Cmod is an external modulator capacitor. It is mandatory for operation of the CSD sensing method and required only if CSD sensing is used.
If Cmod is too large, especially when larger than 10 nF, it can cause excessive noise in cases where sensor parasitic capacitance is small. In these cases, the voltage swing on Cmod is too small and approaches the comparator input ambiguity. When this happens, raw count noise increases dramatically, and the worst case is the comparator not able to turn on.
On the other hand, for smaller Cmod values, especially when smaller than 1 nF, the modulator induces non-linearity. On raw counts, the non-linearity reflects as flat spots. For more information about flat spots, see the “Flat Spots” section in AN85951.
The recommended value is 2.2 nF, 5 V, X7R or an NP0 capacitor. 2.2 nF is a good trade-off between comparator input noise (limiting when Cmod is too large) and modulator-induced non-linearity (limiting when Cmod is too small).