Datasheet

CY8C21x34/B CapSense
®
Design Guide Document No. 001-66271 Rev. *B 29
4. CapSense Performance Tuning with
User Modules
Optimal user module parameter settings depend on board layout, button dimensions, overlay material, and
application requirements. These factors are discussed in Design Considerations. Tuning is the process of identifying
the optimal parameter settings for robust and reliable sensor operation.
4.1 General Considerations
4.1.1 Signal, Noise, and SNR
A well-tuned CapSense system reliably discriminates between ON and OFF sensor states. To achieve this level of
performance, the CapSense signal must be significantly larger than the CapSense noise. The measure that
compares signal to noise is the signal-to-noise ratio (SNR). Before discussing the meaning of SNR for CapSense, it is
first necessary to define signal and noise in the context of touch sensing.
4.1.1.1 CapSense Signal
The CapSense signal is the change in the sensor response when a finger is placed on the sensor, as demonstrated
in Figure 4-1. The output of the sensor is a digital counter with a value that tracks the sensor capacitance. In this
example, the average level without a finger on the sensor is 5925 counts. When a finger is placed on the sensor, the
average output increases to 6060 counts. Because the CapSense signal tracks the change in counts due to the
finger, Signal = 6060 – 5925 = 135 counts.
Figure 4-1. Example of CapSense Signal