Direct Interface Circuit for Differential Capacitive Sensors

This paper proposes and analyses a novel interface circuit for differential capacitive sensors with linear or hyperbolic relationship. In such a new circuit, the sensor is directly connected to a microcontroller without using any analog integrated circuit in the signal path. The circuit relies on measuring the discharging time of several RC circuits that include the two capacitances of the differential capacitive sensor. For high values of the nominal capacitance (C₀) of the sensor, the main source of non-linearity is the quantization of the discharging-time measurement, whereas for low values of C₀, it is the stray capacitances. In spite of its simplicity, the interface circuit performs satisfactorily. According to the experimental results, the maximum non-linearity error is 0.03% FSS for C₀ = 150 pF and 0.36% FSS for C₀ = 20 pF, which are acceptable for many applications.