From: jl@glen--canyon.com   
      
   On Thu, 29 Jan 2026 20:01:27 -0000 (UTC), Phil Hobbs   
    wrote:   
      
   >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   
   >>   
      
   Our visitor lady yesterday is an EE grad student at Georgia Tech.   
   Someone mentioned thermal issues on PCBs and we had to explain what   
   that meant.   
      
   Interns come in not knowing anything about thermals or packaging or   
   assembly drawings... how electronics is actually done.   
      
   Picosecond electronics needs tiny parts that run at high currents and   
   get hot. Simon should be grateful that he doesn't have to deal with   
   that too.   
      
      
   John Larkin   
   Highland Tech Glen Canyon Design Center   
   Lunatic Fringe Electronics   
      
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