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.
Reply by Phil Hobbs●August 19, 20202020-08-19
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.
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.nethttp://hobbs-eo.com
Reply by John Larkin●August 19, 20202020-08-19
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.
Reply by Phil Hobbs●August 18, 20202020-08-18
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.nethttp://hobbs-eo.com
Reply by Chris Jones●August 18, 20202020-08-18
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.
Reply by Phil Hobbs●August 18, 20202020-08-18
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.nethttp://hobbs-eo.com
Reply by John Larkin●August 18, 20202020-08-18
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?
Reply by Phil Hobbs●August 18, 20202020-08-18
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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.
>
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.nethttp://hobbs-eo.com
Reply by ●August 18, 20202020-08-18
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