From: user@example.net   
      
   On 1/29/2026 4:24 PM, Phil Hobbs wrote:   
   > On 2026-01-29 15:32, Liz Tuddenham wrote:   
   >> Phil Hobbs  wrote:   
   >>   
   >>> Hi, all,   
   >>>   
   >>> I'm doing a high-accuracy version of the laser noise canceller   
   >>> .   
   >>>   
   >>> 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.   
   >   
   > Cheers   
   >   
   > Phil Hobbs   
   >   
      
   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   
      
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