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A CMOS delta-sigma (ΣΔ) pH-to-digital converter has been developed for continuous monitoring of H+-ion concentrations. The SnO2/ITO glass, fabricated sputtering SnO2 on the conductive ITO glass, was used as a pH-sensitive membrane of extended gate field effect transistor (EGFET) operational amplifier. The ΣΔ pH-to-digital converter, constructed by using EGFET-OP to realize switched-capacitor (SC) ΣΔ converter, converted the H+-ion concentration into digital form. This chip, fabricated in a 0.18-μm CMOS 1P6M process, operated at a 1.8 V supply voltage and normal sampling rate of 6.25 MHz. The gain error of the converter in the H+-ion concentration range between pH2 and pH12 is less than 2%, and the measurement error over the H+-ion concentration range of pH2 to pH12 can reach as small as ±0.02 pH.
Date of Publication: Feb. 2010