common centriod & interdigitated pattern

[HSD_analog]

When do we go for common centroid layouts and when do we go for interdigitated patterns in analog layouts?
I ve read somewhere that Diff amps usually go for common centroid and current mirroring , interdigitated is sufficient ..
Y is this so?

Thanks
HSD

 

[ricklin]

Well, some senior told me that in deep sub-micron, common centroid even interdigitated may be just a superstition. I think that more sensitive is power supply floor plan and dummy device surrounding the diff pair or current mirror.

 

[yxo]

Common centroid is used to decrease thermal, stress and similar gradients. Submicron process doesn't canceled them. You should do it!

 

[elbadry]

Classically, common centroid was used to improve matching. This had a good effect in old processes with large dimensions and poor process control.

In more modern processes, common-centroid is still useful. However, the routing overhead of using such a structure doesn't justify its use ( given the smaller device dimensions and better process control ). It can be used in the most demanding applications. No need to use it in the less-demanding ones ( e.g. non-critical LDOs, bandgaps, opamps )

Stress effects are becoming more pronounced. These makes the use of dummy devices an absolute necessity, even in the less-demanding designs. Besides, you'd better use multipliers rather than fingers for accurate current-mirroring ( Otherwise the different stresses for different no. of finger will cause serious systematic mismatches )

I just want to add that the above is not based on solid science but more of a speculation