An Intelligent ISFET Sensory System With Temperature and Drift Compensation for Long-Term Monitoring

Abstract

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.

Temperature influence on pH-ISFET sensor operating in weak and moderate inversion regime: Model and circuitry

“…The double layer capacitance C DL consists of the two series capacitances C Gouy and C Helm , where C Gouy represents the equivalent Gouy-Chapman capacitance and C Helm the Helmholtz capacitance, which has already been developed by site-binding theory and the electrical double-layer theory [17,18]. It should be noted that C Gouy and C Helm are temperature dependent, however, since these are second-order effects, we will assume C Gouy and C Helm to be independent of temperature [5]. Thus, it has been possible to investigate the pH-ISFET flat-band voltage shift with pH and the variation of the potential at the electrolyte/gate dielectric interface.…”

“…Commercially available sensors are generally very expensive devices, especially if reasonable precision and reliability are requested. Several investigations have demonstrated that the pH-ISFET sensor presents some drawbacks related to thermal dependency and long-term drift [4][5][6]. These factors limit the performances of pH-ISFET sensors, leading to unacceptable results in critical measurements.…”

Remedial and adaptive solutions of ISFET non‐ideal behaviour

“…Casans et al (2004) showed an instrumentation system designed to compensate long-term drift on ISFETs. Smart compensation (Chen and Chan, 2008) is processed by two approaches: 1 It utilizes a dynamic biasing current temperature compensation technique to provide optimum biasing current for ISFET dynamically. This results in an almost zero temperature coefficient for nonlinear biasing point at different pH values.…”

An on-chip system to monitor the pH of cell culture media

“…ISFETs have to be calibrated over a period of time to compensate for the drift in the threshold voltage. Several methods have been used to compensate for the drift [12] [13] [14]. Our group previously reported a new method to mitigate drift.…”