Effect of Cmod value

Question: What is the effect of changing the value of capacitor(Cmod) in Capsense Sigma Delta (CSD)?



The effect of Cmod is explained in the following two cases:

  1.   If Cmod value is low, then high frequency noise will find a low impedance path to the ground via Cmod and will get coupled to the system very easily.
  2.   If the value is too high, the voltage swing about Vref will be lesser as the capacitor will discharge lesser within the given discharge time. If this happens, the signal might get lost in the comparator noise itself as the signal strength (in terms of amplitude) is low.

Due to these reasons, an optimum value of Cmod is required. We have observed during characterization that the system performs quite well with a 2.2nF capacitor.