Quasi-synchronous thermocompensation for ISFET-based ionometric devices. Part 2: Implementation

Abstract

This paper is a continuation of the previously published work by the same authors, where general principles of the ionometric transducer design utilizing solid-state ion-sensitive electrodes (ion-sensitive field effect transistors, ISFETs) that can simultaneously serve as temperature sensors were laid out. In that part of the work, a possibility of using such transducer as a basis for ionometric device that performs automatic compensation of the temperature dependence of electrode potential without the need for a dedicated thermometric measuring path in the device structure was demonstrated with the circuit simulation results. Combination of the two functions (ionometric and thermometric) in a single sensor is achieved by separating the sensor operation modes in time, and dynamically switching between them by controlling the ISFET bias voltage.
In the present part, a practical implementation of the secondary transducer for ionometric sensors based on ISFET is considered and described. The proposed transducer provides the possibility of programmatic control of the ISFET bias voltage magnitude and polarity, thus allowing to use the ISFET as a temperature sensor. Consecutive switching between ionometric and thermometric modes of sensor operation, along with subsequent algorithmic processing of the obtained data by a microprocessor incorporated into the transducer structure, allows to compensate the temperature dependence of the ISFET electrode potential. Circuit diagrams for the main components of transducer — namely, the programmable voltage source for ISFET biasing and the transimpedance amplifier for the sensor output readout — are presented, as well as the experimental estimation of the ISFET sensor thermometric properties and the efficiency of thermocompensation.