A monolithic low-power highly linear pH measurement circuit with wide input detection range and easy calibration

In this paper, a novel topology for a monolithic low-power, highly miniaturized electrochemical pH sensor readout circuit with wide measurement range of pH values is presented. The readout circuit converts the output voltage of a pH sensor, which corresponds to the pH value of a solution under measurement, to a square-wave signal with its frequency being proportional to the pH sensor output voltage. A simple calibration method is employed in the design which makes the output frequency stay constant over process, supply voltage and temperature variations. The prototype system is designed and fabricated using a standard 0.13-µm CMOS process and the measurement results show that the sensor has good linearity to cover the entire pH value range from 0–14. The proposed sensor consumes 12.8 µW of power with 1.25 V supply voltage for a typical pH value of 7 while occupying a die area of 0.017 mm2. These features make the readout circuit with the pH sensor a promising solution for continuous monitoring of pH values in a wide range of applications such as real-time monitoring of physiological parameters and environmental monitoring.