Capacitive Sensing and Terminal CPU Solutions in Mobile Phones (1) 2

The basic form of a capacitive sensor is a pair of adjacent electrode plates. There is inherent capacitance between these adjacent plates, which is proportional to the thickness of the plates and inversely proportional to the distance between the plates. In an ideal situation, this is the only capacitance that the sensor can measure. When a conductor (such as a finger) is placed near two plates, it adds a parallel capacitance to the inherent capacitance. At this point, the measurable capacitance value is the sum of the intrinsic capacitance value and the coupling capacitance value from the finger to the sensor. When a finger is placed on top of a capacitive sensor, the capacitance will increase. After removing the finger, the capacitance will decrease. Intelligent measurement of changes in capacitance values can determine the presence of fingers. diode

Figure 1: Typical relaxation oscillator topology.

Capacitive sensors are composed of two adjacent copper pads or copper wires with a certain distance on a regular electronic circuit printed circuit board (PCB). This is a basic capacitive digital switch. Similarly, the linear slider is composed of an array of capacitive sensors on the PCB. Typically, one copper electrode plate of the sensor is grounded, leaving only one active electrode plate. In fact, edge capacitance increases the inherent capacitance of the sensor, typically ranging from 10pF to 30pF. A typical finger coupling capacitance value is 1pF to 2pF. The challenge of implementing a capacitive sensor is to design a circuit that can measure a capacitance change of 3 (caused by fingers).

Relaxation oscillator is a simple and effective capacitance measurement circuit. A typical relaxation oscillator circuit topology (Figure 1) consists of four components: synchronous comparator, current source, discharge switch, and capacitive sensor.diode