Sensors and Signal Conditioning, 2nd Edition

5.4   SPECIFIC SIGNAL CONDITIONERS FOR CAPACITIVE SENSORS

Capacitive sensors suit monolithic integration, but bridge circuits with resistors and coils are difficult to integrate. This has led to the development of specific signal conditioners for capacitive sensors suitable for monolithic integration, also amenable to implementation by discrete components. Some conditioners include the capacitive sensor in a variable oscillator (Section 8.3), and others are integrators built from switched capacitors that obtain an output voltage from a difference in electric charge.

The circuit in Figure 5.28 applies the charge redistribution method. There is an autozero phase and a measurement phase. In the autozero phase (Figure 5.28a), a reference dc voltage source charges the sensor C_x at V_r, and the reference capacitor C_r and the integrating capacitor C_i discharge to ground. The op amp output is zero. In the measurement phase, C_x is grounded, C_r is connected to V_r, and C_i closes the op amp feedback loop, hence working as integrator. If C_x = C_r, the charge stored in C_x redistributes between them and the op amp output remains at 0 V. But if C_x ≠ C_r, there is a net charge flow through C_i and the op amp output voltage is proportional to C_x – C_r. The output stage is a sample-and-hold amplifier that keeps the last voltage output during the next autozero phase. Reference 11 describes a method to reduce charge injection errors in this circuit.

Figure 5.28    The charge redistribution method measures ...