Relationship between Difference Counts and Sensor Capacitance - KBA82706

Version 4

    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.