A Low-Noise Switched-Capacitor Front End for Capacitive Sensor

This paper presents the analysis and design of an integrated low-noise switched-capacitor front-end circuit for capacitive sensors. The interface has been implemented with a relaxation oscillator. To prevent overload of the input amplifier for large input signals, a negative-feedback circuit controls the charge-transfer speed. It is shown; that this negative feedback can also be used to optimize the noise performance of the interface. The circuit has been designed and implemented in 0.7 mum standard CMOS technology. The effects of any additive and multiplicative interface errors have been reduced by applying three-signal auto-calibration. Experimental results show that application of negative feedback yields a resolution of more than 16 bits for measurement time of about 100 ms, which is at least 1 bit better than that of an earlier designed interface.