Linear relationship between difference counts and Sensor Capacitance

Question: What is the relation between difference counts and added finger capacitance in case of CSD of 21x34 devices?

 

Answer:

The relation between CSD raw counts and sensor capacitance is as given below:

Raw_Count  = (Vdd/Vref - 1).Rb.fs.(2n - 1).C(Here,Vref is the comparator reference as selected in the user module configuration window in PSoC Designer, Rb is the bleed resistance used on the board, fs is the average switching frequency of sensors and depends on the CSD configuration used like PRS, prescalar etc. and "n" is the resolution as set in the user module configuration window).

If Cp is the sensor capacitance and Cf is the finger capacitance, the difference counts would be:

Diff_count = Raw_CountCp+Cf - Raw_CountCp

                   = ( Vdd/Vref - 1).Rb.fs.(2n - 1).Cf     

Note that the above relation holds true as long as the capacitance Cp+Cf does not lead to raw_counts being saturated and also assumes that the sensors are fully charged and discharged within the time 1/fs max. Thus, the linear relationship shown above does not hold true if Cp+Cf or Cpis so high that the sensors do not charge and discharge to Vdd and Vref respectively within the time 1/fs max. (If PRS is used as clock source, average switching frequency is fIMO/4 but maximum switching frequency is fIMO/2.).