From: liz@poppyrecords.invalid.invalid   
      
   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.   
      
   [...]   
      
   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?   
      
      
   --   
   ~ Liz Tuddenham ~   
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