Characterization of an ISFET with Built-in Calibration Registers through Segmented Eight-Bit Binary Search in Three-Point Algorithm Using FPGA

Abstract

Sensors play the most important role in observing changes in an environment they are a part. They detect even the smallest changes and send the information to other electronic devices. Making sure that these sensors provide an accurate output is equally crucial, as the data it measures and collects are used for analysis. Until now, calibrating sensors has been done manually by following a sequence of procedures, and is usually performed on-site or in a laboratory prior to deployment. To eliminate the manual procedure in the calibration (at the very least), an ion-sensitive field-effect transistor (ISFET) with a built-in calibration registers circuit was created through segmented eight-bit binary search in a three-point algorithm using a field-programmable gate array (FPGA). The circuit was created using a three-point calibration algorithm and three standard buffers (pH 4, pH 7, and pH 10). The block diagram, schematic diagram, and the number of logic gates were derived after synthesizing the Verilog program in Xilinx/FPGA. An average of 0.30% error was computed to prove the reliability of the created circuit using FPGA. Having an ISFET with built-in calibration registers will alleviate the work of experts in performing calibrations. This would follow the plug and play standard, hence its being a calibration-ready ISFET device. With this feature, it could be used as a pH level meter or a remote sensor node in several applications.