How to use mos capacitor in analog circuits

[s55]

I understand the MOS capacitor or the MOSCAP is used for some benefits, such as high capacitance density and more. Then, the capacitance of the MOSCAP changes with the gate voltage as explained in the C-V plot of the following link:

https://www.chegg.com/homework-help/questions-and-answers/figure-shows-capacitance-voltage-c-v-curve-measured-fabricating-mos-capacitor-using-p-si-s-q52025435

From the above curve, there is big difference of capacitance (e.g., ~80pF in different document) between the bottom of depletion (deep depletion) and inversion (or accumulation) region.

Here is my question. I'm trying to use the MOSCAP as the binary-weighted capacitor array (instead of using regular poly capacitors) in my charge-redistribution SAR ADC design. In my ADC, I think there shoud be serious problem that different ADC input voltages (Vin) applied across the MOSCAP array cause different capacitances of each MOSCAP as shown in the above link (e.g., 0 [V] of Vin corresponds to the depletion, while 1[V] of Vin maps to inversion region). This results in wrong binary search process. Am I correct?

If I'm correct, then how do we use the MOSCAP? Is there any specific situation only that we can use the MOSCAP, for example the MOSCAP between voltage supply and ground or something like this?

 

[AMS012]

You are right. Moscap can not be used at places where the linearity is important. Meaning, if the voltage across it is time varying, then the non linear cap takes non linear current resulting in harmonic voltage generation.

So, wherever the linearity is very important like in high resolution ADCs etc., moscaps cannot be used. If the linearity requirement is not stringent or not at all important like supply decap, moscaps can be used as they give high density. Of course, the value or density depends on the average voltage across it.

 

[s55]

Thanks a lot, AMS012

 

[timof]

Just to add a little bit to the explanation by AMS012:

MOSCAPs are often used in situations when a non-linearity of the capacitance does not matter much, such as in decoupling capacitors.