Calculating the Resolution for CY8C21x34 CapSense CSD - KBA82521

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    Question: How do you calculate the resolution of a capacitive touch sensing application designed using the CY8C21x34 CapSense® CSD?

     

    Answer:

    Resolution is equal to the inverse of sensitivity. Sensitivity is calculated using the following formula:

    Where:

                     
            
         Counts = Digital capacitance measurement result returned by the User Module
       
         N = Resolution of the User Module
       
         RB = Bleed resistor value
       
         FSW-AVG = Average switching frequency of the sensor
       
         RefVal = Reference value of the User Module
      

    For example, given the following User Module settings:

                     
            
         N = 14
       
         RB = 10 kΩ
       
         FSW-AVG = 375 kHz (Prescaler = 15)
       
         RefVal = 0
      

    Resolution = 0.00543 pF

    Note: Although the calculation indicates that a change as small as 0.00543 pF can be detected, raw-count noise limits the use of such high resolution. CapSense is not recommended for measuring absolute capacitances.

    Note: The “sensitivity” calculated above is the capacitive measurement module sensitivity, not the system sensitivity to button/sensor activation.