[sdiy] fast opamp with low offset

[ASSI]

On Saturday 03 April 2010, David G. Dixon wrote:
> Thanks List, for all the excellent suggestions.  However, for my
> application, even 0.1mV offset would require trimming :(
[...]

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), 
clamping the summing node during the reset or by draining the cap to a 
real instead of virtual ground (or an op-amp produced copy thereof) or 
by canceling the charge injection via a replica switch.

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.


Achim.
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