Self-Capacitance Measurement

Self-capacitance touch sensors use a single sensor electrode to measure the apparent capacitance between the electrode and the ground of the touch sensor circuit.

Figure 1. Self-Capacitance Sensor Model

The base capacitance is formed by the combination of parasitic, sensor, and ground return capacitance. In combination, these form the ‘untouched’ or default capacitance that is measured during calibration and is used as a reference to detect a capacitance change indicating touch contact.

Figure 2. Self-Capacitance Model with Touch Contact

When a touch contact is applied, the apparent sensor capacitance is increased by the introduction of a parallel path to earth through the ‘Human Body Model’ (HBM). The touch capacitance Ct forms a series combination with the HBM capacitance Ch and ground to earth capacitance Cg. This increase is referred to as the touch ‘delta’.

Note: The HBM resistance Rh does not affect the touch sensitivity.
Ct
Ch
Cg

In series capacitors, the dominant effect is that of the smallest capacitor.

Figure 3. Series Capacitor Combination
Ct=C1C2C1+C2

Ct is much smaller than Ch, and in most applications, Ct is also much smaller than Cg, hence Ct determines the change in the measured capacitance.

Example:

Ct = 1 pF, Ch = 100 pF, Cg = 100 pF

CTotal = 0.98 pF

However, in an application where Cg is very low, e.g., 2 pF, the sensitivity will be significantly reduced.

Ct = 1 pF, Ch = 100 pF, Cg = 2 pF

CTotal = 0.662 pF

The measured touch delta is reduced by ~33%