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 where CF is finger's capacitance and CP is the sum of the distributed capacitance on the sensor. This is how sensor can calculate if the finger is on the sensor as seen in figure 1

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Figure 1

Capacitance sensor activation

There are several integrated capacitance-to-digital converters (CDCs) that can interface with external capacitance sensors. They can implement functions such as capacitive buttons, scroll bars or wheels. Two of these are the AD7142 and the AD7143.

When the sensor is not active the AD714x sensors measures the capacitance value as ambient value as seen from figure 2. As previously explained in Position sensor page, the capacitive increases when the finger is touching screen or a button and it decrease when finger is moving away of the screen or button.

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Figure 2

Figure 2 shows a situation, where the ambient capacitance value does not change. The ambient capacitance changes constantly and is unpredictably due to changes in humidity and temperature. If the ambient capacitance value changes sufficiently, it can affect the sensor activation.

Sources

1. http://www.analog.com/library/analogdialogue/archives/40-10/cap_sensors.html
2. http://www.capacitive-sensing.com/capacitive-sensor-theory.html

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