Relationship between Difference Counts and Sensor Capacitance - KBA82706

Version: *A

 

Question:

What is the relationship between difference counts and finger capacitance in CY8C21x34/B family of CapSense Plus Controllers?

 

Answer:

The relationship between CSD raw counts (raw counts) and sensor capacitance (CS) in CY8C21x34/B is:

1.png

 

Where:
Vref = The comparator reference (selected in the user module configuration window in PSoC Designer)
Rb  = The external bleed resistor
fs = The average switching frequency of the sensors (depends on the CSD configuration settings such as PRS and prescalar)
n = The resolution (set in the user module configuration window)

 

The difference count is calculated by subtracting the raw count without a finger on the sensor (CS = Cp) from the raw count with a finger present on the sensor (CS = CP + CF):

 

2.png

 

By substituting the equation for raw counts you can see the linear relationship between difference counts and finger capacitance:

 

3.png

 

Note: This linear relationship holds true as long as raw counts do not saturate and assumes that the sensors are fully charged and discharged to VDD and Vref respectively within 1/fs max. If PRS is selected as the clock source, the average switching frequency is fIMO/4 but the maximum switching frequency is fIMO/2.