A flexible low-power high-resolution integrated interface for capacitive sensors

This paper presents a low-power high-resolution CMOS interface for capacitive sensors. The circuit is based on the use of a switched-capacitor charge amplifier, which converts the input capacitance to a voltage, which modulates the period of a relaxation oscillator. Auto-calibration is used to eliminate the undesired effects of transfer-parameter drift. The interface is suited for capacitive sensors with capacitance values from 1 pF up to 220 pF. Moreover, the measurement time can be set from about 100 μs up to 50 ms. For the 10 pF range, parasitic capacitances up to 680 pF can be handled while the settling accuracy is more than 14 bits. For a measurement time of 1s, the resolution can be as high as 20 bits. In the 10 pF range, for parasitic capacitance up to 680 pF, the measured nonlinearity error is less than 5×10^-5. All these features have been achieved with a chip consuming only 3 mm² of silicon area and 5 mW of power.