A Current-Mode Interface With Mixed Single/Dual Slope Integration for Differential Capacitive Sensors

Abstract

A current-mode interface (CMI) with mixed single/dual slope integration (MSDSI) for differential capacitive sensors (DCSs) is presented in this paper. The proposed design is based on the integration of the reference current flowing through the capacitors of the DCS. The integration on the capacitor of a smaller capacitance is performed with a dual slope. Contrary, the integration on the capacitor of a larger capacitance is performed with only one slope. The normalized differential capacitance (NDC) defined as the difference-to-sum ratio of the DCS capacitances is proportional to the duration of only one time interval which is digitized using the counting method. There is no need for post-processing in the proposed NDC-to-time-to-digital conversion. It has been prototyped using discrete off-the-shelf components mounted on a printed circuit board, with a single supply voltage of 3.3 V. The measured NDC is in the range |NDC| < 0.612, with the constant sum of DCS capacitances of 970 pF. Achieved full scale error is smaller than 0.3 %, with a range of conversion speed from 1770 to 2755 NDC-to-time-to-digital conversions per second.