Sensors Related Ph.D. and D.Sc. Theses

Interface Circuits in SOI-CMOS for High-Temperature Wireless Micro-Sensors

Author: Lemi Toygur

Adviser: Dr. Steven L. Garverick, Case Western Reserve University

Defense: 2004

Pages: 134

Abstract:

This thesis explores the use of commercially available SOI-CMOS technology for use in high-temperature sensor interface circuits for operation at temperatures up to 300 °C. A fully depleted technology was chosen for its inherently low leakage current and critical sensor interface circuits were developed, specifically, a transimpedance amplifier and analog-to-digital converter. Since the ultimate goal is a high-temperature wireless microsensor, low power consumption and stable oscillator frequency were key issues in this work.

Oscillator topologies for MEMS resonators having very high series resistance were examined in regards power consumption and in-circuit quality factor. The transresistance topology was determined to be the best candidate for operation with MEMS resonators, but bandwidth and transimpedance gain must be very high to achieve a loop gain greater than one, as required for oscillation. An SOI transresistance was designed to meet the requirements of a particular SiC resonator. This IC was fabricated, packaged in a DIP, and tested. The amplifier, itself, oscillated due to parasitic coupling capacitance between input and output in the packaging, as proved by a variety of measurements and simulations. In future work, an unpackaged SOI-IC will be used to eliminate the parasitic coupling. Ideally, the MEMS resonator should be integrated with the IC.

To convert the analog signal to digital with 8-bit of accuracy reliably, a robust, 1st-order sigma-delta converter was designed. The sigma-delta converter is fully differential with discrete-time integrator and comparator, and also uses chopper stabilization, dynamic element matching and dithering to achieve high performance with relatively poor components. State-of-the-art performance has been achieved. With a power supply voltage of 3.3 V, SNR reached the theoretical maximum of 50 dB at room temperature, and was above 40 dB and 30 dB, respectively, up to 250 °C and 275 °C. Design weaknesses were identified in the course of testing, so it is believed that this performance can be improved.

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