88-μ A 1-MHz stray-insensitive CMOS current-mode interface IC for differential capacitive sensors

The paper describes an innovative technique to implement a low-power high-speed CMOS interface circuit for differential capacitive sensors. The proposed approach comprises a capacitance to current converter providing current-summing and current-differencing capability. It also exploits an autotuning feedback loop to control the common-mode current, thereby ensuring virtually the same maximum sensitivity and measure accuracy ir-respectively of the input parasitic capacitance. Therefore, the main limitation of all previous current-mode techniques is nearly eliminated. Besides, as an additional distinctive aspect, the proposed solution is suitable for both linear-and hyperbolic-type capacitive sensors. To validate the idea an interface circuit was designed in a 65-nm CMOS technology powered from a 2.5-V supply and dissipating 88-mu A standby current. Measurements show that relative capacitive sensor variations up to +/- 900 fF (+/- 100% of the nominal value) even in presence of a large parasitic capacitance of 2.5 pF are detected in less than 1 mu s with a sensitivity of about 5 nA/fF and with an relative error lower than +/- 1.5%, without requiring digital calibration.