Non-inverting CCII-based astable multivibrator and its application as uncalibrated wide-range capacitive sensor interface

Abstract

A novel wide-range capacitive sensor interface employing a non-inverting Second Generation Current Conveyor (CCII)-based astable multivibrator is here presented. With respect to typical Capacitance-to-time (C-T) conversion techniques, the circuit has been designed employing a reduced number of both active (only one CCII) and passive (three resistances and the capacitive sensor) devices, in order to keep power consumption reduced and allowing the development of a long-life portable tool. Test results, conducted on PCB, through the commercial AD844, have shown that the circuit is able to estimate large capacitance variations within [100 pF–10 μF] (about 5 decades), maintaining a reduced relative error. This makes the interface uncalibrated since a number of capacitive sensors can be employed maintaining the same oscillator features and making it suitable in those applications where the sensor baseline cannot be accurately estimated or is not well known a priori.