformer-coupling the dpot.  The first

It\'ll need a fair amount of inductance to look like 1k ohms at 200
kHz and above, probably a couple of mH, and a turns ratio of 2:1 or
so to reduce the voltage swing.

Anybody here done something like that?

Cheers

Phil Hobbs

You might use a digital capacitor to tweak HF response.

https://www.mouser.com/Passive-Components/Capacitors/Trimmer-Variable-Capacitors/_/N-5g7r?keyword=digital


Those are very pretty.  I sort of need at least a 10V swing, though.

Cheers

Phil Hobbs

Slide it a little closer to the summing point maybe?



Hard to do without destroying the SNR, because the larger Johnson noise
of the small segment of the feedback resistor can flow through the
lead-lag network.  The same thing happens if you try splitting the
resistor and bootstrapping the midpoint via a lower-resistance
string--the SNR is the same as if you just used the first resistor by
itself.

If transformer coupling a dpot doesn\'t work, I\'ll probably wind up
switching capacitors to ground with a DG409 or something like that. The
caps would be in the hundreds of pF, in series with 120 ohms or so,
which wouldn\'t tax the HV mux\'s capabilities too much.

I\'d really like to use a dpot though, because that can be programmed at
test time whereas using a mux would mean I\'d need a micro on a hitherto
all-analogue board.

Mini-Circuits sells transformers that work down to 4 kHz at 50 ohms, so
ISTM they should work at 40 kHz at 500 ohms.  We\'ll see.

Looks like it should work if the coupling is high enough (see
attached)--hopefully these will work better since there\'s no custom symbol).

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

 

[John Larkin]

On Tue, 18 Aug 2020 14:03:11 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-08-18 11:27, Phil Hobbs wrote:
On 2020-08-18 11:05, jlarkin@highlandsniptechnology.com wrote:
On Tue, 18 Aug 2020 10:58:30 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-08-18 10:19, jlarkin@highlandsniptechnology.com wrote:
On Mon, 17 Aug 2020 17:30:47 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

So I\'m doing a spin of my swoopy nanoamp photoreceiver board, to
make it smaller and more versatile.

The original QL01 has two pots, one for offset and one for HF
boost. The HF boost is a lead-lag network that attenuates the
voltage the FB resistor sees, reducing the effect of its parallel
capacitance.

The first one can easily be a dpot, but the second one is a bit
more of a challenge.  It\'s not too hard to level-shift
unidirectional I2c up to the op amp\'s output--a PNP CB stage with
the base grounded, followed by an NPN CB stage with the base about
3V above the negative supply rail, as in the LTspice files below
(i2cls.lib and I2Cshifter.asc.  The dpot can be an AD5273BRJZ1,
whose 6 MHz bandwidth will work fine.

Problem is, at high frequency most of the signal swing appears
across the dpot, so if I do it this way I\'ll have to reduce the
supplies.

I\'m considering two things: first, leaving the resistors fixed and
switching the caps in and out with a high-voltage mux, or else
(more interestingly) transformer-coupling the dpot.  The first

It\'ll need a fair amount of inductance to look like 1k ohms at 200
kHz and above, probably a couple of mH, and a turns ratio of 2:1 or
so to reduce the voltage swing.

Anybody here done something like that?

Cheers

Phil Hobbs

You might use a digital capacitor to tweak HF response.

https://www.mouser.com/Passive-Components/Capacitors/Trimmer-Variable-Capacitors/_/N-5g7r?keyword=digital


Those are very pretty.  I sort of need at least a 10V swing, though.

Cheers

Phil Hobbs

Slide it a little closer to the summing point maybe?



Hard to do without destroying the SNR, because the larger Johnson noise
of the small segment of the feedback resistor can flow through the
lead-lag network.  The same thing happens if you try splitting the
resistor and bootstrapping the midpoint via a lower-resistance
string--the SNR is the same as if you just used the first resistor by
itself.

If transformer coupling a dpot doesn\'t work, I\'ll probably wind up
switching capacitors to ground with a DG409 or something like that. The
caps would be in the hundreds of pF, in series with 120 ohms or so,
which wouldn\'t tax the HV mux\'s capabilities too much.

I\'d really like to use a dpot though, because that can be programmed at
test time whereas using a mux would mean I\'d need a micro on a hitherto
all-analogue board.

Mini-Circuits sells transformers that work down to 4 kHz at 50 ohms, so
ISTM they should work at 40 kHz at 500 ohms.  We\'ll see.


Looks like it should work if the coupling is high enough (see
attached)--hopefully these will work better since there\'s no custom symbol).

Cheers

Phil Hobbs

Seems to me that the transformer doesn\'t do much. Couldn\'t you use the
dpot as a pot, with one end grounded, one end driven from the amp
(with a resistor maybe) and the wiper driving the compensation?

 

[Phil Hobbs]

On 2020-08-18 14:52, John Larkin wrote:

On Tue, 18 Aug 2020 14:03:11 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-08-18 11:27, Phil Hobbs wrote:
On 2020-08-18 11:05, jlarkin@highlandsniptechnology.com wrote:
On Tue, 18 Aug 2020 10:58:30 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-08-18 10:19, jlarkin@highlandsniptechnology.com
wrote:
On Mon, 17 Aug 2020 17:30:47 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

So I\'m doing a spin of my swoopy nanoamp photoreceiver
board, to make it smaller and more versatile.

The original QL01 has two pots, one for offset and one
for HF boost. The HF boost is a lead-lag network that
attenuates the voltage the FB resistor sees, reducing
the effect of its parallel capacitance.

The first one can easily be a dpot, but the second one
is a bit more of a challenge. It\'s not too hard to
level-shift unidirectional I2c up to the op amp\'s
output--a PNP CB stage with the base grounded, followed
by an NPN CB stage with the base about 3V above the
negative supply rail, as in the LTspice files below
(i2cls.lib and I2Cshifter.asc. The dpot can be an
AD5273BRJZ1, whose 6 MHz bandwidth will work fine.

Problem is, at high frequency most of the signal swing
appears across the dpot, so if I do it this way I\'ll
have to reduce the supplies.

I\'m considering two things: first, leaving the resistors
fixed and switching the caps in and out with a
high-voltage mux, or else (more interestingly)
transformer-coupling the dpot. The first

It\'ll need a fair amount of inductance to look like 1k
ohms at 200 kHz and above, probably a couple of mH, and
a turns ratio of 2:1 or so to reduce the voltage swing.

Anybody here done something like that?

Cheers

Phil Hobbs

You might use a digital capacitor to tweak HF response.

https://www.mouser.com/Passive-Components/Capacitors/Trimmer-Variable-Capacitors/_/N-5g7r?keyword=digital





Those are very pretty. I sort of need at least a 10V swing, though.

Cheers

Phil Hobbs

Slide it a little closer to the summing point maybe?



Hard to do without destroying the SNR, because the larger
Johnson noise of the small segment of the feedback resistor can
flow through the lead-lag network. The same thing happens if you
try splitting the resistor and bootstrapping the midpoint via a
lower-resistance string--the SNR is the same as if you just used
the first resistor by itself.

If transformer coupling a dpot doesn\'t work, I\'ll probably wind
up switching capacitors to ground with a DG409 or something like
that. The caps would be in the hundreds of pF, in series with
120 ohms or so, which wouldn\'t tax the HV mux\'s capabilities too
much.

I\'d really like to use a dpot though, because that can be
programmed at test time whereas using a mux would mean I\'d need
a micro on a hitherto all-analogue board.

Mini-Circuits sells transformers that work down to 4 kHz at 50
ohms, so ISTM they should work at 40 kHz at 500 ohms. We\'ll
see.


Looks like it should work if the coupling is high enough (see
attached)--hopefully these will work better since there\'s no
custom symbol).


Seems to me that the transformer doesn\'t do much.

From a compensation POV, that\'s a feature rather than a bug.

The transformer more than halves the voltage applied to the dpot, so I
can use AD5273BRJZ1\'s with a 6V supply and hang one end on the +3.3V
rail. It also eliminates the need to level-shift the I2C.

Couldn\'t you use the dpot as a pot, with one end grounded, one end
driven from the amp (with a resistor maybe) and the wiper driving
the compensation?

I\'d have to drop the power supplies way down to avoid frying the dpot,
so the feedback resistor would have to go way down as well, which would
hurt the SNR at low currents. The Johnson noise of the feedback
resistor equals the shot noise when it\'s dropping 2kT/e (50 mV at room
temperature). With 10M, that\'s 5 nA, but with 2M it\'s 25 nA, and I care
about that quite a lot.

I have some of these on order. Both have 10 kHz minimum frequency quoted.

<https://www.minicircuits.com/WebStore/dashboard.html?model=T-626%2B>
which is 1:1:1, , and
<https://www.minicircuits.com/WebStore/dashboard.html?model=ADT4-6T%2B>
which is 1:4

Using my trusty AADE, I measured a T622 1:1:1 (100 kHz-200 MHz). It was
35 uH per winding open, 35 nH with one winding shorted. Since

L_sc/L_oc = 1-k**2,

that works out to k = 0.9995.

If the ratios are the same, the 10 kHz 4:1 will be about 350 uH on the
primary and 1.4 mH on the secondary. That leads to a bit of tilt on the
pulse tops unless I halve the impedance level of the lead-lag network.
I can probably use a fixed resistor there to do most of the work, and
run the dpot with the two ends shorted, which gives vernier action near
half-scale.

It might be easier to fix the tilt in the second stage by giving a bit
of a LF gain cut.

Fun.

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

 

[Chris Jones]

On 19/08/2020 01:27, Phil Hobbs wrote:

I\'d really like to use a dpot though, because that can be programmed at
test time whereas using a mux would mean I\'d need a micro on a hitherto
all-analogue board.

If you\'re going to program it at test, could you put two or three DPOTs
in series (so less voltage across each one), and power them either with
photovoltaic isolators, or some fancy op-amp bootstrapping circuit. Yes
the SPI level shifters could get interesting, but those could be a
one-off monstrosity residing in your tester, behind the pogo pins or
whatever.

 

[Phil Hobbs]

On 2020-08-18 20:15, Chris Jones wrote:

On 19/08/2020 01:27, Phil Hobbs wrote:
I\'d really like to use a dpot though, because that can be programmed
at test time whereas using a mux would mean I\'d need a micro on a
hitherto all-analogue board.

If you\'re going to program it at test, could you put two or three DPOTs
in series (so less voltage across each one), and power them either with
photovoltaic isolators, or some fancy op-amp bootstrapping circuit. Yes
the SPI level shifters could get interesting, but those could be a
one-off monstrosity residing in your tester, behind the pogo pins or
whatever.

Not a bad idea--I\'m not at all above such things. In a tester, it could
be three Bus Pirates hung off USB isolators, so nobody would even have
to know.

This is one of those things that comes up a fair amount though, so it
would be useful to have a solution that would work in a box with a uC as
well.

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

 

[John Larkin]

On Mon, 17 Aug 2020 17:30:47 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

So I\'m doing a spin of my swoopy nanoamp photoreceiver board, to make it
smaller and more versatile.

The original QL01 has two pots, one for offset and one for HF boost.
The HF boost is a lead-lag network that attenuates the voltage the FB
resistor sees, reducing the effect of its parallel capacitance.

The first one can easily be a dpot, but the second one is a bit more of
a challenge. It\'s not too hard to level-shift unidirectional I2c up to
the op amp\'s output--a PNP CB stage with the base grounded, followed by
an NPN CB stage with the base about 3V above the negative supply rail,
as in the LTspice files below (i2cls.lib and I2Cshifter.asc. The dpot
can be an AD5273BRJZ1, whose 6 MHz bandwidth will work fine.

Problem is, at high frequency most of the signal swing appears across
the dpot, so if I do it this way I\'ll have to reduce the supplies.

I\'m considering two things: first, leaving the resistors fixed and
switching the caps in and out with a high-voltage mux, or else (more
interestingly) transformer-coupling the dpot. The first

It\'ll need a fair amount of inductance to look like 1k ohms at 200 kHz
and above, probably a couple of mH, and a turns ratio of 2:1 or so to
reduce the voltage swing.

Anybody here done something like that?

Cheers

Phil Hobbs

Oh, there\'s another trick: a tiny surface-mount dip switch and 4 caps
makes a 4-bit capacitive DAC.

Or a tiny screwdriver-adjusted variable capacitor.

Both are cheap and low noise and require no software.

https://www.digikey.com/products/en?keywords=490-2008-1-ND

We pay 26 cents for that one.

 

[Phil Hobbs]

On 2020-08-19 14:24, John Larkin wrote:

On Mon, 17 Aug 2020 17:30:47 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

So I\'m doing a spin of my swoopy nanoamp photoreceiver board, to make it
smaller and more versatile.

The original QL01 has two pots, one for offset and one for HF boost.
The HF boost is a lead-lag network that attenuates the voltage the FB
resistor sees, reducing the effect of its parallel capacitance.

The first one can easily be a dpot, but the second one is a bit more of
a challenge. It\'s not too hard to level-shift unidirectional I2c up to
the op amp\'s output--a PNP CB stage with the base grounded, followed by
an NPN CB stage with the base about 3V above the negative supply rail,
as in the LTspice files below (i2cls.lib and I2Cshifter.asc. The dpot
can be an AD5273BRJZ1, whose 6 MHz bandwidth will work fine.

Problem is, at high frequency most of the signal swing appears across
the dpot, so if I do it this way I\'ll have to reduce the supplies.

I\'m considering two things: first, leaving the resistors fixed and
switching the caps in and out with a high-voltage mux, or else (more
interestingly) transformer-coupling the dpot. The first

It\'ll need a fair amount of inductance to look like 1k ohms at 200 kHz
and above, probably a couple of mH, and a turns ratio of 2:1 or so to
reduce the voltage swing.

Anybody here done something like that?

Cheers

Phil Hobbs

Oh, there\'s another trick: a tiny surface-mount dip switch and 4 caps
makes a 4-bit capacitive DAC.

I\'m not at all above that sort of thing either. I could even use a hex
dip switch to mimic a variable cap.

Or a tiny screwdriver-adjusted variable capacitor.

Both are cheap and low noise and require no software.

Agreed. Part of what we\'re doing is to make our gizmos attractive for
people to license, and as you know there\'s a lot of prejudice out there
against manual trims. That\'s not completely unreasonable when the
person doing the adjusting has no idea how the circuit works--it\'s easy
to encapsulate the know-how in the test software.

For protos and proof-of-concept systems I often use gimmick caps to
adjust amplifier peaking.

https://www.digikey.com/products/en?keywords=490-2008-1-ND

We pay 26 cents for that one.

Long obsolete, unfortunately.

Our existing trimpot works okay but doesn\'t play that well with ATE.
(You used to be able to get GPIB-controlled screwdrivers for that sort
of thing.)

Plus, needing to hang an AC tweak on some out-of-the-way circuit node is
something that comes up a fair amount round here.

The transformers will be here today, so maybe next week I\'ll give it a
whirl. We\'re ordering boards probably tomorrow, so it\'ll have to be
trimpots for this batch.

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

 

[John Larkin]

On Wed, 19 Aug 2020 15:51:50 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-08-19 14:24, John Larkin wrote:
On Mon, 17 Aug 2020 17:30:47 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

So I\'m doing a spin of my swoopy nanoamp photoreceiver board, to make it
smaller and more versatile.

The original QL01 has two pots, one for offset and one for HF boost.
The HF boost is a lead-lag network that attenuates the voltage the FB
resistor sees, reducing the effect of its parallel capacitance.

The first one can easily be a dpot, but the second one is a bit more of
a challenge. It\'s not too hard to level-shift unidirectional I2c up to
the op amp\'s output--a PNP CB stage with the base grounded, followed by
an NPN CB stage with the base about 3V above the negative supply rail,
as in the LTspice files below (i2cls.lib and I2Cshifter.asc. The dpot
can be an AD5273BRJZ1, whose 6 MHz bandwidth will work fine.

Problem is, at high frequency most of the signal swing appears across
the dpot, so if I do it this way I\'ll have to reduce the supplies.

I\'m considering two things: first, leaving the resistors fixed and
switching the caps in and out with a high-voltage mux, or else (more
interestingly) transformer-coupling the dpot. The first

It\'ll need a fair amount of inductance to look like 1k ohms at 200 kHz
and above, probably a couple of mH, and a turns ratio of 2:1 or so to
reduce the voltage swing.

Anybody here done something like that?

Cheers

Phil Hobbs

Oh, there\'s another trick: a tiny surface-mount dip switch and 4 caps
makes a 4-bit capacitive DAC.

I\'m not at all above that sort of thing either. I could even use a hex
dip switch to mimic a variable cap.

Or a tiny screwdriver-adjusted variable capacitor.

Both are cheap and low noise and require no software.

Agreed. Part of what we\'re doing is to make our gizmos attractive for
people to license, and as you know there\'s a lot of prejudice out there
against manual trims. That\'s not completely unreasonable when the
person doing the adjusting has no idea how the circuit works--it\'s easy
to encapsulate the know-how in the test software.

For protos and proof-of-concept systems I often use gimmick caps to
adjust amplifier peaking.

https://www.digikey.com/products/en?keywords=490-2008-1-ND

We pay 26 cents for that one.

Long obsolete, unfortunately.

Maybe that\'s why we have a ton of them. Probably an EOL buy.

Our existing trimpot works okay but doesn\'t play that well with ATE.
(You used to be able to get GPIB-controlled screwdrivers for that sort
of thing.)

Plus, needing to hang an AC tweak on some out-of-the-way circuit node is
something that comes up a fair amount round here.

The transformers will be here today, so maybe next week I\'ll give it a
whirl. We\'re ordering boards probably tomorrow, so it\'ll have to be
trimpots for this batch.

Cheers

Phil Hobbs

I still think you could use the dpot as a grounded pot, but I don\'t
entirely understand your situation.