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Interdigitation question

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AMSA84

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Hi guys,

When doing interdigitation, the first diffusion is better to be the source or drain? Because of the capacitances.

I am doing a layout a differential pair. I can split him in 4 or 5. If I split in 4, each finger has 5um and I can do a interdigitation pattern as: AABBAABB or ABBAABBAAB. The first pattern starts with SOURCE, the second with DRAIN. The axis of symmetry for the first pattern is different for the transistor A and B and the axis of symmetry for the second one is the same.

Kind regarsd.
 

It depends if you need better common mode rejection or better bandwidth. Reducing capacitance at the sources extends the bandwidth of the common mode rejection, reducing it at the drains it helps to push parasitic poles (or dominant poles if that is the case) to higher frequencies.
 

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