pipeline ADC- parasitic input capacitance HELP!

[ljy4468]

hi all
I've a question about amp parasitic input capacitance.
In pipelined ADC 1.5bit stage, in the MDAC amplification mode,
feedback factor is 0.5.(neglect input parasitic capacitance).

but without neglecting opamp's parasitic capacitance, feedback factor is
much smaller.

Now, I've a question about that.
For calculate input parasitic capacitance,
for NMOS input driver, cap is about L*W*(mos capacitance per um2)
that's ok
but PMOS input driver, Same calculation?????
Pmos's body is vdd.So very confused.
Somebody helps me!!!!

 

[gigle]

The same....

But for both nmos and pmos device the Cgs in saturation region is (2/3)WLCox, not just WLCox.

And you have to take (1+beta)Cgd into account. Usually you can not ignore this component. beta is the gain from gate to drain of input device.