17.3.1.2 Mutual Capacitance CVD Sensor

The mutual drive CVDTXx signal is a square wave that is driven low during all of Sample A and high during all of Sample B (so, it is in-phase with the external sensor’s starting voltage for each sample). The ‘mutual capacitance’ is formed by the AC coupling between the sensor CVDANx and a low impedance CVDTXx signal, as shown in Figure 17-4. This TX signal can be common for all sensors because only one sensor at a time is sampled. CTX-AN is the amount of coupling between the TX trace and the sensor without any external influences.

When a user capacitively couples to the system, it pulls charge away from CTX-AN. CBODY-GND tends to be incredibly large with respect to the other magnitudes of capacitance in the circuit model. Thus, if the user and the board share or strongly couple their grounds, the pulled TX charge will dissipate through the human body model coupling path into the board’s ground. This path is shown with a dashed line in Figure 17-4. The effect of this dissipation will be to decrease the mutual capacitance between TX and the sensor.

Figure 17-4. Mutual Capacitance CVD Sensor

As shown on Figure 17-5, the Capacitive Voltage Divider (CVD) technique uses two opposite polarity measurements to achieve a differential, self-capacitive reading (Sample A – Sample B). When the self-capacitance of a sensor increases, the first measurement of the CVD (Sample A) will decrease, and the second measurement of the CVD (Sample B) will increase.

If a TX square wave is driven in-phase with the CVD waveform, a mutual capacitance component is introduced to the final signal level. When the TX signal is driven high, the AC coupling between TX and the sensor will cause the final settled voltage to increase; when the TX signal is driven low, the AC coupling will cause the final settled voltage to decrease. It ‘pulls’ the signal in the direction it’s headed. As the mutual capacitance between TX and the sensor gets stronger or weaker, the strength of the pulling on the signal likewise gets stronger and weaker.

Figure 17-5. CVD Pin Waveform with Mutual TX Drive

To reduce the base capacitance of a sensor and increase the sensitivity the neighbor, sensors in the 2D matrix can be used as active guards. This feature can be enabled by GRDBx[1:0] and GRDAx[1:0] bits in the records registers ITCRECx.