Capacitive Sensors and their Interface Circuits

Abstract

An innovative technique has been developed to interface to capacitive sensors. This is a new technique where the unknown capacitance is connected with a resistance to generate a relative change of frequency in a CMOS inverter circuit. This change is proportional to the unknown capacitance. This frequency is then measured using a logical Frequency Locked Loop circuit which is programmed onto a general purpose FPGA board. This technique produces a nearly linear response, is tunable over dierent capacitance ranges and possesses a fast response time.