Analysis of a Direct Microcontroller Interface for Capacitively Coupled Resistive Sensors

A novel approach to directly interface a capacitively coupled resistive sensor to a microcontroller is presented in this article. The existing measurement schemes for such sensors are complex. In addition, the coupling capacitance often also holds important data. The proposed simple measurement system, for such series RC sensors, is capable of measuring both the resistance and the coupling capacitance. A detailed analysis on the effect of the nonidealities on the resistance measurement showed that it is independent of the accuracy of the charging capacitor, supply voltage, and preset threshold voltage. The performance of the proposed scheme has been evaluated by building suitable prototypes. Initially, a setup was designed such that the measurement was not limited by the nonidealities of the microcontroller. The test results from this showed a maximum error of 0.28% and 0.96% for the resistance and capacitance measurement, respectively. The subsequent study with the microcontroller interface exhibited a maximum error of 0.91% (resistance) and 2.94% (capacitance). Noise and resolution studies have also been conducted and the results are presented. The accuracy of the prototype is promising, with a measurement time of 5 ms per parameter. This is a practical, low-power, low-cost measurement system as it provides digital data on the resistance and capacitance, in series, using only a microcontroller, and a couple of passive components.