Datasheet

CY8C20xx6A/H CapSense Design Guide Doc. No. 001-65973 Rev. *A 10
2.2.2 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 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 one external component, C
MOD
Figure 2-4. CSD Block Diagram
. This is an Integration capacitor that holds the charge
transferred through the equivalent resistor.
Figure 2-5. CSD Equivalent Circuit


=



×

Equation 3
Where:
f
SW
C
= switching frequency
P
For an in depth discussion of Cypress’s CSD sensing method refer to
= sensor capacitance
PSoC® CY8C20x66, CY8C20x66A,
CY8C20x46/96, CY8C20x46A/96A, CY8C20x36, CY8C20x36A Technical Reference Manual (TRM).