Paul, I think yer bored and needed something to post :-). I was worried that people might get the idea that common centroids weren't needed so I mentioned that they were useful not once, but twice. I was going to mention thermal gradients, but it just seemed to be getting more off topic to give yet another reason. By the way, you forgot temperature dependent stess, esp. in plastic packages. ;)
The main point was to
answer the question, and to give the reason why. Monte Carlo isn't pessimistic; you shouldn't expect common centroid to decrease the random offsets caused by device mismatch - that is just an oft repeated myth.
Anyway, are you sure you didn't mean V
T mismatch in the post below? The effect of mobility mismatch in a diff pair is minimized by biasing in weak inversion, whereas the effects of V
T mismatch is bias independent (for a diff pair).
Paul wrote on Apr 29th, 2006, 5:56am:Indeed, matching rules are not only targeting Vt mismatches. In differential pairs, which you would usually bias in moderate or weak inversion, mobility mismatch is the dominant contribution to the overall device mismatch.