An Integrated Interface for Leaky Capacitive Sensor with Emphasize on Humidity Sensor

This paper presents the analysis and design of an integrated interface for leaky capacitive sensors with emphasize on humidity sensor. For such an application, we developed a novel type of integrated interface circuit. With this circuit it is possible to measure the capacitance of a humidity sensor, at the presence of shunting resistors, with sufficient accuracy. For the signal processing, a relaxation oscillator is used with a modified front-end which is a novel capacitance to voltage converter, to optimize the interface circuit for immunity to leakage. The circuit has been designed and implemented in 0.7 mum standard CMOS technology. Experimental results show that a 220 kOmega shunt resistance (4.5 muS) causes an error that corresponds to about 1.25% in relative humidity which is also agree with our simulation result. The measured resolution is 14 bit for measurement time of 30 ms and the measured non-linearity is about 2times10^-4 . The interface chip consumes about 1 mA and takes a chip area of 2.8 mm².