User guide

Getting Started with CapSense Document No. 001-64846 Rev. *A 11
Figure 2-5. Output of Sensing Algorithm
2.3.3 CapSense with Sigma Delta Modulator (CSD)
Cypress’s CSD method uses a switched capacitor circuit on the front end of the system to convert the sensor
capacitance to an equivalent resistor, as shown in Equation 3. A Sigma-Delta modulator converts the current
measured through the equivalent resistor into a digital count. When a finger is on the sensor, the capacitance
increases and the equivalent resistance decreases. This causes an increase in the current through the resistor,
resulting in an increase in the digital count.
The CSD method requires a single dedicated pin and a single external component, C
mod
, or two dedicated pins and
two external components, C
mod
and R
B
Figure 2-6, depending on what CapSense part family is selected. shows the
CSD configuration using two external components and two dedicated pins for the part family CY8C21x34
Figure 2-6. CSD Block Diagram