Self-capacitance refers to a capacitive measurement using a single sensor
electrode to measure the apparent capacitance between the electrode and the DC ground of
the touch sensor MCU circuit.
At power-on or Reset, a baseline measurement of the capacitance is recorded
and assumed to be the ‘Out Of Touch’ capacitance. Reference capacitance is the
combination of Cp in parallel to the series pair Cg and Cx.
When a touch contact is applied, the capacitance is increased by the
introduction of a parallel path to earth, via the series combination of Ct and Ch. The
increase is compared to the touch threshold, and if exceeded, the sensor is indicated to
be ‘In Touch’.
Note: Cx, the human
body capacitance, varies by person and surroundings and is typically in the order of
100 pF to 200 pF. The touch contact Ct, however, is more consistent and much smaller
at typically 1 pF to 5 pF, depending primarily on the design and construction of the
touch sensor and secondly on the size of the finger used to activate the
sensor.
As the dominant component in a pair of series capacitors is the smaller one,
in this case, Ct, a well-designed and tuned sensor shows very consistent sensitivity to
touch contact with little dependence on the user.
The online versions of the documents are provided as a courtesy. Verify all content and data in the device’s PDF documentation found on the device product page.
