Design of Interface Circuits for Capacitive Sensing Applications by Fatemeh Aezinia M.A.Sc., University of Tehran, 2006 B.Sc., University of Tehran, 2003 Thesis Submitted In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in the School of Mechatronic Systems Engineering Faculty of Applied Sciences Fatemeh Aezinia 2014 SIMON FRASER UNIVERSITY Summer 2014 Approval Name: Fatemeh Aezinia Degree: Doctor of Philosophy Title of Thesis: Design of Interface Circuits for Capacitive Sensing Applications Examining Committee: Chair: Gary Wang Professor Behraad Bahreyni, P. Eng. Senior Supervisor Assistant Professor Shawn Stapleton, P. Eng. Supervisor Professor School of Engineering Science Mehrdad Moallem, P. Eng. Supervisor Professor Ash Parameswaran, P. Eng. Internal Examiner Professor School of Engineering Science Kambiz Moez, P. Eng. External Examiner Associate professor, Department of Electrical and Computer Engineering University of Alberta Date Defended/Approved: August 08, 2014 ii Partial Copyright Licence iii Abstract This thesis focuses on the design of integrated readout circuits for differential capacitive sensing applications. Such circuits are needed especially for interfacing with microsensors where capacitive transduction is predominantly used. The result of this research is the development of common framework for interface circuitries suitable for different sensing applications. These interface circuits were designed and fabricated in standard Complementary Metal-Oxide-Semiconductor (CMOS) processes and can be integrated into the design of various sensing systems. The proposed circuits in this work are characterized by high dynamic range, low power consumption, and adjustable sensing range. Such circuits promote easy-to-use user interfaces while having a low cost. Three different circuit designs were proposed and form the highlights of this thesis. The first interface circuit is a novel realization of a synchronous demodulation technique. The main advantage of the proposed circuit compared to state-of-the-art is that it has a high sensing dynamic range of 112 and is capable of measuring capacitance as small as 30 with a total power consumption of 8. Low power consumption is one of the most important design criteria for portable sensing systems besides accuracy and precision. Following this requirement, low power consumption is the main criterion in the second circuit proposed in this work. This circuit uses a switch-based capacitance-to-voltage converter that is designed and fabricated in 0.35 CMOS technology. This circuit had a low power consumption of 600. Its simple structure offers area and power advantages over the more complex circuits. In addition, its ratiometric sensing feature provides an adjustable sensing range which can be tuned for different applications. This circuit can detect capacitances as small as 230 in 1 range of capacitance. To reduce the effect of parasitics on the circuit performance and improve the linearity, the design of the second circuit was enhanced. By using an additional block and an analog divider, the sensitivity of the circuit to parasitics was significantly reduced. On the other hand, a time based output allowed for the elimination of the analog buffers. The fabricated circuit consumed a total power of only 720 and was fabricated in iv 0.35 CMOS technology. Another advantage of this circuit over the previous designs is that the pulse-width output signal of this circuit can be more easily digitized. The proposed circuits in this thesis have been tested with different types of sensors including humidity, motion, and variable MEMS capacitors. For all of them also, the measurement results are found to be in good agreement with the analytic and simulation results. These circuits can be used as standalone chips or can be integrated into the design of larger sensing systems. Keywords: Interface circuit; capacitive sensors; wide dynamic range; low power consumption v Dedication To my mother, father, and my husband for their endless love and support vi Acknowledgements I would not have been able to make it to this point without the support of my supervisor Dr. Behraad Bahreyni. His patience, generous help, and wise suggestions helped me a lot during my PhD studies. Other than his thoughtful guidance throughout my research work, he taught me a lot about technical writing and technical presentation skills. I also thank, Dr. Shawn Stapleton and Dr. Mehrdad Moallem for their helpful comments and technical suggestions through my proposal defence. I also want to thank my examiners, Dr. Ash Parameswaran and Dr. Kambiz Moez for accepting to be on my committee despite their busy schedule and giving thoughtful comments and advice. I would also like to thank anonymous reviewers of my research papers for their comments which helped me improve the quality of my works. Thanks to the computing system staffs at SFU, especially Chao Cheng, and fabrication team at CMC Microsystems for their helps in computing problem solving and their supports through the hard time before the deadlines. Further, I thank faculty, staff, and all graduate students in both School of Engineering Science and Mechatronic Systems Engineering at SFU. Every one of these people has helped me in my studies. Also thanks to NSERC Canada, Nokia Corporation, and IMRIS Company which supported part of this work through Grant. I would also like to thank all my friends who gave me the energy all the time to work and all members of IMUTS lab for their supports and encouragements. Finally, I wish to thank my beloved parents and sisters for their never ending love that always filled my heart with energy. Last but not least, I thank my love, Mani, for his emotional support as a husband, his sincere suggestions as a friend, and his technical advices as a colleague throughout my PhD program. And I start this thesis in the name of God... vii Table of Contents Approval .............................................................................................................................ii Partial Copyright Licence .................................................................................................. iii Abstract .............................................................................................................................iv Dedication .........................................................................................................................vi Acknowledgements .......................................................................................................... vii Table of Contents ............................................................................................................ viii List of Tables ..................................................................................................................... x List of Figures....................................................................................................................xi List of Acronyms ............................................................................................................. xvii 1. Introduction ............................................................................................................ 1 1.1. Background .............................................................................................................. 1 1.2. Motivation ................................................................................................................. 3 1.3. Organization of the thesis ........................................................................................ 4 2. Literature review..................................................................................................... 5 2.1. Capacitive sensing ................................................................................................... 5 2.1.1. Basic configuration of capacitive sensors ..................................................... 5 2.1.2. Differential capacitive sensing ...................................................................... 8 2.1.3. Capacitive sensing based on coplanar electrodes ....................................... 8 2.2. Applications of capacitive sensing systems ........................................................... 10 2.3. Interface electronics for capacitive microsensors .................................................. 19 2.3.1. Capacitance to voltage converters (C2V) ................................................... 19 2.3.2. Capacitance to frequency converters (C2F) ............................................... 22 2.3.3. Capacitance to current converters (C2C) ................................................... 24 2.3.4. Capacitance to pulse-width converters (C2PW) ......................................... 26 2.3.5. Capacitive to digital converters (C2D) ........................................................ 26 2.4. Synchronous demodulation-based circuits ............................................................ 28 3. Differential capacitive sensing circuit with extended dynamic range ............ 31 3.1. Conventional synchronous demodulator topology ................................................. 32 3.2. Expanding the dynamic range of circuits based on synchronous demodulation .......................................................................................................... 43 3.3. Circuit design ......................................................................................................... 43 3.4.