Datasheet

30 Document No. 001-66271 Rev. *B CY8C21x34/B CapSense
®
Design Guide
4.1.1.2 CapSense Noise
CapSense noise is the peak-to-peak variation in sensor response when a finger is not present, as shown in
Figure 4-2. In this example, the output waveform without a finger is bounded by a minimum of 5912 counts and a
maximum of 5938 counts. Because the noise is the difference between the minimum and the maximum values of this
waveform, Noise = 5938 – 5912 = 26 counts.
Figure 4-2. Example of CapSense Noise
4.1.1.3 CapSense SNR
CapSense SNR is the simple ratio of signal and noise. Continuing with the example, if the signal is 135 counts and
noise is 26 counts, and then SNR is 135:26, which reduces to an SNR of 5.2:1. The minimum recommended SNR for
CapSense is 5:1, which means the signal is five times larger than the noise. Filters are commonly implemented in
firmware to reduce noise. See Software Filtering for more information.
4.1.2 Charge/Discharge Rate
To achieve maximum sensitivity in the tuning process, the sensor capacitor must be charged and discharged fully
during each cycle. The charge/discharge path switches between two states at a rate set by the Clock parameter in
the User Module.
The charge/discharge path includes series resistance that slows down the transfer of charge. The rate of change for
this charge transfer is characterized by an RC time constant involving the sensor capacitor and series resistance, as
shown in Figure 4-3.