A low-cost and accurate interface for conductivity sensors

This paper presents a low-cost and accurate interface for four-electrode conductivity sensors. The interface mainly consists of an analog front-end, a multiplexer and a voltage-to-time converter. The analog front-end is used to provide a controlled excitation voltage signal for the sensor and to convert the sensor signal (conductivity) into a voltage signal. The voltage-to-time period converter acts as an asynchronous converter for the sensor signals (voltage) employing a relaxation oscillator which output is a period-modulated signal. Experimental results over a conductance range of 0.1 μS to 20 mS show a random error of 0.32 μS and a systematic error of 1.32 μS for a measurement time of 400 ms.