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.