ON-CHIP TOUCH SENSOR READOUT CIRCUIT USING PASSIVE SIGMA-DELTA MODULATOR CAPACITANCE-TO-DIGITAL CONVERTER A Thesis Presented to The Graduate Faculty of The University of Akron In Partial Fulfillment of the Requirements for the Degree Master of Science Bo Liu December, 2016 ON-CHIP TOUCH SENSOR READOUT CIRCUIT USING PASSIVE SIGMA-DELTA MODULATOR CAPACITANCE-TO-DIGITAL CONVERTER Bo Liu Thesis Approved: Accepted: _______________________________ _____________________________ Advisor Interim Department Chair Dr. Kye-Shin Lee Dr. Joan Carletta _______________________________ ______________________________ Committee Member Interim Dean of the College Dr. Joan Carletta Dr. Donald P. Visco, Jr. _______________________________ ______________________________ Committee Member Dean of the Graduate School Dr. Robert Veillette Dr. Chand Midha ______________________________ Date ii ABSTRACT This work describes a capacitive touch sensor readout circuit using a passive sigma-delta modulator-based capacitance-to-digital converter. With the proposed approach, the panel condition (touched or un-touched) can be effectively converted into a binary signal by using the panel capacitance as the summing element of a first order passive sigma-delta modulator. In addition, the proposed touch sensor readout circuit does not require an analog-to-digital converter (ADC), since a digital output corresponding to the touch panel condition can be simply obtained by counting the number of modulator output pulses within the detection period. This along with the passive architecture leads to a compact and low power on-chip touch sensor readout circuitry. Furthermore, due to the noise shaping property of the sigma-delta modulator, it is possible to achieve a signal-to-noise ratio (SNR) and detection time that are comparable to those of other touch sensor readout circuits. The proposed touch sensor readout circuit is implemented using CMOS 0.35 µm technology and has a core area of 0.1 mm2. The operation of the sensor readout circuit is verified with a 10.4″ projective capacitance type touch panel, where the measured SNR is 31.4 dB and the measured power consumption is 65 µW. iii DEDICATION Dedicated to my parents and friends iv ACKNOWLEDGEMENTS I would like to thank my advisor Dr. Kye-shin Lee for guiding me throughout my Master’s degree. Without his patience, his encouragement and the financial support for the prototype IC and PCB, it would not have been possible for me to present this thesis project. I would like to acknowledge my thesis committee Dr. Joan Carletta and Dr. Robert Veillette, for their technical input and support throughout my Master’s degree projects and research assistant work. I would also like to express my sincere gratitude to the Electrical Engineering Department of the University of Akron for giving me an opportunity to be in this program and supporting my degree through a research assistantship. v TABLE OF CONTENTS Page LIST OF TABLES ........................................................................................................... viii LIST OF FIGURES ........................................................................................................... ix CHAPTER I. INTRODUCTION ......................................................................................................... 1 1.1 Background and Motivation ................................................................................. 1 1.2 Goals of the Thesis ............................................................................................... 2 1.3 Thesis Organization.............................................................................................. 3 II. LITERATURE REVIEW............................................................................................... 4 2.1 Capacitive Touch Screen Types ........................................................................... 4 2.2 Conventional Capacitive Touch Sensor Readout Circuit Structures ................... 6 2.2.1 Readout Circuits Based on Relaxation Oscillators ....................................... 7 2.2.2 Readout Circuit Based on Charge Transfer ................................................ 10 2.2.3 Readout Circuit Based on Series Capacitance Voltage Division ................ 12 2.3 Most Recent Work Related to Capacitive Type Touch Sensor Readout Circuit ............................................................................................................................ 13 2.4 Sigma-delta Modulator-Based Capacitance Sensors.......................................... 17 III. PROPOSED TOUCH SENSOR READOUT CIRCUIT ........................................... 23 3.1 Sensor Readout Circuit Model ........................................................................... 23 vi 3.2 Passive Sigma-delta Modulator Based Capacitance-to-Digital Converter ........ 27 3.3 CDC System Level Analysis .............................................................................. 32 IV. EFEFECT OF NON-IDEALITIES............................................................................ 34 4.1 Parasitic Capacitance.......................................................................................... 35 4.2 Capacitor Error ................................................................................................... 39 4.3 Comparator Offset .............................................................................................. 41 4.4 Supply Sensitivity .............................................................................................. 42 V. CIRCUIT IMPLEMENTATION ............................................................................... 44 5.1 Passive Sigma-delta Modulator .......................................................................... 44 5.2 Comparator ......................................................................................................... 45 5.3 Panel Capacitance (Channel) Selector ............................................................... 48 VI. EXPERIMENTAL RESULTS .................................................................................. 49 VII. CONCLUSIONS AND FUTURE WORK ................................................................ 58 BIBLIOGRAPHY ............................................................................................................. 60 vii LIST OF TABLES Table Page 2.1 Switch S1, S2 and S3 Position with Comparator Output Level and Switch Control Clock Edge ........................................................................................................................ 21 3.1 Cp Detection Range with Cs and M. (Unit: pF)........................................................... 33 3.2 Cp Resolution with Cs (M = 33) .................................................................................. 33 4.1 Npulse with Cp for Different Cpx and Cpy ...................................................................... 39 4.2 Npulse with Cp for ±10% Cs Error ................................................................................. 40 4.3 Npulse with Cp for Comparator Offset Vos .................................................................... 41 4.4 Npulse with Supply Ripple of Different Amplitude and Frequency (CP=20 pF) .......... 42 6.1 Performance Comparison with the State-of-the-art Capacitive Touch Sensor Readout Circuits .............................................................................................................................. 56 viii LIST OF FIGURES Figure Page 2.1 Capacitive Touch Screen Touch Sensing Scheme ........................................................ 4 2.2 Projected Capacitance Touch Screens (a) Self-Capacitance Touch Screen; (b) Mutual Capacitance Touch Screen .................................................................................................. 5 2.3 Relaxation Oscillator-Based Touch Sensor Circuit using Schmitt Trigger ................... 8 2.4 Schmitt Trigger Relaxation Oscillator-Based Touch Sensor Readout Circuit Timing Diagram (a) With Small Panel Capacitance; (b) With Large Panel Capacitance ............... 8 2.5 Relaxation Oscillator Touch Sensor Readout Circuit using Comparator ..................... 9 2.6 Relaxation Oscillator-Based Touch Sensor Readout Circuit Timing Diagram ........... 10 2.7 Charge Transfer-Based Touch Sensor Readout Circuit ...............................................11 2.8 Charge Transfer-Based Touch Sensor Readout Circuit Timing Diagram ................... 12 2.9 Series Capacitance Voltage Divider Based Touch Sensor Readout Circuit ................ 13 2.10 One C-VC Block with Time Interleaved Scheme [11] ............................................. 14 2.11 Charge Sensing Circuit Block Diagram [12] ............................................................ 16 2.12 Charge Sensing Circuit Timing Diagram .................................................................. 16 2.13 Conventional Sigma-delta Modulator Structure ....................................................... 17 2.14 Differential Sigma-delta Modulator-Based CDC [4] ................................................ 19 2.15 Single-ended Sigma-delta Modulator-Based CDC [5] ............................................. 20 2.16 Equivalent Circuit Depending on VCOMP Level (a) Equivalent Circuit for VCOMP = “H”; (b) Equivalent Circuit for VCOMP = “L” .................................................................... 21 3.1 Proposed Touch Sensor Readout Circuit ...................................................................