1.1 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 1-1. 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 1-2. 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’.

: The HBM resistance Rh does not affect the touch sensitivity.
Ct
  • May be approximated as a parallel plate capacitor comprising the touch sensor electrode and the user’s fingertip separated by a dielectric in the form of an overlay material
  • A user’s fingertip placed onto a solid surface may be approximated as a disc with the diameter between 5-10 mm. 8 mm is estimated as a typical user’s fingertip diameter and is used in the examples from this document.
  • A smaller sensor or thicker cover will reduce the touch capacitance value
Ch
  • Human body model capacitance
  • Self-capacitance of the human body with respect to earth
  • 100 pF to 200 pF for an adult depending on physique
Cg
  • Capacitance of the coupling between the application DC ground and earth
  • Depends on application type and power system
  • Ranging from ~1 pF in a small battery-powered device to infinite capacitance (short circuit) where the DC ground is connected directly to earth

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

Equation 1-1. Series Capacitor Combination
C t = C 1 C 2 C 1 + C 2

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%