Cypress’ CapSense Successive Approximation Algorithm
By Robert Jania, Cypress Semiconductor Global Marketing Manager (RJO)
Successive Approximation algorithm (CSA) is Cypress’s new capacitive sensing algorithm for the CY8C20x34 PSoC® device
family. CSA enables the implementation of an array of capacitive sensors through switched capacitor circuitry, an analog
multiplexer and digital counting functions. The hardware configuration works in conjunction with high-level software routines
from the CSA User Module found in PSoC Designer™ to compensate for environmental and physical sensor variations.
What is Capacitive Sensing?
A capacitive sensor is a pair of adjacent electrodes (Figure 1). When a conductive object is placed in proximity of the two
electrodes, the capacitance is changed. The base capacitance is often referred to as the parasitic capacitance (CP). The
physical sensor itself is typically a copper trace constructed on a PCB. Sensors can be any conductive material. For example,
one could use indium tin oxide (ITO) and print the pattern on a transparent substrate.
Figure 1. Capacitive Sensor
What is Successive Approximation?
The successive approximation method, developed by Cypress, uses the PSoC’s switched capacitor circuitry to convert
capacitance into a voltage. The voltage is then measured using a counter and a single-slope ADC. The capacitance
measurement is achieved by converting the capacitance to its equivalent resistance, using the equivalent resistance to set and
maintain voltage on a capacitor, then measuring the sampled voltage using an adjustable current source.
In addition to the sensor capacitor, CSA uses an internal capacitor (CInternal) and an optional external modification capacitor
(CMod). The block diagram for CSA appears in Figure 2.
Figure 2. CSA Block Diagram
The circuitry brings the sensor