Autobalancing Analog Front End for Full-Range Differential Capacitive Sensing

The authors here propose an analog interface for differential capacitance estimation, able to reveal and quantify capacitive variations. Different from other works in the literature, the proposed interface can operate on a full-range scale. This solution takes inspiration from a preliminary work of the same authors, whose working principle is based on a modified De-Sauty AC bridge configuration; in particular, two (differential) capacitors and two resistors are employed, one of which is implemented by a voltage-controlled resistor (VCR). In this paper, the electronic interface has been redesigned to obtain full-range operation and a linear input-output characteristic. The implemented autobalancing mechanism controls the VCR generating a reference voltage following the sensor output. Simulation results in two different circuit configurations performed by OrCAD PSpice have shown a very good agreement with a theoretical model implemented in MATLAB/Simulink environment. In addition, an evaluation of the goodness of the experimental measurements at steady and unsteady conditions is given. Experimental tests on a discrete-component prototype have been carried out using an automated test equipment including National Instruments tools and a dedicated software in the LabVIEW environment. Measurements have shown satisfactory results, the standard deviation being on the order of 0.01, in the worst case. The linearity error has been evaluated as well, resulting in lower than 1% full scale.