MIMcap substrate shielding

[sharkies]

Hi,

I'm trying to shield the mimcap from the substrate. This is for my ADC design.
I have two-options. first is to put NWell beneath the cap. the other one is to put deep nwell beneath it.
Obviously, deep-nwell is better. But is it really necessary to shield it with a deep-nwell?
What is the common practice for mimcaps?

 

[shar2013]

What you refer to as shielding , is basically lowering the bottom plate (of MIM) parasitic capacitance. nwell adds a series capacitance, and a deep n-well adds two more.
In practice if you have the flexibility try to arrange you ADC design such that MIM is a shunt device which means the bottom plate is grounded, thus avoiding this issue.
Deep n-well is an expensive process so it is best avoided.
However if the die has already a deep nwell then use it , by all means.

Good Luck