# Sensitivity and Capacitance Range for the CY8C21x34 CSD - KBA82522

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Question: How do you calculate the sensitivity and capacitance range for a capacitive touch sensing application designed using the CY8C21x34 CapSense® CSD?

Sensitivity is calculated using the digital capacitive measurement result returned by the User Module, referred to as Counts. Capacitance measurement range is calculated using sensitivity.

Counts are calculated using Equation 1.

Where:

 N = Resolution of the User Module         RB = Bleed resistor value         CSENSOR = Capacitance of the sensor (Parasitic Capacitance, CP + Finger Capacitance, CF)         FSW-AVG = Average switching frequency of the sensor         RefVal = Reference value of the User Module

Sensitivity is calculated using Equation 2.

The upper limit of the capacitance measurement range is calculated using Equation 3.

For example, given the following User Module settings:

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

Sensitivity = 184.3 Counts/pF

Capacitance measurement range (upper limit) = 88.9 pF

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

Note: It is not possible to measure capacitance values all the way down to zero because there will always be some parasitic capacitance, CP, and pin capacitance measured by the sensor.