I agree with Boe and Tiger9999, even though they are opposite opinions
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. In general layout asymmetries doesn't affect the spread (standard deviation) of a mismatch. Instead it creates a systematic error, that is, it affects the average (mean) offset. At least that is true in the older processes.
For example, an opamp with
good layout might have an offset with a
mean of zero and a standard deviation of 1 mV. On the other hand an opamp with
poor layout might have an offset with a
mean of 2 mV, but the standard deviation will still be 1 mV. The mean offset is not accounted for by the model so it is important to have a good layout.
The one exception that I know of is having metal over the device - in some process this messes up the anneal and results in a larger spread. There may be other exceptions with the newer processes.