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Journal of Sensors
Volume 2017, Article ID 6158689, 8 pages
https://doi.org/10.1155/2017/6158689
Research Article

A 7 μW Offset- and Temperature-Compensated pH-to-Digital Converter

1Electronic Instrumentation Laboratory, Delft University of Technology, Mekelweg 4, 2628 CD Delft, Netherlands
2NXP Semiconductors, Interleuvenlaan 80, 3001 Leuven, Belgium
3AMS AG, Coveliersstraat 15, 2600 Antwerpen, Belgium

Correspondence should be addressed to Michiel A. P. Pertijs; ln.tfledut@sjitrep.p.a.m

Received 29 August 2016; Revised 20 November 2016; Accepted 7 December 2016; Published 29 January 2017

Academic Editor: Lucio Pancheri

Copyright © 2017 Saleh Heidary Shalmany et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

This paper demonstrates a micropower offset- and temperature-compensated smart pH sensor, intended for use in battery-powered RFID systems that monitor the quality of perishable products. Low operation power is essential in such systems to enable autonomous logging of environmental parameters, such as the pH level, over extended periods of time using only a small, low-cost battery. The pH-sensing element in this work is an ion-sensitive extended-gate field-effect transistor (EGFET), which is incorporated in a low-power sensor front-end. The front-end outputs a pH-dependent voltage, which is then digitized by means of a co-integrated incremental delta-sigma ADC. To compensate for the offset and temperature cross-sensitivity of the EGFET, a compensation scheme using a calibration process and a temperature sensor has been devised. A prototype chip has been realized in a 0.16 μm CMOS process. It occupies 0.35 × 3.9 mm2 of die area and draws only 4 μA from a 1.8 V supply. Two different types of custom packaging have been used for measurement purposes. The pH sensor achieves a linearity of better than ±0.1 for pH values ranging from 4 to 10. The calibration and compensation scheme reduces errors due to temperature cross-sensitivity to less than ±0.1 in the temperature range of 6°C to 25°C.