sheldon
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Kanu,
In general, op amps are not intended to operate "slammed" into the rail. They often do not recover well from this condition. Next, you need to be careful, there are two terms to offset: systematic and random. You seem to saying that the systematic offset is 4mV when you need a resolution of 1mV. So you have a big problem, you need to improve the balance in your design so the systematic offset is much smaller than the required resolution, ~100uV. Then you need to run Monte Carlo to determine the random offset. As Raja noted, you probably need to look at circuit techniques to eliminate random offset, after eliminating the systematic offset. If the process has bipolars you might want to try them, some nice 1 mil circular emitter might get you the offset that you need if you can live with the base current and lower ft.
Best Regards,
Sheldon
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