An integrated switched-capacitor front-end for capacitive sensors with a wide dynamic range

This paper presents the analysis and design of an integrated switched-capacitor front-end circuit for capacitive sensor with a wide dynamic range. The interface has been implemented with a relaxation oscillator. A negative-feedback circuit controls the charge-transfer speed, to prevent the overload of the input amplifier for large input signals. It has been shown, that proper functioning of this circuit is limited to a certain range of one of the parasitic input capacitances. Solutions to extend this range have been proposed. 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 linearity of about 50times10~6 (14 bits) for the capacitor range of 1 pF to 300 pF, with a 16 bits resolution for a measurement time of 100 ms.