[sdiy] fast opamp with low offset

[David G. Dixon]

> May I suggest again to fight the problem rather than the symptoms?
> 
> You say the problem is the charge injection from the reset switch or
> more precisely the feedback reaction of the 2164 to a fast voltage
> change at it's output.
> 
> This might for instance be countered by using a different switch
> (JFET, or perhaps a modern analog switch with low charge injection)

That's a good idea, which I've also considered but haven't done anything
with as yet.  I've got 25 of these bloody PN4391s, and I want to use them!

> clamping the summing node during the reset

Yes, I've tried this and it does work somewhat, but not really enough.  The
clamp still only limits the swing to +/-0.6V or so, which is still well
outside of the acceptable range.  I don't think this is a viable option on
its own, but it may relax the RC requirement for the filter, especially
given the severe restriction on the resistor value.

> or by draining the cap to a
> real instead of virtual ground (or an op-amp produced copy thereof)

Would that simply entail connecting the virtually grounded end of the JFET
to actual ground instead?  This would be easy enough to try, but it doesn't
work in simulation...?

> by canceling the charge injection via a replica switch.

H&H discuss using complementary switches in an integrator.  I don't have it
at hand, as I'm not at home, but it may be worth looking into.  I suppose
this is essentially your first suggestion.

> You could also try to isolate the 2164 via another current mirror, but
> the mirror would have to be very precise over some decades of current.
> Your I-V-I idea does that in a way, but as you said you'd need
> insanely speced components for it to work correctly.

Yes, I've also thought about mirrors, but I think something simpler should
suffice.  The solution doesn't have to be elegant.  It just has to work!