In case of double guard ring for PMOS which should come first whether N+ first or P+

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Akshaya katkar

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In case of double guard ring for PMOS which should come first whether N+ first and then P+ or reverse...... please explain
 

The N+ first, it sits in the same well and doubles as bulk contact. You always need the bulk, so why not make a ring of it. Then the P+ to pick up substrate noise. You can then add another N+/NW ring around that. This will force the passing noise deeper into the substrate (more resistive), before it gets up again, and is picked up by the P+ ring.
 
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