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.