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-mu m 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 mu W of power with 1.25 V supply voltage for a typical pH value of 7 while occupying a die area of 0.017 mm(2). 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.