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

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 degrees C and 0.05 pH at 40 degrees C with dual compensation. The compensation results show a maximum time drift of 0.003 pH/hour (0.166 mV/hour) at 23 degrees C and an average temperature drift of 0.00049 pH/hour/degrees C (0.0245 mV/hour/degrees C) for a reference temperature increase from 23 degrees C to 40 degrees 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.