Interfacing Differential Capacitive Sensors to Microcontrollers: A Direct Approach

Abstract

This paper is a continuation of a previous work with regard to the direct connection of differential sensors to microcontrollers without using intermediate electronics between them. This paper focuses on the measurement of differential capacitive sensors, whereas the previous work dealt with the resistive counterparts. The proposed circuit is analyzed, and the main limitation seems to be the fact that the magnitude of the input parasitic capacitances of the microcontroller is similar to or even higher than the sensor capacitances. Methods to overcome this limitation are proposed, particularly when measuring low-value differential capacitive sensors such as microelectromechanical system (MEMS) sensors. Experimental tests of the circuit have been carried out by measuring a commercial capacitive accelerometer working as a tilt sensor. Although such a sensor has a low value (1.5 pF) and low sensitivity (0.105 pF/g), the measurement has shown a nonlinearity error of 1% full-scale span (FSS), which is a remarkable value considering the simplicity of the circuit.