Capacitive sensor interface with improved dynamic range and stability

This paper presents a capacitive sensor interface (CSI) based on the charge balancing method. Compared with previously reported works, the circuit is optimized with respect to the dynamic range while dissipating only 45 mW of electric power in proximity to the sensor area. The implemented conversion principle uses resistance, resistance ratio and time as references, thus achieving good initial accuracy and very high stability over temperature and time. The interface is realized at the PCB level with off-the-shelf components, and measures up to six sensors consecutively. The capacitance measurement range is 8 pF. The resolution of the converter is 0.18 fF (~16 bits dynamic range) with 0.35 ms conversion time.