From: pcdhSpamMeSenseless@electrooptical.net   
      
   john larkin  wrote:   
   > On Thu, 29 Jan 2026 13:01:31 -0500, 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?   
   >>   
   >> Thanks   
   >>   
   >   
   > Would the Johnson noise of a resistor wreck the input, like if you   
   > lowpass filter the summing junction into the offset servo?   
      
   The TIA stage is an AD825 with an LMH6321 buffer and a 5k feedback   
   resistor.  A 200k resistor from the SJ to the chopamp won’t perturb that   
   much, and I’ll probably split it in half, with 1nF or to ground just to   
   make sure.   
      
   The chopamp runs on +-2.5V and the AD825’s Vos range is +-5mV over   
   temperature, so I’ll put a 200:1 voltage divider to match the ranges   
   better. Hopefully that’ll reduce the transient funnies to a tolerable   
   level.   
      
      
   >   
   > Can you do anything useful with the other end of the photodiode? Seems   
   > a shame to waste that current.   
      
   The other end goes to a DC-coupled bootstrap. That keeps the load impedance   
   stable, which avoids AC cancellation errors due to different RC time   
   constants.   
      
   >   
   > I just finished a 4-layer double-sided-parts pcb layout and discovered   
   > that I forgot to include the threshold generator circuit. I can   
   > squeeze in the parts somewhere but routing will be nasty.   
   >   
      
   Blech, I admire your patience.  Simon teases me that my circuits have   
   everything connected to everything else, by traces that all have to be   
   short.  ;)   
      
   Cheers   
      
   Phil Hobbs   
   >   
   > John Larkin   
   > Highland Tech Glen Canyon Design Center   
   > Lunatic Fringe Electronics   
   >   
      
      
      
   --   
   Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /   
   Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics   
      
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