Relationship between Difference Counts and Sensor Capacitance - KBA82706

[Anonymous]

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:

Where:
V_ref = The comparator reference (selected in the user module configuration window in PSoC Designer)
R_b  = The external bleed resistor
f_s = 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 (C_S = C_p) from the raw count with a finger present on the sensor (C_S = C_P + C_F😞

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

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 V_DD and V_ref respectively within 1/f_{s max}. If PRS is selected as the clock source, the average switching frequency is f_IMO/4 but the maximum switching frequency is f_IMO/2.