This paper presents a new intelligent ISFET sensory system dedicated to a precision pH sensory function, as well as a long-term monitoring capability without being jeopardized by temperature and drift fluctuations in the water-quality monitoring environment. It includes the novel compensation technique for counteracting a simultaneous change of temperature and drift, the design of the sensory system incorporating hardware and software co-design for enhancing the performance stability of a standard ISFET device and a new programmable current source with reduced dynamic current error for a dedicated temperature compensation process. The proposed ISFET sensory system, with about a 0.01 pH resolution in system design at a 3.3 V supply, has been validated by the experiments, exhibiting a maximum accuracy error of 0.02 pH at 23 C and 0.05 pH at 40 C with dual compensation. The compensation results show a maximum time drift of 0.003 pH/hour (0.166 mV/hour) at 23 C and an average temperature drift of 0.00049 pH hour C (0.0245 mV hour C) for a reference temperature increase from 23 C to 40 C, with the value of the pH solution ranging from 4 to 9 in six-hour measurements. These measured results outperform those of the reported drift reduction techniques, suggesting that the ISFET sensory system using novel compensation can provide significant immunity against temperature change, time drift, and temperature drift, which are favorable towards robust measurements in environmental monitoring applications.Index Terms-Intelligent sensor, ISFET drift, ISFET interface circuit, ISFET temperature compensation, long-term monitoring, microcontroller sensor system, precision current mirror, water quality monitoring.

This paper presents a new ion-sensitive field-effect transistor (ISFET ) readout circuit including a novel nonlinear temperature compensation method that is based on the theoretical work for formulating a body-effect-based ISFET drain current expression, the derivation of an unified temperature-dependent ISFET threshold voltage expression, and the use of iterative method for solving design parameters in nonlinear equations. Regarding the basic readout circuit, it comprises only one source follower and one current source to establish a self-biased configuration for a single ISFET device. Due to elimination of body effect, it displays linear transfer characteristic in the experimental result. Incorporating temperature compensation further improves the thermal stability of the ISFET device in pH sensing function. This has been validated by the experimental results on pH values ranging from 4 to 9 in a temperature range of 22 C to 50 C from the measurement setup. The pH7 parameter is used as a reference in the method. The proposed works are attractive in terms of circuit simplicity, temperature-compensated performance, cost and compatibility for smart sensor operation.Index Terms-CMOS circuit, ion-sensitive field-effect transistor (ISFET) interface circuit, ISFET sensor, temperature compensation, zero temperature coefficient. from 2001 to 2003. Currently, he has joined Broadcom, Singapore, as a Design Engineer, with the responsibility for the mixed-signal integrated circuit design. His research interests cover circuit theory, integrated analog circuits and systems, sensor characterization and interface circuits design.

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