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12 Document No. 001-64846 Rev. *I Getting Started with CapSense

Figure 2-4. CapSense System Equivalent Model

With a finger on the sensor surface, C

equals the sum of C

and C













Equation 2

2.3 CapSense Sensing Technology

2.3.1 Sensing Methods

There are a number of capacitive sensing methods currently in use across the electronics industry. Some of them

include:

 Charge Transfer: The change in sensor capacitance introduced by a finger touch results in the change in charge

transfer between the sensor capacitor and the reference capacitor. These incremental charge packets are

transferred until a reference voltage is achieved on the reference capacitor, which indicates the presence of a

touch.

 Relaxation Oscillator: A sensor capacitor is used to set its frequency directly. The capacitance introduced by the

finger touch is detected on the sensor by tracking the change in the frequency of the oscillator. When there is a

finger touch, the sensor capacitor increases and frequency of the oscillator decreases.

 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‘s CapSense devices measure sensor capacitance using either CapSense with Sigma Delta modulator

(CSD) or CapSense Successive Approximation (CSA_EMC). Both methods are variants of the ADC method.

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.

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