Mutual capacitance touch sensors use a pair of electrodes for each sensor node and measure the capacitance between them. The sensor is formed where the electrodes are placed close together, usually with interleaved segments to optimize the length of the parallel conductors forming the base capacitance of the sensor node.
Figure 1. Mutual Capacitance Sensor
When a touch contact is placed over the sensor, the user’s fingertip
interacts with the electric field between the X (transmit) and Y (receive) electrodes.
To model touch effects in the circuit, the sensor capacitance Cxy is replaced
with an equivalent overall capacitance formed by two capacitors in series each of value
2Cxy.
Figure 2. Mutual Capacitance Sensor with
Touch Contact
The touch contact is a complex interaction of two competing effects:
- 1.The finger forms a third electrode in the X-Y capacitor and increases the coupling between X and Y. This is modeled by the capacitor labeled Cxyt.
- 2.The touch capacitance Ct forms a ground return path via Ch - human body model (HBM) capacitance - and Cg (ground-to-earth capacitance), which reduces the amount of charge transferred from X to Y, causing an apparent decrease in the X–Y capacitance.
Note: The HBM resistance
Rh does not affect touch sensitivity because each capacitance must be
fully charged or discharged during the measurement.
Ct
- The series capacitance between the sensor and fingertip
Cxyt
- Parallel capacitance between X and Y due to the fingertip
Ch
- Human body model
- 100 pF to 200 pF
Cg
- Coupling between the application DC ground and earth
- Depends on application type and power system
- As little as ~1 pF in a small battery-powered device and infinite capacitance/short circuit where the DC ground is connected directly to earth
As in self-capacitance sensors, Ct is much smaller than Ch or Cg for most applications; the measured touch delta is dominated by Ct, which is controlled by the sensor design.
The equivalent XY capacitance is:
Figure 3. Equivalent XY capacitance
C e q = 4 C x y 2 4 C x y + C f + C x y t
where Cf is the series combination of Ct, Ch and Cg.