A direct-digital interface circuit for sensors representable using parallel R-C model

摘要

In this paper, a new and high accuracy, dual-purpose Direct-Digital Converter (DDC) is presented. The proposed Switched-Capacitor (SC) DDC can be used to interface sensors whose electrical equivalent is (a) a capacitor Cx or (b) a resistor Rx or (c) Cx and Rx in parallel. Irrespective of (a)-(c), the DDC gives digital outputs proportional to Cx and Rx of the sensor. Capacitance-to-Digital Converters (CDC) and Resistance-to-Digital Converters (RDC) have been reported earlier, but no accurate dual-purpose DDC has been reported. In comparison with most of the CDCs/RDCs, the DDC presented uses no noticeable additional hardware. In the DDC, the conversion is obtained based on a SC dual-slope technique, which guarantees high immunity to interference. The DDC uses a dc reference voltage for excitation and helps to achieve high accuracy. A prototype of the proposed DDC has been developed. The prototype was tested for Rx from 50 kΩ to 1 MΩ and Cx from 50 pF to 250 pF. The results obtained from the DDC confirmed its usefulness.