A high-performance interface for grounded conductivity sensors

This paper presents a high-performance interface for grounded conductivity sensors. The interface mainly consists of a sensor driver, an analog front-end, a multiplexer and a voltage-to-time converter. The sensor driver and analog front-end provide a controlled excitation voltage for the sensor and convert the sensor signal (conductance) into a voltage signal. The voltage-to-time converter acts as an asynchronous converter that employs a relaxation oscillator to convert the sensor signals (voltages) into a period-modulated output voltage. The analysis and experiments are performed to optimize the interface circuit with respect to the range of measurable conductance. With a prototype, over a wide conductance range, from 0.01 mu S to 1 mS, the experimental results show random errors with a standard deviation of less than 5.6 nS for a measurement time of 160 ms, and a systematic error of less than 22 nS.