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Sensors 2014, 14(10), 18784-18799; doi:10.3390/s141018784

A Linearity-Enhanced Time-Domain CMOS Thermostat with Process-Variation Calibration

Department of Electronic Engineering, National Kaohsiung First University of Science and Technology, Kaohsiung 811, Taiwan
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Received: 13 July 2014 / Revised: 13 September 2014 / Accepted: 29 September 2014 / Published: 10 October 2014
(This article belongs to the Section Physical Sensors)
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Abstract

This study proposes a linearity-enhanced time-domain complementary metal-oxide semiconductor (CMOS) thermostat with process-variation calibration for improving the accuracy, expanding the operating temperature range, and reducing test costs. For sensing temperatures in the time domain, the large characteristic curve of a CMOS inverter markedly affects the accuracy, particularly when the operating temperature range is increased. To enhance the on-chip linearity, this study proposes a novel temperature-sensing cell comprising a simple buffer and a buffer with a thermal-compensation circuit to achieve a linearised delay. Thus, a linearity-enhanced oscillator consisting of these cells can generate an oscillation period with high linearity. To achieve one-point calibration support, an adjustable-gain time stretcher and calibration circuit were adopted for the process-variation calibration. The programmable temperature set point was determined using a reference clock and a second (identical) adjustable-gain time stretcher. A delay-time comparator with a built-in customised hysteresis circuit was used to perform a time comparison to obtain an appropriate response. Based on the proposed design, a thermostat with a small area of 0.067 mm2 was fabricated using a TSMC 0.35-μm 2P4M CMOS process, and a robust resolution of 0.05 °C and dissipation of 25 μW were achieved at a sample rate of 10 samples/s. An inaccuracy of −0.35 °C to 1.35 °C was achieved after one-point calibration at temperatures ranging from −40 °C to 120 °C. Compared with existing thermostats, the proposed thermostat substantially improves the circuit area, accuracy, operating temperature range, and test costs. View Full-Text
Keywords: thermostat; CMOS; time-domain; linearity enhancement thermostat; CMOS; time-domain; linearity enhancement
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. (CC BY 4.0).

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MDPI and ACS Style

Chen, C.-C.; Lin, Y. A Linearity-Enhanced Time-Domain CMOS Thermostat with Process-Variation Calibration. Sensors 2014, 14, 18784-18799.

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