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I'd think the additional parasitic cap due to the grounded PMOS is small/negligible compared to the n-well-substrate parasitic cap. So it shouldn't change a lot.... in reality you are creating extra parasitic RC in series. Your N-well is acts as high resistance in series with active capacitor.
I am agree, but still such dummys are avoided with critical low noise signal path. What may be the reason?..... So it shouldn't change a lot.
Where did you come to know that? No reason given? I can't imagine any. I'd think more capacitance between VDD & GND should help decoupling noise?... such dummys are avoided with critical low noise signal path. What may be the reason?
sorry,I may not give reference for this...but, still, i'll tell, PMOS dummy's never used in the lower rail. ( here S,D are connected to GND)( capacitive benifit mentioned may not available here).But it will be vulnerable to breakdown and leakage current may increase. You may find this in any of layout designed in <180nm.Where did you come to know that? No reason given? I can't imagine any. I'd think more capacitance between VDD & GND should help decoupling noise?
thanks to all of you!
As S,D and G are all connected to gnd, the dummy PMOS is worked as a decouple capacitor and will not affect the signal path, and the only disadvantage is the leakage current, am I right?
This dummy PMOS isn't in an active state: Vgs=0 !... dummys should not be used in active state
... and what about source followers?(dummy PMOS gate should not be at lower potential and Dummy NMOS gate in higher potential.)
Right; but Chao's application is quite ok -- apart from a possible higher leakage current -- as stated by Chao above.PMOS dummys are used in the upper rail voltage ( D, S are shorted to VDD) while NMOS dummys are used in the lower rail. This is the best way to use dummys.
... and what about source followers?
Yes, you're right, those aren't dummies, sorry!Source followers are different case
E.g. a PMOS cap in inversion state -- a very effective (high cap/area) capacitor for rail decoupling & LF filter applications, s. below. Shouldn't be labeled as dummy, either.I don't see much application where you need to connect PMOS directly to ground.