2.2.1 Touch Capacitance Model

Unlike for self-capacitance measurements, there is no simple way to approximate the expected touch capacitance for a given mutual sensor layout. The parallel plate approximation is not applicable as the ‘plates’ in this case are segments of the X and Y electrodes, which are much smaller than the touch cover. The user’s touch contact is dominated by edge and point fields between the electrode pair and the fingertip.

Figure 2-3. Electric Fields in Mutual Capacitance Sensors

When designing mutual capacitance sensors, the node layout may be optimized to suit application requirements such as:

  • Maximum sensitivity
  • Best noise tolerance
  • Best water rejection
  • Minimum default sensor capacitance (some acquisition technologies have a limit on the sensor capacitance)
  • Minimum power consumption
  • Minimum touch latency

All applications will require a trade-off between these properties, as achieving one will mean compromising others.

As an example, the strongest sensitivity is achieved using high interleaving of electrodes. However, achieving minimum sensor capacitance requires a larger spacing between X and Y.

Excess sensor capacitance increases acquisition time and power consumption.

Increasing X-Y separation reduces default X-Y capacitance, but it also reduces the lengths of parallel segments between the electrodes.

Figure 2-4. 0.5 mm vs. 1 mm XY Spacing

When a user touches the sensor with larger spacing, a smaller total length of parallel segments is covered by the touch. This translates to a reduced X-Y field interaction and hence a proportional reduction in sensor sensitivity.

Figure 2-5. Touch Contact 12 mm Key