Capacitive Sensing and Terminal CPU Solutions in Mobile Phones 2

Typical relaxation oscillator topology structure. 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. At the beginning, the discharge switch is in an open circuit state, and the current source linearly charges the capacitive sensor. The voltage on the capacitive sensor rises diagonally until it exceeds the threshold of the comparator. The output of the comparator changes from low level to high level, causing the discharge switch to close. The capacitive sensor rapidly discharges to ground through this low impedance channel. This process will cause the output of the comparator to go from high to low, and then repeat the cycle. The output frequency (fout) depends on the charging current and the value of the capacitive sensor, and the relationship is as follows: a typical design sets the oscillator frequency in the range of 20kHz to 200kHz, and then feeds this frequency into an intelligent measurement circuit. By measuring the change in frequency, the presence of fingers can be determined. There are two widely used methods for measuring the frequency of a relaxation oscillator: one is to measure the frequency, and the other is to measure the period.  diode