PERFORMANCE OF A PLANAR DIFFERENTIAL-CONDUCTIVITY SENSOR FOR UREA

A detailed study of the performance of a planar thin-film differential-conductivity sensor for urea is described. The basic transducer consists of two identical interdigitated conductivity cells forming a differential pair. Special attention has been paid to the influence of the design parameters on the characteristics of the conductivity cells. The performance of the sensor is characterized by its differential gain, which is proportional to the cell constant of the individual cells. A common-mode rejection ratio (CMRR) is defined to quantify the capability of the differential sensor to suppress the influence of background conductivity changes on the output signal. This common-mode rejection reflects the selectivity of the transducer. CMRR values of over 40 dB can easily be obtained, comparable with a selectivity coefficient of 100 and more. Baseline stability is shown to be closely related to the temperature behaviour of the cells. In combination with the urease-containing protein membrane, a urea-specific sensor is obtained showing a linear response between 0.25 and 10 mM, with a typical sensitivity of 34 mV mM(-1) of urea.