A threshold voltage variation calibration algorithm for an ISFET-based low-cost pH sensor system

To utilize an ISFET in a low-cost pH sensor system, this paper presents an algorithm to calibrate VTH variation of ISFET by one-time measurement of IDS-pH curves of the reference ISFET and per-instance measurement of one IDS-pH point of the instance ISFET. In our algorithm, the IDS-pH curves of the reference ISFET are measured at various conditions once, and then a single IDS-pH point of the instance ISFET is measured per instance for 6-pH PBS. From these two data, the calibrated IDs-pH curve of the instance ISFET is acquired by fitting the reference IDS-pH curve closest to the measured single IDS-pH point of the instance ISFET into three-degree polynomial formula. In our experiment with PBS solution to verify our algorithm, our algorithm reduced mean absolute errors of the measured pH by 21 times from 4.31 to 0.2 compared with non-calibrated ones.