Thermal Noise in ADCs

[niloufar-navidi]

Hi everyone,

When we want to calculate ADCs sampling capacitor size we choose C such that thermal noise level is less (or equal) than quantization noise

KT/C<Δ²/12

Which leads to

C>12KT×2^2b/V_FS²

My question is that if "C" here is the total capacitance of the capacitor array or unit cell capacitor? and why?

For example if in the case of an eight bit ADC we find C>20fF from C>12KT×2^2b/V_FS²
should we divide 20fF by 2⁸ to find the minimum unit cell size or C is the minimum unit cell size itself?

 

[crutschow]

C is the minimum unit cell size itself?

I believe that is the case.
20fF is a very small value of capacitance.