Figure 1: Typical capacitive sensor pad construction
When a finger comes in contact with the overlay covering a sensor pad, it forms a simple parallel plate capacitor called finger capacitance (CF
). Even without a finger touching the overlay, the capacitive sensing controller measures some parasitic capacitance (CP
is the sum of the distributed capacitance on the sensor. This includes the capacitance of the sensor pad due to its proximity to circuit ground, the trace connecting the capacitive sensing controller input pin and the sensor pad, vias, and the capacitive sensing controller input pin.
The capacitive sensing controller converts the capacitance measured at its input pin to digital counts using an A-to-D converter. The controller uses a DSP algorithm to continuously monitor the digital counts and identify increases in sensor capacitance due to a finger touch.
To accurately detect a finger touch, the A-to-D converter’s resolution has to be tuned to maintain a specific sensitivity level. If the CP is high, the resolution of A-to-D converter should be increased. Higher resolution results in a longer conversion time, which increases the average power consumption of a capacitive sensing application. To reduce power consumption, reduce the sensor(s) CP so that you can use a lower resolution A-to-D converter along with sleep mode.
The main components of CP are trace capacitance and sensor capacitance. CP is a nonlinear function of the annular gap between the sensor pad and ground, the distance between the trace and ground, trace length and width, and the sensor pad diameter. There is no simple relationship between CP and PCB layout features but, in general, increasing the annular gap and decreasing the trace length and width will reduce CP. Unfortunately, widening the gap between the sensor pad and ground will decrease noise immunity. To achieve optimal CP and noise immunity, follow the capacitive sensing controller manufacturer’s PCB layout best practices.