A Monolithic Low-Power Highly Linear pH Measurement System With Power Conditioning System for Medical Application

Abstract

This paper presents a monolithic pH measurement system with a power conditioning system for supply power derived from harvested energy. The proposed system includes a low-power highly linear pH readout circuits with wide pH values (0-14) and a power conditioning unit based on low dropout (LDO) regulator. The readout circuit provides a squarewave output with the frequency being highly linear corresponding to the input pH values. To achieve that, a simple operational transconductance amplifier is employed to linearly convert the pH sensor output voltage to a current. To overcome the process variations, a simple calibration method is employed in the design which makes the output frequency stay constant over the process, supply voltage, and temperature variations. The prototype circuit is designed and fabricated in a standard 0.13-μm CMOS process and demonstrates a good linearity to cover the entire pH value range from 0 to 14, while the voltage regulator provides a stable supply voltage 1.25 V for the system. The proposed sensor consumes 12.8 μW of power for a typical pH value of 7 while occupying a die area of 0.017 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The total die area including the readout circuit and the LDO is 160 μm × 110 μm, which makes this system a suitable candidate for low-power miniaturized sensor systems. Both electrical test and in vitro measurement are performed, and a commercial pH meter is employed to make a performance comparison with this paper. The test results of the prototype circuit closely match the measurement results obtained by its commercial counterpart.