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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 for the sensor and to convert the sensor signal (conductance) into a voltage signal. The voltage-to-time period converter acts as an asynchronous converter for the sensor signals (voltage), which employs a relaxation oscillator and outputs a period-modulated signal. Experimental results over a conductance range of 0.1 μS to 20 mS show a random error of 1.6×10-5 and a systematic error of 6.6×10-5 for a measurement time of 110 ms.