vivkr
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Hi Marc,
I would say that systematic mismatch is an issue if you want precise currents, because you may know the systematic mismatch at one corner (process, voltage, temperature), but it is unlikely to be the same across all (P,V,T) corners.
Whether the mismatch is critical or not is largely dependent on the application. For instance, it would be fatal in a current-steering DAC, but may be a little less serious in an opamp.
However, a significant amount of systematic mismatch is a sign of a poorly designed mirror, and may be an indirect sign that your output impedance etc. are poor. You should ask yourself why there is systematic mismatch. You have the same VGS, same (W/L) or a fixed ratio. The only variation comes from the VDS variation, and if this causes significant deviation in the mirror ratio, then you will run into trouble with some other critical parameter such as gain or PSRR.
So pay attention to the mismatch is what I would say.
Regards Vivek
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