From: pcdhSpamMeSenseless@electrooptical.net   
      
   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  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.   
      
   >   
   > 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   
      
   --   
   Dr Philip C D Hobbs   
   Principal Consultant   
   ElectroOptical Innovations LLC / Hobbs ElectroOptics   
   Optics, Electro-optics, Photonics, Analog Electronics   
   Briarcliff Manor NY 10510   
      
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