From: pcdhSpamMeSenseless@electrooptical.net   
      
   Liz Tuddenham  wrote:   
   > 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?   
      
   The snooper’s main rationale is that it’s not in the main signal   
   path—it’s   
   just a bag hung off one side to balance out the offsets of the otherwise   
   very nice AD822 TIA and AD8605 servo/driver.   
      
   The signal paths are much faster, up to 10 MHz.   
      
   Cheers   
      
   Phil Hobbs   
      
   --   
   Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /   
   Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)