Mutual capacitance refers to a capacitive measurement using a pair of sensor
electrodes to measure the apparent capacitance between them. Typically, one electrode
acts as the Driver (X), while the other is the receiver (Y). Each physical location
where an X electrode transfers charge to a Y electrode is a sensor node, and this is the
location of touch sensitivity.
As with self-capacitance, a baseline measurement of the capacitance is recorded and
assumed to be the ‘Out Of Touch’ capacitance. Reference capacitance is the
apparent capacitance between the X electrode and the Y electrode. Unlike
self-capacitance, the reference capacitance does not depend on an earth return.
Interaction between a mutual capacitance sensor and the human body is more complex. It
may be modeled by considering two separate touch contacts to the X and Y electrodes,
where each is capacitively coupled to the body, resistively connected to each other
inside the body and capacitively coupled to earth via the human body capacitance.
A touch contact has two competing effects:
The introduction of a conductive
plate (finger) to both X and Y electrodes increases the capacitance between X and Y.
This occurs if any conductive part is placed over the sensor.
The addition of another capacitance
(Ch + Cg) at the XY node provides an alternative path for the energy emitted by the
X electrode, reducing the amount of charge accumulated on the sensor. This effect is
manifested as an apparent reduction in the XY capacitance and occurs only if the
body of material connected to the conductive part has a significant
self-capacitance.
When a real touch contact is placed, the second (reducing) effect is much greater than
the first (increasing) effect, and so a touch contact on a mutual capacitance sensor is
indicated by an apparent reduction in sensor capacitance.
This apparent change in capacitance (delta) is compared to the configured
touch threshold, and if it exceeds the threshold, then the sensor is deemed to be in
detect.
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