An Auto-calibrated Measurement System for One-dimensional Matrices of Impedimetric Sensors

Impedance measurement ICs are widely used in embedded systems for single-sensor measurements. The impedance value of the target sensor is calculated by comparing its measurement signal with one of a given reference element. This is known as the calibration concept. To automate the calibration process and to enable the multi-sensor measurements, multiplexers are recommended. But the switch on-resistances of multiplexers cause additional measurement deviations, unless expensive multiplexers with very low switch on-resistances or buffer functions are used. To overcome the need for specially designed multiplexers, a circuit structure based on two multiplexers in the feedback loop of one voltage follower is proposed in this work, with DC bias circuits. This solution is experimentally validated in a prototype system based on the AD5933 chip for a one-dimensional sensor matrix. Compared to the common single multiplexer system, the proposed approach has suppressed the average measurement deviation from 6.01 % to 0.06 % in the impedance magnitude measurements, and from 10.75 % to 0.13 % for the resistive targets.