User guide

10 Document No. 001-64846 Rev. *A Getting Started with CapSense®
Figure 2-4. CapSense System Equivalent Model
With a finger on the sensor surface, C
X
equals the sum of C
P
and C
F


= 

+ 

.
2.3 CapSense Sensing Technology
Equation 2
2.3.1 Sensing Methods
There are a number of capacitive sensing methods currently in use across the electronics industry. Some of the
major sensing methods include:
 Charge Transfer: Incremental packets of charge are transferred between the sensor capacitor and a reference
capacitor until a reference voltage is reached. The number of charge transfers is used to track changes in sensor
capacitance.
 Relaxation Oscillator: A sensor capacitor is used to directly set its frequency. An internal oscillator tracks the
changes in frequency set by the sensor capacitor to detect the capacitance change caused by a finger.
 TX-RX: A source waveform is driven on the TX end of a mutual capacitance system and senses the response on
the RX end. The received signal reflects changes in sensor capacitance.
 ADC: A current source generates a linear voltage ramp on a capacitor. This voltage is input to an analog
comparator circuit. The comparator’s output is monitored and a counter increments whenever it transitions from
high to low.
Cypress CapSense devices measure sensor capacitance using either CapSense with Sigma Delta modulator (CSD),
or CapSense Successive Approximation (CSA). Both methods are variants of the ADC method. The Electromagnetic
Compatible variation of CSA (CSA_EMC) provides far superior noise immunity compared to the other sensing
methods.
2.3.2 Capacitance Conversion
The CapSense algorithm converts the sensor capacitance into a digital count, called raw count. The raw count is
interpreted as either a TOUCH or NO TOUCH state for the sensor. The numerical value of the raw count is the digital
representation of the sensor capacitance, and increases as the capacitance increases. Sensitivity is a measure of
how much the output will change for a given change on the input. The sensitivity of the CapSense sensor has units of
counts-per-pF.