From: pcdhSpamMeSenseless@electrooptical.net   
      
   john larkin  wrote:   
   > On Thu, 29 Jan 2026 17:22:39 -0500, Phil Hobbs   
   >  wrote:   
   >   
   >> 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.   
   >>   
   > Couldn't the offset servo loop be mega-slow? I think it's correcting   
   > 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   
      
   --   
   Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /   
   Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics   
      
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