Hello all!

A general configuration of RF suppression circuitry on a differential input is as follows.


All the resources I have read till date warn of keeping CD>=10*CC in order to have a decent CMR. This is so because an Op-Amp itself amplifies the differential changes in the inputs, and suppresses common-mode changes in the inputs. What if the case is reversed, that is, CC>=CD, as when a X2Y capacitor is used, like in this configuration?


For an equivalent X2Y capacitor, CD is approximately half of CC.

Thank you in advance.

 

You could do as you suggest, but due to practical component tolerances you will probably very badly degrade performance. The normal recommendation for the ratio of CD to CC is in recognition of the fact that precise matching of the two RCC filters is critical to maintenance of common mode rejection. If the RCC filters are not precisely matched, the degradation can be partially restored by "short circuiting" the two amp inputs together at some frequency sufficiently lower than the corner frequency of the RCC filters. If high precision components were available, the ratio of CD to CC could be smaller.

Very few capacitors are available with a tolerance even as good as ±1%. For example, if you check DigiKey, only three values in film type are in stock and the highest capacitance is 22 nF. More values are available in C0G ceramic, generally the only ceramic type suitable. A 100 nF through-hole type will cost a few dollars each and is physically large.

The effects of imprecision in the components can be calculated or simulated. In simulation, all you need to simulate is the network of the two resistors and three capacitors - simply look at the voltage across the CD capacitor.

 

Thanks, ebp, that made it a lot clearer.