The noise performance of a high-speed capacitive-sensor interface based on a relaxation oscillator and a fast counter

This paper presents the analysis and experimental results on the noise performances of a capacitive-sensor interface. The interface is able to measure low capacitance values in the order of picofarads and is implemented with a simple relaxation oscillator, a fast counter, and a microcontroller. The goal is to find the criteria to implement a low-noise system, so that, even with a short measuring time, low noise can be obtained. Experimental results are performed in order to prove the validity of the theoretical analysis. The achieved resolution, with a measuring time of 20 ms, was better than 14.2 /spl times/ 10/sup -7/ for the measurement of a capacitance value of 2.2 pF.