An Accurate Period-Modulator-Based Interface for Grounded Capacitive Sensors

This paper presents a high-precision capacitance-to-digital converter (CDC) based on period-modulation (PM) for grounded capacitive sensors. With a symmetrical design, the performance of the CDC is significantly improved by applying zoom-in and auto-calibration techniques. The nonidealities of the CDC circuit which are mainly caused by charge injection of switches and associated parasitic effects is reduced by utilizing dummy switches at asymmetrical paths of the applied auto-calibration. The proposed interface is designed as an integrated circuit using a standard 0.18μm CMOS technology. A worst-case capacitance error less than 0.2fF for a 10pF sensor capacitor with maximum variation of 200fF, and parasitic capacitance of up to 20pF is obtained. The achieved latency is 125μs and the CDC consumes 170μA from a 2V power supply.

Keywords:

Capacitive-to-voltage converter (CVC) , Zoom-in technique , Grounded capacitive sensor