[volt-nuts] HP3458 ADC integrator

[Attila Kinali]

On Sat, 24 Feb 2018 12:59:21 +1300 (NZDT)
Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:

> 1) Reverse the supplies on U4 (in real circuit reversed supplies will fry the chip.)
> 2) Add 10k in series with reset switch
> 3) Swap connections to U4 inputs.
> 4) Use series shunt switch for reset switch as used in an actual 3458A.
> Once done together with swapping in LT1220 for LT1225 rest works as expected and output slews due to gate current of J1 and input bias current of U1 

Ok, I played around a bit more and I think I now mostly understand what's
going on. The main problem was really the stability of the opamps chosen.
For the integrator to work, the added up gain and phase shift must still
fulfill the stability condition (obvious, yes, but.. well I am stupid at times)
I am not sure how the intermediate steps in a 3 opamp integrator work,
but I guess that should imediatly be obvious once a complete stability
analyis is done.

Thanks everyone who provided insight and feedback.

			Attila Kinali

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