A CMOS ISFET interface circuit with dynamic current temperature compensation technique

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 degrees C to 50 degrees 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.