Common centroid and Interdigitated placement

Hi All!

Just would like to ask: Is the common centroid and interdigitated placement technique mandatory in every layout?

I mean, I understand that it's very practical to apply these techniques for good device matching, but should there be any instance that it's far better not to apply?

Thanks in advance!

An example: usually the transistors of a simple current mirror should be matched by one of these techniques, but if you need further slave mirror transistors far apart from the master mirror transistor, i think it's better to place this slave mirror transistor near the circuit which it has to supply, and just route the gate bias to it, instead of geometrically good matching, and routing the current.

For simple current mirrors, matching isn't particularly important, because their currents in most cases don't match very well anyway, because of their - in most cases - different Vds voltages. (If exact matching is needed in such a case, a control loop is necessary.)


And there will be devices that don't have to match to another device, but this is trivial, of course.

An amplifier or comparator that doesn't need the best
Vio achievable, might be made somewhat simpler and
more compact. But everybody says they want 0 mV
and it's hard to quantify before the fact, what the
unaddressed effects of simple layout w.r.t. process
and thermal gradients in the silicon might be.

Just like you don't -have- to cook pork before you
eat it, all the time. Only when there's tapeworm
eggs in it. But are you capable to tell, or do you
want to just be sure you did the best job practical?