the purpose of the nwell under a finger capacitor

Hello,

I am designing a PLL, and the capacitor of the loop filter is using a finger capacitor(~200pf). But in the PDK, the finger capacitor is built on top of a nwell.

I think the nwell is used to isolate substrate noise, right?

But what voltage should I apply to this nwell, leave it floating or connect to VDD?

If connect it to VDD, then I need a quiet VDD which I don't have.

Somebody can help?

Thanks

The nwell makes the parasitic bottom plate yours to control
(unlike psub, which is a global net of indeterminate impedance
and noise-content, given or received). You could return the
NWell current to wherever makes the most sense, or even
turn the parasitic into a bonus density-wise if you're lucky.

If the cap is DC-blocking a signal, then the nwell wants to
be returned to the receiver rail of greatest importance, in
the sense of supply noise etc. For example, if you are DC
blocking a low-side-sourced, psub-referred signal that goes
to a high side, common source amplifier, then tie the nwell
to that high side rail so that (vdd-vss) noise transients are
largely bucked ohmically and not directly amplified as if they
were valid signal.

I suggest to play with this as any other design degree of
freedom with an eye to common-mode and supply noise
(inject w/ current pulse, and inspect) and pick the best
outcome of the options you can think to try.