advantages and disadvantages of common centroid and interdigitated matching in layout

[rameshiloveu]

can anyone please explain

advantages and disadvantages of both common centroid and interdigitated matching in layout ?

 

[MSLayout]

Hi,

Advantages of inter-digitization gives each finger of the gate the same effects. Keep in mind that you must add spares onto the end so that the end finger "sees" the same as a finger in the middle of the row.
Keeping the inputs of a differential pair the same length in also expected.
The disadvantages of inter-digitization is that for high speed circuits it can add parasitics and could kill the circuit. In this case mirroring and matching is best.

Common centroid is very good in case of gradient shift on the chip. Gates A and B encounter the same field effects. One disadvantage of common centroid is that for the lower technologies it can be
harmful for the gates and is generally not used. Another disadvantage of common centroid on a large set of gates is the routing that is required to connect them. This could add capacitance to the signal.

 

[rameshiloveu]

Hi,

Thanks for the reply.
It helped a lot.
Two doubts are still there.
Could you please explain these two lines little elaborately
1. The disadvantages of inter-digitization is that for high speed circuits it can add parasitics and could kill the circuit.
2. One disadvantage of common centroid is that for the lower technologies it can be
harmful for the gates and is generally not used.

Regards,

 

[jirika]

... that for high speed circuits it can add parasitics ...

There can be slight chance to have higher parasitic cap due to interdigitating but it can be minimal with a little care.

...disadvantage of common centroid on a large set of gates is the routing that is required to connect them.

This is a general problem not a specific to common centroid placement.