Hi, all,
I'm doing a high-accuracy version of the laser noise canceller ><https://electrooptical.net/www/canceller/WithoutTears.pdf>.
In particular, to get better cancellation accuracy, I want to get rid of
the input offset voltages of a couple of op amps.
One approach to this is to use a chopamp integrator to snoop the summing >junction, and dork the noninverting input to force the summing junction
to average 0.00000V.
This is nice conceptually, but there are a couple of worries:
1. Chopamps kick out nasty switching spikes, which will have to be
decoupled sufficiently well.
2. Weird-ass composite amplifiers always have weird settling behavior.
I haven't done this lately, but I'm thinking of a TLV2333.
Any wisdom?
Thanks
Phil Hobbs
On Thu, 29 Jan 2026 13:01:31 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
Hi, all,
I'm doing a high-accuracy version of the laser noise canceller
<https://electrooptical.net/www/canceller/WithoutTears.pdf>.
In particular, to get better cancellation accuracy, I want to get rid of
the input offset voltages of a couple of op amps.
One approach to this is to use a chopamp integrator to snoop the summing
junction, and dork the noninverting input to force the summing junction
to average 0.00000V.
This is nice conceptually, but there are a couple of worries:
1. Chopamps kick out nasty switching spikes, which will have to be
decoupled sufficiently well.
2. Weird-ass composite amplifiers always have weird settling behavior.
I haven't done this lately, but I'm thinking of a TLV2333.
Any wisdom?
Thanks
Would the Johnson noise of a resistor wreck the input, like if you
lowpass filter the summing junction into the offset servo?
Can you do anything useful with the other end of the photodiode? Seems
a shame to waste that current.
I just finished a 4-layer double-sided-parts pcb layout and discovered
that I forgot to include the threshold generator circuit. I can
squeeze in the parts somewhere but routing will be nasty.
John Larkin
Highland Tech Glen Canyon Design Center
Lunatic Fringe Electronics
john larkin <jl@glen--canyon.com> wrote:
On Thu, 29 Jan 2026 13:01:31 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
Hi, all,
I'm doing a high-accuracy version of the laser noise canceller
<https://electrooptical.net/www/canceller/WithoutTears.pdf>.
In particular, to get better cancellation accuracy, I want to get rid of >>> the input offset voltages of a couple of op amps.
One approach to this is to use a chopamp integrator to snoop the summing >>> junction, and dork the noninverting input to force the summing junction >>> to average 0.00000V.
This is nice conceptually, but there are a couple of worries:
1. Chopamps kick out nasty switching spikes, which will have to be
decoupled sufficiently well.
2. Weird-ass composite amplifiers always have weird settling behavior.
I haven't done this lately, but I'm thinking of a TLV2333.
Any wisdom?
Thanks
Would the Johnson noise of a resistor wreck the input, like if you
lowpass filter the summing junction into the offset servo?
The TIA stage is an AD825 with an LMH6321 buffer and a 5k feedback
resistor. A 200k resistor from the SJ to the chopamp won?t perturb that >much, and I?ll probably split it in half, with 1nF or to ground just to
make sure.
The chopamp runs on +-2.5V and the AD825?s Vos range is +-5mV over >temperature, so I?ll put a 200:1 voltage divider to match the ranges
better. Hopefully that?ll reduce the transient funnies to a tolerable
level.
Can you do anything useful with the other end of the photodiode? Seems
a shame to waste that current.
The other end goes to a DC-coupled bootstrap. That keeps the load impedance >stable, which avoids AC cancellation errors due to different RC time >constants.
I just finished a 4-layer double-sided-parts pcb layout and discovered
that I forgot to include the threshold generator circuit. I can
squeeze in the parts somewhere but routing will be nasty.
Blech, I admire your patience. Simon teases me that my circuits have >everything connected to everything else, by traces that all have to be
short. ;)
Cheers
Phil Hobbs
John Larkin
Highland Tech Glen Canyon Design Center
Lunatic Fringe Electronics
Hi, all,
I'm doing a high-accuracy version of the laser noise canceller <https://electrooptical.net/www/canceller/WithoutTears.pdf>.
In particular, to get better cancellation accuracy, I want to get rid of
the input offset voltages of a couple of op amps.
One approach to this is to use a chopamp integrator to snoop the summing junction, and dork the noninverting input to force the summing junction
to average 0.00000V.
This is nice conceptually, but there are a couple of worries:
1. Chopamps kick out nasty switching spikes, which will have to be
decoupled sufficiently well.
2. Weird-ass composite amplifiers always have weird settling behavior.
I haven't done this lately, but I'm thinking of a TLV2333.
Any wisdom?
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
Hi, all,
I'm doing a high-accuracy version of the laser noise canceller
<https://electrooptical.net/www/canceller/WithoutTears.pdf>.
In particular, to get better cancellation accuracy, I want to get
rid of the input offset voltages of a couple of op amps.
One approach to this is to use a chopamp integrator to snoop the
summing junction, and dork the noninverting input to force the
summing junction to average 0.00000V.
This is nice conceptually, but there are a couple of worries:
1. Chopamps kick out nasty switching spikes, which will have to be
decoupled sufficiently well.
2. Weird-ass composite amplifiers always have weird settling
behavior.
I haven't done this lately, but I'm thinking of a TLV2333.
Any wisdom?
At that level of accuracy, beware thermocouple effects.
If you are compensating a slow drift in offset, chop slowly and sinusoidally, then the 'spikes' will matter less.
On 2026-01-29 15:32, Liz Tuddenham wrote:
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
Hi, all,
I'm doing a high-accuracy version of the laser noise canceller
<https://electrooptical.net/www/canceller/WithoutTears.pdf>.
In particular, to get better cancellation accuracy, I want to get
rid of the input offset voltages of a couple of op amps.
One approach to this is to use a chopamp integrator to snoop the
summing junction, and dork the noninverting input to force the
summing junction to average 0.00000V.
This is nice conceptually, but there are a couple of worries:
1. Chopamps kick out nasty switching spikes, which will have to be
decoupled sufficiently well.
2. Weird-ass composite amplifiers always have weird settling
behavior.
I haven't done this lately, but I'm thinking of a TLV2333.
Any wisdom?
At that level of accuracy, beware thermocouple effects.
It's all on one board, and the power level is low, so that shouldn't be
a huge issue, I don't think.ÿ Gradients on the board should be way under
1K in the quarter-inch or so separating the two amps.ÿ I'll certainly
put the power buffer some distance away.
If you are compensating a slow drift in offset, chop slowly and
sinusoidally, then the 'spikes' will matter less.
I'm not the one doing the chopping--the spikes come from the CMOS
switches inside the chopamp.
The noise canceller works by splitting a larger photocurrent using a BJT diff pair, and adjusting the split ratio until the current in one arm exactly cancels a smaller photocurrent derived from the same laser.
There are various fine points, but because the diff pair is a highly
linear current splitter, the fluctuations split the same as the DC, so
by adjusting the DC to zero, one in principle obtains cancellation of
the fluctuations at all frequencies.ÿ A slow servo loop lets you do AC- coupled measurements down at the shot noise even with noisy lasers.
With a bit of math, you can use the delta V_BE of the diff pair to do
the same thing inside the feedback loop bandwidth.
An offset voltage in either the TIA or the integrating servo amp causes
the cancellation to be in error by
delta I = V_os / R_F.
With a 5k ohm R_F, a millivolt of offset makes 200 nA of current
imbalance.ÿ With a 100-uA photocurrent, that limits the cancellation performance to
Amax = 20*log(100uA/500nA) = 54 dB.
It's better than that at higher photocurrent, but I'm chasing an honest
70 dB with this box, so the offsets have to be down in the tens of microvolts at most.
Cheers
Phil Hobbs
On Thu, 29 Jan 2026 20:01:27 -0000 (UTC), Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
john larkin <jl@glen--canyon.com> wrote:
On Thu, 29 Jan 2026 13:01:31 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
Hi, all,
I'm doing a high-accuracy version of the laser noise canceller
<https://electrooptical.net/www/canceller/WithoutTears.pdf>.
In particular, to get better cancellation accuracy, I want to get rid of >>>> the input offset voltages of a couple of op amps.
One approach to this is to use a chopamp integrator to snoop the summing >>>> junction, and dork the noninverting input to force the summing junction >>>> to average 0.00000V.
This is nice conceptually, but there are a couple of worries:
1. Chopamps kick out nasty switching spikes, which will have to be
decoupled sufficiently well.
2. Weird-ass composite amplifiers always have weird settling behavior. >>>>
I haven't done this lately, but I'm thinking of a TLV2333.
Any wisdom?
Thanks
Would the Johnson noise of a resistor wreck the input, like if you
lowpass filter the summing junction into the offset servo?
The TIA stage is an AD825 with an LMH6321 buffer and a 5k feedback
resistor. A 200k resistor from the SJ to the chopamp won?t perturb that
much, and I?ll probably split it in half, with 1nF or to ground just to
make sure.
The chopamp runs on +-2.5V and the AD825?s Vos range is +-5mV over
temperature, so I?ll put a 200:1 voltage divider to match the ranges
better. Hopefully that?ll reduce the transient funnies to a tolerable
level.
Can you do anything useful with the other end of the photodiode? Seems
a shame to waste that current.
The other end goes to a DC-coupled bootstrap. That keeps the load impedance >> stable, which avoids AC cancellation errors due to different RC time
constants.
I just finished a 4-layer double-sided-parts pcb layout and discovered
that I forgot to include the threshold generator circuit. I can
squeeze in the parts somewhere but routing will be nasty.
Blech, I admire your patience. Simon teases me that my circuits have
everything connected to everything else, by traces that all have to be
short. ;)
Our visitor lady yesterday is an EE grad student at Georgia Tech.
Someone mentioned thermal issues on PCBs and we had to explain what
that meant.
Interns come in not knowing anything about thermals or packaging or
assembly drawings... how electronics is actually done.
Picosecond electronics needs tiny parts that run at high currents and
get hot. Simon should be grateful that he doesn't have to deal with
that too.
On 1/29/2026 4:24 PM, Phil Hobbs wrote:
On 2026-01-29 15:32, Liz Tuddenham wrote:
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
Hi, all,
I'm doing a high-accuracy version of the laser noise canceller
<https://electrooptical.net/www/canceller/WithoutTears.pdf>.
In particular, to get better cancellation accuracy, I want to
get rid of the input offset voltages of a couple of op amps.
One approach to this is to use a chopamp integrator to snoop
the summing junction, and dork the noninverting input to force
the summing junction to average 0.00000V.
This is nice conceptually, but there are a couple of worries:
1. Chopamps kick out nasty switching spikes, which will have to
be decoupled sufficiently well.
2. Weird-ass composite amplifiers always have weird settling
behavior.
I haven't done this lately, but I'm thinking of a TLV2333.
Any wisdom?
At that level of accuracy, beware thermocouple effects.
It's all on one board, and the power level is low, so that
shouldn't be a huge issue, I don't think. Gradients on the board
should be way under 1K in the quarter-inch or so separating the two
amps. I'll certainly put the power buffer some distance away.
If you are compensating a slow drift in offset, chop slowly and
sinusoidally, then the 'spikes' will matter less.
I'm not the one doing the chopping--the spikes come from the CMOS
switches inside the chopamp.
The noise canceller works by splitting a larger photocurrent using
a BJT diff pair, and adjusting the split ratio until the current in
one arm exactly cancels a smaller photocurrent derived from the
same laser.
There are various fine points, but because the diff pair is a
highly linear current splitter, the fluctuations split the same as
the DC, so by adjusting the DC to zero, one in principle obtains
cancellation of the fluctuations at all frequencies. A slow servo
loop lets you do AC- coupled measurements down at the shot noise
even with noisy lasers.
With a bit of math, you can use the delta V_BE of the diff pair to
do the same thing inside the feedback loop bandwidth.
An offset voltage in either the TIA or the integrating servo amp
causes the cancellation to be in error by
delta I = V_os / R_F.
With a 5k ohm R_F, a millivolt of offset makes 200 nA of current
imbalance. With a 100-uA photocurrent, that limits the
cancellation performance to
Amax = 20*log(100uA/500nA) = 54 dB.
It's better than that at higher photocurrent, but I'm chasing an
honest 70 dB with this box, so the offsets have to be down in the
tens of microvolts at most.
So is the idea to LPF the crap out of the summing junction voltage,
send to a chopper amp used as an integrator, and then LPF the crap of
the chopper amp output sent to the TIA amp non-inverting input?
Is it spikes going forward to the TIA non-inverting input or going
backwards to the summing junction itself that's the most concern?
The non-inverting input is the devil and I don't really like it
anywhere but bolted to ground in precision applications but I guess
there aren't a lot of other places to inject a correction that isn't
going to disturb the summing junction worse
On 2026-01-29 16:40, bitrex wrote:
On 1/29/2026 4:24 PM, Phil Hobbs wrote:
On 2026-01-29 15:32, Liz Tuddenham wrote:
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
Hi, all,
I'm doing a high-accuracy version of the laser noise canceller
<https://electrooptical.net/www/canceller/WithoutTears.pdf>.
In particular, to get better cancellation accuracy, I want to
get rid of the input offset voltages of a couple of op amps.
One approach to this is to use a chopamp integrator to snoop
the summing junction, and dork the noninverting input to force
the summing junction to average 0.00000V.
This is nice conceptually, but there are a couple of worries:
1. Chopamps kick out nasty switching spikes, which will have to
be decoupled sufficiently well.
2. Weird-ass composite amplifiers always have weird settling
behavior.
I haven't done this lately, but I'm thinking of a TLV2333.
Any wisdom?
At that level of accuracy, beware thermocouple effects.
It's all on one board, and the power level is low, so that
shouldn't be a huge issue, I don't think. Gradients on the board
should be way under 1K in the quarter-inch or so separating the two
amps. I'll certainly put the power buffer some distance away.
If you are compensating a slow drift in offset, chop slowly and
sinusoidally, then the 'spikes' will matter less.
I'm not the one doing the chopping--the spikes come from the CMOS
switches inside the chopamp.
The noise canceller works by splitting a larger photocurrent using
a BJT diff pair, and adjusting the split ratio until the current in
one arm exactly cancels a smaller photocurrent derived from the
same laser.
There are various fine points, but because the diff pair is a
highly linear current splitter, the fluctuations split the same as
the DC, so by adjusting the DC to zero, one in principle obtains
cancellation of the fluctuations at all frequencies. A slow servo
loop lets you do AC- coupled measurements down at the shot noise
even with noisy lasers.
With a bit of math, you can use the delta V_BE of the diff pair to
do the same thing inside the feedback loop bandwidth.
An offset voltage in either the TIA or the integrating servo amp
causes the cancellation to be in error by
delta I = V_os / R_F.
With a 5k ohm R_F, a millivolt of offset makes 200 nA of current
imbalance. With a 100-uA photocurrent, that limits the
cancellation performance to
Amax = 20*log(100uA/500nA) = 54 dB.
It's better than that at higher photocurrent, but I'm chasing an
honest 70 dB with this box, so the offsets have to be down in the
tens of microvolts at most.
So is the idea to LPF the crap out of the summing junction voltage,
send to a chopper amp used as an integrator, and then LPF the crap of
the chopper amp output sent to the TIA amp non-inverting input?
Right, except that the output doesn't need filtering, just a voltage >divider.
Is it spikes going forward to the TIA non-inverting input or going
backwards to the summing junction itself that's the most concern?
The output is just ordinary noisy--53 nV in 1 Hz. Chopamp inputs kick
out evil microamp-level spikes of low duty cycle--the 70 pA bias current
spec is basically the bits of the spikes that don't average to zero.
The non-inverting input is the devil and I don't really like it
anywhere but bolted to ground in precision applications but I guess
there aren't a lot of other places to inject a correction that isn't
going to disturb the summing junction worse
Well, it's got a 10k:50R voltage divider to help keep it still. The
total adjustment range is thus +-12 mV or so, comfortably larger than
the +-5 mV max offset over temperature.
The voltage divider reduces the loop bandwidth by the same factor of 200
for a given time constant, so to get a 5-Hz snoop loop bandwidth, it
needs a time constant of
tau = 1/200 / (2 pi * 5 Hz) = 160 us
so the integrator has 200k * 820 pF.
Since the 820 pF is connected between the inverting input and the
(low-Z) output, it'll suck in most of the spikies, but just in case, I'm >splitting the 200k in half and bypassing the midpoint with 1 nF to ground.
Cheers
Phil Hobbs
On Thu, 29 Jan 2026 17:22:39 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
On 2026-01-29 16:40, bitrex wrote:Couldn't the offset servo loop be mega-slow? I think it's correcting thermals.
On 1/29/2026 4:24 PM, Phil Hobbs wrote:
On 2026-01-29 15:32, Liz Tuddenham wrote:
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
Hi, all,
I'm doing a high-accuracy version of the laser noise canceller
<https://electrooptical.net/www/canceller/WithoutTears.pdf>.
In particular, to get better cancellation accuracy, I want to
get rid of the input offset voltages of a couple of op amps.
One approach to this is to use a chopamp integrator to snoop
the summing junction, and dork the noninverting input to force
the summing junction to average 0.00000V.
This is nice conceptually, but there are a couple of worries:
1. Chopamps kick out nasty switching spikes, which will have to
be decoupled sufficiently well.
2. Weird-ass composite amplifiers always have weird settling
behavior.
I haven't done this lately, but I'm thinking of a TLV2333.
Any wisdom?
At that level of accuracy, beware thermocouple effects.
It's all on one board, and the power level is low, so that
shouldn't be a huge issue, I don't think. Gradients on the board
should be way under 1K in the quarter-inch or so separating the two
amps. I'll certainly put the power buffer some distance away.
If you are compensating a slow drift in offset, chop slowly and
sinusoidally, then the 'spikes' will matter less.
I'm not the one doing the chopping--the spikes come from the CMOS
switches inside the chopamp.
The noise canceller works by splitting a larger photocurrent using
a BJT diff pair, and adjusting the split ratio until the current in
one arm exactly cancels a smaller photocurrent derived from the
same laser.
There are various fine points, but because the diff pair is a
highly linear current splitter, the fluctuations split the same as
the DC, so by adjusting the DC to zero, one in principle obtains
cancellation of the fluctuations at all frequencies. A slow servo
loop lets you do AC- coupled measurements down at the shot noise
even with noisy lasers.
With a bit of math, you can use the delta V_BE of the diff pair to
do the same thing inside the feedback loop bandwidth.
An offset voltage in either the TIA or the integrating servo amp
causes the cancellation to be in error by
delta I = V_os / R_F.
With a 5k ohm R_F, a millivolt of offset makes 200 nA of current
imbalance. With a 100-uA photocurrent, that limits the
cancellation performance to
Amax = 20*log(100uA/500nA) = 54 dB.
It's better than that at higher photocurrent, but I'm chasing an
honest 70 dB with this box, so the offsets have to be down in the
tens of microvolts at most.
So is the idea to LPF the crap out of the summing junction voltage,
send to a chopper amp used as an integrator, and then LPF the crap of
the chopper amp output sent to the TIA amp non-inverting input?
Right, except that the output doesn't need filtering, just a voltage
divider.
Is it spikes going forward to the TIA non-inverting input or going
backwards to the summing junction itself that's the most concern?
The output is just ordinary noisy--53 nV in 1 Hz. Chopamp inputs kick
out evil microamp-level spikes of low duty cycle--the 70 pA bias current
spec is basically the bits of the spikes that don't average to zero.
The non-inverting input is the devil and I don't really like it
anywhere but bolted to ground in precision applications but I guess
there aren't a lot of other places to inject a correction that isn't
going to disturb the summing junction worse
Well, it's got a 10k:50R voltage divider to help keep it still. The
total adjustment range is thus +-12 mV or so, comfortably larger than
the +-5 mV max offset over temperature.
The voltage divider reduces the loop bandwidth by the same factor of 200
for a given time constant, so to get a 5-Hz snoop loop bandwidth, it
needs a time constant of
tau = 1/200 / (2 pi * 5 Hz) = 160 us
so the integrator has 200k * 820 pF.
Since the 820 pF is connected between the inverting input and the
(low-Z) output, it'll suck in most of the spikies, but just in case, I'm
splitting the 200k in half and bypassing the midpoint with 1 nF to ground. >>
john larkin <jl@glen--canyon.com> wrote:
On Thu, 29 Jan 2026 17:22:39 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
On 2026-01-29 16:40, bitrex wrote:Couldn't the offset servo loop be mega-slow? I think it's correcting
On 1/29/2026 4:24 PM, Phil Hobbs wrote:
On 2026-01-29 15:32, Liz Tuddenham wrote:
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
Hi, all,
I'm doing a high-accuracy version of the laser noise canceller
<https://electrooptical.net/www/canceller/WithoutTears.pdf>.
In particular, to get better cancellation accuracy, I want to
get rid of the input offset voltages of a couple of op amps.
One approach to this is to use a chopamp integrator to snoop
the summing junction, and dork the noninverting input to force
the summing junction to average 0.00000V.
This is nice conceptually, but there are a couple of worries:
1. Chopamps kick out nasty switching spikes, which will have to
be decoupled sufficiently well.
2. Weird-ass composite amplifiers always have weird settling
behavior.
I haven't done this lately, but I'm thinking of a TLV2333.
Any wisdom?
At that level of accuracy, beware thermocouple effects.
It's all on one board, and the power level is low, so that
shouldn't be a huge issue, I don't think. Gradients on the board
should be way under 1K in the quarter-inch or so separating the two
amps. I'll certainly put the power buffer some distance away.
If you are compensating a slow drift in offset, chop slowly and
sinusoidally, then the 'spikes' will matter less.
I'm not the one doing the chopping--the spikes come from the CMOS
switches inside the chopamp.
The noise canceller works by splitting a larger photocurrent using
a BJT diff pair, and adjusting the split ratio until the current in
one arm exactly cancels a smaller photocurrent derived from the
same laser.
There are various fine points, but because the diff pair is a
highly linear current splitter, the fluctuations split the same as
the DC, so by adjusting the DC to zero, one in principle obtains
cancellation of the fluctuations at all frequencies. A slow servo
loop lets you do AC- coupled measurements down at the shot noise
even with noisy lasers.
With a bit of math, you can use the delta V_BE of the diff pair to
do the same thing inside the feedback loop bandwidth.
An offset voltage in either the TIA or the integrating servo amp
causes the cancellation to be in error by
delta I = V_os / R_F.
With a 5k ohm R_F, a millivolt of offset makes 200 nA of current
imbalance. With a 100-uA photocurrent, that limits the
cancellation performance to
Amax = 20*log(100uA/500nA) = 54 dB.
It's better than that at higher photocurrent, but I'm chasing an
honest 70 dB with this box, so the offsets have to be down in the
tens of microvolts at most.
So is the idea to LPF the crap out of the summing junction voltage,
send to a chopper amp used as an integrator, and then LPF the crap of
the chopper amp output sent to the TIA amp non-inverting input?
Right, except that the output doesn't need filtering, just a voltage
divider.
Is it spikes going forward to the TIA non-inverting input or going
backwards to the summing junction itself that's the most concern?
The output is just ordinary noisy--53 nV in 1 Hz. Chopamp inputs kick
out evil microamp-level spikes of low duty cycle--the 70 pA bias current >>> spec is basically the bits of the spikes that don't average to zero.
The non-inverting input is the devil and I don't really like it
anywhere but bolted to ground in precision applications but I guess
there aren't a lot of other places to inject a correction that isn't
going to disturb the summing junction worse
Well, it's got a 10k:50R voltage divider to help keep it still. The
total adjustment range is thus +-12 mV or so, comfortably larger than
the +-5 mV max offset over temperature.
The voltage divider reduces the loop bandwidth by the same factor of 200 >>> for a given time constant, so to get a 5-Hz snoop loop bandwidth, it
needs a time constant of
tau = 1/200 / (2 pi * 5 Hz) = 160 us
so the integrator has 200k * 820 pF.
Since the 820 pF is connected between the inverting input and the
(low-Z) output, it'll suck in most of the spikies, but just in case, I'm >>> splitting the 200k in half and bypassing the midpoint with 1 nF to ground. >>>
thermals.
I think several hertz is the sweet spot, because there are situations such
as turn-on where it?ll start way out of whack and have to recover.
It?s more the fixed part of the offset than the drift that?s the problem, >except at extreme temperatures.
Cheers
Phil Hobbs
On 2026-01-29 15:32, Liz Tuddenham wrote:
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
Hi, all,
I'm doing a high-accuracy version of the laser noise canceller
<https://electrooptical.net/www/canceller/WithoutTears.pdf>.
In particular, to get better cancellation accuracy, I want to get
rid of the input offset voltages of a couple of op amps.
One approach to this is to use a chopamp integrator to snoop the
summing junction, and dork the noninverting input to force the
summing junction to average 0.00000V.
This is nice conceptually, but there are a couple of worries:
1. Chopamps kick out nasty switching spikes, which will have to be
decoupled sufficiently well.
2. Weird-ass composite amplifiers always have weird settling
behavior.
I haven't done this lately, but I'm thinking of a TLV2333.
Any wisdom?
At that level of accuracy, beware thermocouple effects.
It's all on one board, and the power level is low, so that shouldn't be
a huge issue, I don't think. Gradients on the board should be way under
1K in the quarter-inch or so separating the two amps. I'll certainly
put the power buffer some distance away.
If you are compensating a slow drift in offset, chop slowly and sinusoidally, then the 'spikes' will matter less.
I'm not the one doing the chopping--the spikes come from the CMOS
switches inside the chopamp.
As the charges injected into a CMOS switch from switching on and
switching off are usually unequal, could you use a balanced circuit
which would more-or-less cancel them? The actual spikes could be
slugged by a long time constant and the long-term inequalities would
balance out. ...or is that gettig too complicated?
On Fri, 30 Jan 2026 04:24:18 -0000 (UTC), Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
john larkin <jl@glen--canyon.com> wrote:
On Thu, 29 Jan 2026 17:22:39 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
On 2026-01-29 16:40, bitrex wrote:Couldn't the offset servo loop be mega-slow? I think it's correcting
On 1/29/2026 4:24 PM, Phil Hobbs wrote:
On 2026-01-29 15:32, Liz Tuddenham wrote:
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
Hi, all,
I'm doing a high-accuracy version of the laser noise canceller >>>>>>>> <https://electrooptical.net/www/canceller/WithoutTears.pdf>.
In particular, to get better cancellation accuracy, I want to
get rid of the input offset voltages of a couple of op amps.
One approach to this is to use a chopamp integrator to snoop
the summing junction, and dork the noninverting input to force >>>>>>>> the summing junction to average 0.00000V.
This is nice conceptually, but there are a couple of worries:
1. Chopamps kick out nasty switching spikes, which will have to >>>>>>>> be decoupled sufficiently well.
2. Weird-ass composite amplifiers always have weird settling
behavior.
I haven't done this lately, but I'm thinking of a TLV2333.
Any wisdom?
At that level of accuracy, beware thermocouple effects.
It's all on one board, and the power level is low, so that
shouldn't be a huge issue, I don't think. Gradients on the board
should be way under 1K in the quarter-inch or so separating the two >>>>>> amps. I'll certainly put the power buffer some distance away.
If you are compensating a slow drift in offset, chop slowly and >>>>>>> sinusoidally, then the 'spikes' will matter less.
I'm not the one doing the chopping--the spikes come from the CMOS >>>>>> switches inside the chopamp.
The noise canceller works by splitting a larger photocurrent using >>>>>> a BJT diff pair, and adjusting the split ratio until the current in >>>>>> one arm exactly cancels a smaller photocurrent derived from the
same laser.
There are various fine points, but because the diff pair is a
highly linear current splitter, the fluctuations split the same as >>>>>> the DC, so by adjusting the DC to zero, one in principle obtains
cancellation of the fluctuations at all frequencies. A slow servo >>>>>> loop lets you do AC- coupled measurements down at the shot noise
even with noisy lasers.
With a bit of math, you can use the delta V_BE of the diff pair to >>>>>> do the same thing inside the feedback loop bandwidth.
An offset voltage in either the TIA or the integrating servo amp
causes the cancellation to be in error by
delta I = V_os / R_F.
With a 5k ohm R_F, a millivolt of offset makes 200 nA of current
imbalance. With a 100-uA photocurrent, that limits the
cancellation performance to
Amax = 20*log(100uA/500nA) = 54 dB.
It's better than that at higher photocurrent, but I'm chasing an
honest 70 dB with this box, so the offsets have to be down in the
tens of microvolts at most.
So is the idea to LPF the crap out of the summing junction voltage,
send to a chopper amp used as an integrator, and then LPF the crap of >>>>> the chopper amp output sent to the TIA amp non-inverting input?
Right, except that the output doesn't need filtering, just a voltage
divider.
Is it spikes going forward to the TIA non-inverting input or going
backwards to the summing junction itself that's the most concern?
The output is just ordinary noisy--53 nV in 1 Hz. Chopamp inputs kick >>>> out evil microamp-level spikes of low duty cycle--the 70 pA bias current >>>> spec is basically the bits of the spikes that don't average to zero.
The non-inverting input is the devil and I don't really like it
anywhere but bolted to ground in precision applications but I guess
there aren't a lot of other places to inject a correction that isn't >>>>> going to disturb the summing junction worse
Well, it's got a 10k:50R voltage divider to help keep it still. The
total adjustment range is thus +-12 mV or so, comfortably larger than >>>> the +-5 mV max offset over temperature.
The voltage divider reduces the loop bandwidth by the same factor of 200 >>>> for a given time constant, so to get a 5-Hz snoop loop bandwidth, it
needs a time constant of
tau = 1/200 / (2 pi * 5 Hz) = 160 us
so the integrator has 200k * 820 pF.
Since the 820 pF is connected between the inverting input and the
(low-Z) output, it'll suck in most of the spikies, but just in case, I'm >>>> splitting the 200k in half and bypassing the midpoint with 1 nF to ground. >>>>
thermals.
I think several hertz is the sweet spot, because there are situations such >> as turn-on where it?ll start way out of whack and have to recover.
It?s more the fixed part of the offset than the drift that?s the problem,
except at extreme temperatures.
Cheers
Phil Hobbs
Trimpot!
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
On 2026-01-29 15:32, Liz Tuddenham wrote:
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
Hi, all,
I'm doing a high-accuracy version of the laser noise canceller
<https://electrooptical.net/www/canceller/WithoutTears.pdf>.
In particular, to get better cancellation accuracy, I want to get
rid of the input offset voltages of a couple of op amps.
One approach to this is to use a chopamp integrator to snoop the
summing junction, and dork the noninverting input to force the
summing junction to average 0.00000V.
This is nice conceptually, but there are a couple of worries:
1. Chopamps kick out nasty switching spikes, which will have to be
decoupled sufficiently well.
2. Weird-ass composite amplifiers always have weird settling
behavior.
I haven't done this lately, but I'm thinking of a TLV2333.
Any wisdom?
At that level of accuracy, beware thermocouple effects.
It's all on one board, and the power level is low, so that shouldn't be
a huge issue, I don't think. Gradients on the board should be way under
1K in the quarter-inch or so separating the two amps. I'll certainly
put the power buffer some distance away.
If you are compensating a slow drift in offset, chop slowly and
sinusoidally, then the 'spikes' will matter less.
I'm not the one doing the chopping--the spikes come from the CMOS
switches inside the chopamp.
[...]
If you are cancelling slowly-changing offsets, the switching could be
done with FETs driven by a very low frequency sinewave, there would be
no spikes and the charging and discharging currents of the gate
capacitances would inject negilgible unwanted charges at low frequency.
If starting transients are a problem, increase the frequency momentarily during start-up.
As the charges injected into a CMOS switch from switching on and
switching off are usually unequal, could you use a balanced circuit
which would more-or-less cancel them? The actual spikes could be
slugged by a long time constant and the long-term inequalities would
balance out. ...or is that gettig too complicated?
Am 30.01.26 um 10:17 schrieb Liz Tuddenham:
As the charges injected into a CMOS switch from switching on and
switching off are usually unequal, could you use a balanced circuit
which would more-or-less cancel them? The actual spikes could be
slugged by a long time constant and the long-term inequalities would
balance out. ...or is that gettig too complicated?
I once had a chopper where it was enough to balance VCC/VEE
(which were not equal!).
Gate switching levels were fixed and came from a Xilinx Coolrunner2.
Gerhard
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