CMOS $\Sigma \Delta$ pH-to-Digital Converter

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.