A Direct Digital Readout Circuit for Impedance Sensors

A new, simple and high-accuracy impedance-to-digital converter (IDC) is proposed in this paper. Conventionally, the parameters of sensors that can be modeled using a parallel combination of a capacitor (C) and a resistor (R) are measured using ac bridges, excited from a sinusoidal source. Recently, with the widespread use of digital systems in instrumentation, capacitance-to-digital converters and resistance-to-digital converters gained a lot of importance. An IDC that accepts sensors having C and R in parallel and provides digital outputs directly proportional to the C and R values is presented in this paper. This can be used not only for sensors whose C and R values vary with the measurand but also when C or R of a sensor needs to be measured keeping the output not affected by parasitic R or C present in parallel with the sensing element. Another application of the IDC is in the measurement of the dissipation factor of dielectric materials. The proposed IDC is based on a dual-slope technique, and hence provides high accuracy and immunity to noise and interference. A prototype of the proposed IDC has been developed and tested in the laboratory. Accuracy of the prototype IDC developed was 0.15% for the measurement of C and 0.04% for the measurement of R. The total conversion time of the prototype converter developed is 3 s, and its total power dissipation is 175.8 mW. The IDC was also interfaced with a polymer-based impedance humidity sensor, measured its C and R values for a range of humidity, computed the humidity, and compared its performance with another instrument, showing the practicality of the proposed IDC.