ON-CHIP TOUCH SENSOR READOUT CIRCUIT USING PASSIVE SIGMA-DELTA MODULATOR CAPACITANCE-TO-DIGITAL CONVERTER. A Thesis. Presented to

Transcription

1 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

2 ON-CHIP TOUCH SENSOR READOUT CIRCUIT USING PASSIVE SIGMA-DELTA MODULATOR CAPACITANCE-TO-DIGITAL CONVERTER Bo Liu Thesis Approved: Accepted: Advisor Dr. Kye-Shin Lee Committee Member Dr. Joan Carletta Committee Member Dr. Robert Veillette Interim Department Chair Dr. Joan Carletta Interim Dean of the College Dr. Donald P. Visco, Jr. Dean of the Graduate School Dr. Chand Midha Date ii

3 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 mm 2. 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

4 DEDICATION Dedicated to my parents and friends iv

5 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

6 TABLE OF CONTENTS Page LIST OF TABLES... viii LIST OF FIGURES... ix CHAPTER I. INTRODUCTION Background and Motivation Goals of the Thesis Thesis Organization... 3 II. LITERATURE REVIEW Capacitive Touch Screen Types Conventional Capacitive Touch Sensor Readout Circuit Structures Readout Circuits Based on Relaxation Oscillators Readout Circuit Based on Charge Transfer Readout Circuit Based on Series Capacitance Voltage Division Most Recent Work Related to Capacitive Type Touch Sensor Readout Circuit Sigma-delta Modulator-Based Capacitance Sensors III. PROPOSED TOUCH SENSOR READOUT CIRCUIT Sensor Readout Circuit Model vi

7 3.2 Passive Sigma-delta Modulator Based Capacitance-to-Digital Converter CDC System Level Analysis IV. EFEFECT OF NON-IDEALITIES Parasitic Capacitance Capacitor Error Comparator Offset Supply Sensitivity V. CIRCUIT IMPLEMENTATION Passive Sigma-delta Modulator Comparator Panel Capacitance (Channel) Selector VI. EXPERIMENTAL RESULTS VII. CONCLUSIONS AND FUTURE WORK BIBLIOGRAPHY vii

8 LIST OF TABLES Table Page 2.1 Switch S 1, S 2 and S 3 Position with Comparator Output Level and Switch Control Clock Edge C p Detection Range with C s and M. (Unit: pf) C p Resolution with C s (M = 33) N pulse with C p for Different C px and C py N pulse with C p for ±10% C s Error N pulse with C p for Comparator Offset V os N pulse with Supply Ripple of Different Amplitude and Frequency (C P =20 pf) Performance Comparison with the State-of-the-art Capacitive Touch Sensor Readout Circuits viii

9 LIST OF FIGURES Figure Page 2.1 Capacitive Touch Screen Touch Sensing Scheme Projected Capacitance Touch Screens (a) Self-Capacitance Touch Screen; (b) Mutual Capacitance Touch Screen Relaxation Oscillator-Based Touch Sensor Circuit using Schmitt Trigger Schmitt Trigger Relaxation Oscillator-Based Touch Sensor Readout Circuit Timing Diagram (a) With Small Panel Capacitance; (b) With Large Panel Capacitance Relaxation Oscillator Touch Sensor Readout Circuit using Comparator Relaxation Oscillator-Based Touch Sensor Readout Circuit Timing Diagram Charge Transfer-Based Touch Sensor Readout Circuit Charge Transfer-Based Touch Sensor Readout Circuit Timing Diagram Series Capacitance Voltage Divider Based Touch Sensor Readout Circuit One C-VC Block with Time Interleaved Scheme [11] Charge Sensing Circuit Block Diagram [12] Charge Sensing Circuit Timing Diagram Conventional Sigma-delta Modulator Structure Differential Sigma-delta Modulator-Based CDC [4] Single-ended Sigma-delta Modulator-Based CDC [5] Equivalent Circuit Depending on V COMP Level (a) Equivalent Circuit for V COMP = H ; (b) Equivalent Circuit for V COMP = L Proposed Touch Sensor Readout Circuit ix

10 3.2 Proposed Readout Circuit Operation Timing Passive Sigma-delta Modulator Z-domain Model Proposed CDC Operation CDC Output Waveforms with Cs=0.75pF (a) Cp = 10pF. (b) Cp = 20pF Proposed CDC, Pulse Number Versus Panel Capacitance with Different Sampling Capacitor Values (M = 33) Proposed CDC with Non-idealities Z-domain Block Diagram of the Passive Sigma-delta Modulator with Parasitic Capacitance Transistor Level Circuit of the Proposed Passive Sigma-delta Modulator Comparator Circuit Comparator Offset Monte-Carlo Simulation Result Chip Micrograph (a) and Measurement Setup (b) Passive Sigma-delta Modulator Output Spectrum DNL and INL Plot of Decimated Sigma-delta Modulator Output Input-output Transfer Characteristic of Decimated Sigma-delta Modulator Three Different Touch Locations for the Panel CDC Output Waveforms Corresponding to Each Touch Location (a) Edge (b) Middle (c) Corner x

11 CHAPTER I INTRODUCTION 1.1 Background and Motivation Due to the increased demand for user friendly interfaces, touch screens and touch panels are becoming essential components for most portable electronic devices such as smart phones, tablets, MP3 players, and ebook readers [1] [3]. During the past two decades, the touch screen market has experienced a major transition from low cost resistive-type touch screens to capacitive-type touch panels that can easily support multitouch features with improved accuracy, durability, and shorter detection time. Basically, the capacitive-type touch panel requires a touch sensor readout circuit that can detect the variation of the touch panel capacitance (usually with the variation between 10 pf and 50 pf). However, in order to be suitable for battery operated portable devices, touch panels require compact and low power sensor readout circuits with high detection accuracy. One low-power approach for detection of changes in capacitance is the sigmadelta modulator (ΣΔM) based capacitance-to-digital converter (CDC). The sigma-delta modulator based CDC can achieve improved SNR due to the noise shaping property, where two different approaches have been proposed in [4] and [5]. The scheme in [5] is immune to environment noise, and insensitive to stray capacitance, but it is not 1

12 suitable for touch panel capacitance (that are basically single capacitance) detection due to the differential configuration. The scheme in [4] can be used for single capacitance detection, with a four phase control clocking scheme, but the on-chip implementation can be limited since the reference capacitor has to be larger than the touch panel capacitance, which can already be as large as 50 pf. In this thesis, a touch sensor readout circuit based on a passive sigma-delta modulator based capacitance-to-digital converter (CDC) is proposed. The proposed CDC uses the panel capacitance as the summing element of the passive sigma-delta modulator avoiding the need for a large reference capacitor, and does not require an ADC, since the digital output format can be obtained by counting the number of CDC pulses within the detection period. These two features both constitute advantages over the scheme in [4] and [5]. The passive sigma-delta modulator architecture used for the proposed touch sensor readout circuit is based on the applications from the previous works [6] [8], where the benefits such as the wide input dynamic range and low power consumption are maintained. As a result, the silicon area and the power consumption can be significantly reduced, while maintaining a comparable SNR and detection time with respect to other touch sensor readout schemes. 1.2 Goals of the Thesis The goal of this thesis is to design an on-chip touch sensor readout circuit using a passive sigma-delta modulator based CDC, which can work with the most popular projected capacitive type touchscreen panels. To achieve this goal, the behavioral model development, circuit implementation, testing and characterization of the proposed touch sensor readout circuit have been done. 2

13 The following contents are the detailed steps that were carried out to complete the thesis. A discrete time model of the touch sensor readout circuit was constructed in Simulink in order to verify the operation. The effect of various non-idealities is considered and analyzed to estimate the actual circuit performance. Each circuit block of the IC was designed and simulated using Cadence Virtuoso to verify the functionality. The readout circuit layout was done and the IC was implemented using CMOS 0.35-µm technology. After the IC was fabricated, a PCB that can interface the IC with a 10.4 projected type capacitive touch panel was built. Using this test setup, the operation of the readout IC was verified and the performance was characterized. 1.3 Thesis Organization The remaining part of this thesis is presented in six chapters. Chapter II presents the background information related to touch screen types, conventional and recent capacitive touch sensor readout circuits. Chapter III describes the proposed passive sigma-delta modulator based touch sensor readout circuit. Chapter IV discusses the effects of non-idealities, which includes parasitic capacitance, capacitor error, comparator offset and power supply sensitivity to show the robustness of the readout circuit. Chapter V presents the circuit implementation divided into three parts, which are sigma-delta modulator, comparator, and panel capacitance selector. Chapter VI presents the experimental setup using an actual 10.4 touch screen panel and the test results. Finally, Chapter VII presents the conclusions. 3

14 CHAPTER II LITERATURE REVIEW 2.1 Capacitive Touch Screen Types A capacitive type touch panel consists of an insulator (glass plate) that is coated by a conductive material such as ITO (indium tin oxide). When a human finger approaches the surface of the screen, the distortion of the screen s electrostatic field changes the capacitance between the sensing electrodes as shown in Figure 2.1 [9]. The most commonly used capacitive touch screens nowadays are projected capacitance touch screens. Figure 2.1 Capacitive Touch Screen Touch Sensing Scheme Projected capacitance touch (PCT) technologies detect touch by measuring the capacitance at each addressable electrode. When a finger approaches an electrode, it 4

15 disturbs the electromagnetic field and alters the capacitance. This capacitance variation can be measured by the readout circuits, and then converted into X-Y coordinates that the system can use to detect the touch. The value of the capacitance at each coordinate can vary from 10 pf to 50 pf. There are two touch screen types for the projected capacitance touch technology, which are self-capacitance and mutual capacitance where the electrode arrangements and the equivalent circuits are shown in Figure 2.2 (a), (b), (c) and (d). Figure 2.2 Projected Capacitance Touch Screens (a) Self-Capacitance Touch Screen; (b) Mutual Capacitance Touch Screen In the self-capacitance implementation, each row and column is an electrode independently as shown in Figure 2.2 (a), and the equivalent circuit is shown in Figure 2.2 (b), therefore rows and columns are individually addressed by the controller. Even 5

16 though the intersection of a row and column represents a unique coordinate pair, the electronic readout circuits are not able to measure each individual intersection as they can only measure each electrode. Self-capacitance technology has one capacitor for each row and one for each column. When a finger approaches the screen, the readout circuits scan through each electrode for capacitive load changes. By determining which row and column are closest to the touch location, the touch location can be determined by a microcontroller. Mutual capacitance type is a common PCT approach that utilizes the fact that if two conductive objects are close together they can hold a charge. A mutual capacitance touch screen is shown in Figure 2.2 (c) and the equivalent circuit is shown in Figure 2.2 (d). Mutual capacitance touch screens intentionally create mutual capacitance between elements of columns and rows in the vicinity where each intersects the other. Every intersection of each row and column forms a mutual capacitor, and every capacitor is independent with each other. When a finger touches near an intersection, some of the electric field between the row and column is coupled to the finger, which reduces the capacitance at the intersection as measured by the system readout circuits. This reduced capacitance that crosses the touch threshold set by the electronics indicates a touch has occurred. Many products such as smart phones combine the mutual and self-capacitance types to realize the touch panel. 2.2 Conventional Capacitive Touch Sensor Readout Circuit Structures There are several capacitive touch sensor readout circuits, which are realized based on relaxation oscillator, charge transfer and series capacitance voltage divider [10] 6

17 [11]. These readout circuits are used for the most commonly used projected mutual capacitance touch screens. They detect a touch event by measuring the change of capacitance at the nearest X-Y coordinates on the touch panel, and each independent capacitance is called panel capacitance. The three schemes are to be discussed in more detail below Readout Circuits Based on Relaxation Oscillators Figure 2.3 shows a relaxation oscillator based touch sensor readout circuit realized using a Schmitt trigger and a feedback resistor. Assuming C P represents the capacitance at a certain touch screen location, the circuit detects the C P variation, where C P changes with the panel condition - touched or untouched. X is the input of the Schmitt trigger, and Y is the inverting output. In this case, a different panel capacitance results in a different RC time constant, which changes the period of the Schmitt trigger. Figure 2.4 shows the operation of the readout circuit. V TH-L and V TH-H are the low and high threshold of the Schmitt Trigger, respectively. Given an initial condition that V X is close to V TH-L but below V TH-L, so V Y is H (high), then V Y starts to charge C P through R. The time required to charge C P until V X reaches V TH-H is proportional to the time constant τ=r C P. When V X reaches V TH-H, V Y becomes L (low), where C P begins to be discharged. The time required to discharge C P until V X reaches V TH-L is also proportional to τ. The sum of the charge and discharge time is denoted as T. Since different panel capacitance results in different τ, different panel conditions (touched or untouched) can be detected by measuring the duration of T. Figure 2.4 shows the waveforms at nodes X and Y with different panel capacitance, (a) small panel 7

18 capacitance, (b) large panel capacitance. As is shown, panel condition can be detected based on different oscillation periods T. Figure 2.3 Relaxation Oscillator-Based Touch Sensor Circuit using Schmitt Trigger Figure 2.4 Schmitt Trigger Relaxation Oscillator-Based Touch Sensor Readout Circuit Timing Diagram (a) With Small Panel Capacitance; (b) With Large Panel Capacitance 8

19 Figure 2.5 shows a relaxation oscillator based touch screen readout circuit that includes a reset switch and a comparator to detect the touch screen panel condition. The comparator output controls the reset switch. At the beginning, V Y = L (low), and the switch is off, so the power supply V DD starts charging C P through R. Figure 2.6 shows the operation timing diagram, where V X keeps increasing as C P is being charged until it reaches the comparator threshold voltage V TH, then the comparator output V Y changes from L to H (high). At this instant the switch is turned on to discharge the panel capacitance C P. Then as V X drops down below V TH, V Y changes from H to L. The oscillation period T changes with panel capacitance variation, because different panel capacitance requires different charge and discharge time, i.e. smaller panel capacitance results in shorter T and larger panel capacitance results in longer T. Thus, by measuring the period of the output waveform T the panel condition can be detected. Figure 2.5 Relaxation Oscillator-Based Touch Sensor Readout Circuit using Comparator 9

20 Figure 2.6 Relaxation Oscillator-Based Touch Sensor Readout Circuit Timing Diagram Readout Circuit Based on Charge Transfer Figure 2.7 shows a charge transfer based touch sensor readout circuit which can detect the touch screen panel condition. The operation is divided into three phases, sampling, summing and reset. The sampling and summing are controlled by a single pole double throw switch SW 1 and the reset is enabled by SW 2. In the sampling phase, SW 1 is connected to position 1, and C P is charged by V DD while C sum retains the charge from last summing phase. In the summing phase, SW 1 is connected to position 2, the charge in C P is shared between C P and C sum, resulting in an increase in V X. This circuit repeats the charge and discharge of C P by connecting SW 1 to position 1 and 2, so the charge in C sum is accumulated. As a result, V X keeps increasing until it reaches the comparator threshold voltage V TH. The reset switch SW 2 is open until the comparator output is high. Once V X reaches V TH, V out changes from L to H, the reset phase begins and the reset switch 10

21 SW 2 is turned on, so C sum is discharged. Switch SW 2 is designed to have small enough on-resistance to allow C sum to be completely discharged before V out becomes L. The waveforms of V X and V out are shown in Figure 2.8. The sampling and summing time of C P which is denoted as T or T is different for different panel capacitance. In Figure 2.8, T corresponds to a smaller C P, and T corresponds to a larger C P. If C P is smaller, the charge in phase 1 is smaller, so the charge sharing between C P and C sum in phase 2 is smaller, which leads to a smaller increment in V X after each clock cycle. Therefore, it needs more cycles (longer time) for V X to reach the comparator threshold voltage V TH than when C P is bigger. Thus by measuring T the panel condition can be detected. Figure 2.7 Charge Transfer-Based Touch Sensor Readout Circuit 11

22 Figure 2.8 Charge Transfer-Based Touch Sensor Readout Circuit Timing Diagram Readout Circuit Based on Series Capacitance Voltage Division Figure 2.9 shows a series capacitance voltage divider based touch sensor readout circuit. This circuit works in two phases, which are series capacitors charging and discharging, respectively. In Phase 1, SW 1 is turned on, SW 2 and SW 3 are turned off, thus C P and C REF are charged by the reference voltage V R. When C P and C REF are fully charged, since C P and C REF are connected in series, the node voltage V A can be written as C V A= C +C P REF REF V. (2.1) R As a result, panel capacitance C P variation will result in different V A levels. In Phase 2, SW 1 is turned off, SW 2 and SW 3 are turned on, thus C P and C REF are discharged and node voltage V A will be reset. 12

23 By setting a specific value of the comparator threshold based on C P and C REF values, the comparator output at the end of each Phase 1 will be logically different based on the touching condition; i. e. a small C P gives a H output and a large C P will lead to a L output. Thus, by detecting the comparator output level, the panel condition can be detected. Figure 2.9 Series Capacitance Voltage Divider Based Touch Sensor Readout Circuit 2.3 Most Recent Work Related to Capacitive Type Touch Sensor Readout Circuits This section summarizes the two most recent capacitive type touch sensor readout circuits in the literature, which use switched capacitor based capacitance-to-voltage converter (C-VC) and capacitance-to-time conversion (CTC) approaches. Work [12] proposes a switched-capacitor integrator-based capacitance-to-voltage converter (C-VC) with time-interleaved scheme. In this work, a 320 channel touch panel 13

24 with 20 row and 16 column electrodes is utilized, where one channel is the intersection of one row and one column electrode. The Y electrodes are divided into four groups and are connected to four C-VCs via four 4-to-1 Analog Multiplexers. Figure 2.10 shows one C- VC block for simplicity. X electrodes [0:19] are sequentially driven column by column by non-overlapping clock generator and switches. The corresponding C-VC block senses the Y coordinate on the touch screen and the charge stored in each panel capacitance will be integrated, and the integrator output level will change based on different touch conditions. At the end of each channel detection, the reset switch RST will be turned on to reset the circuit to get ready for the next channel detection. Larger panel capacitance will result in higher C-VC output level and vice versa, thus the touch location can be detected by monitoring the C-VC output level. The four C-VC outputs are connected to the SAR ADC by a 4-to-1 Mux. The drawback of this approach is that it requires large size off-chip feedback capacitors C INT. Figure 2.10 One C-VC Block with Time Interleaved Scheme [12] 14

25 Another touch sensor readout circuit that can effectively measure the panel capacitance variation using the capacitance-to-time conversion (CTC) method is presented in reference [13]. The core part of this design is the charge sensing circuit shown in Figure The reference current I ref is generated from a current reference circuit and injected into the panel capacitor C P. The timing diagram is shown in Figure Initially the comparator output is high since V C is lower than the comparator threshold level V TH. The time T required for the node voltage V C to reach V TH is derived as T=C V TH P, (2.2) I ref where different panel capacitance results in different T for the comparator output to change from H to L. In this case, T is measured by an external microcontroller. The discharge switch SW 2 is turned on when the comparator output changes to L, so that the panel capacitor could be discharged for the next sensing event. After a certain delay period, the switch SW 1 is turned on and SW 3 is turned off to prevent the reference current from flowing into C P when the panel capacitor is being discharged. SW 1, SW 2 and SW 2 are all implemented with NMOS switches. An additional timing control block generates the control signals for the charge sensing circuit and the panel capacitance switch array to sequentially select the panel capacitance. 15

26 Figure 2.11 Charge Sensing Circuit Block Diagram [13] Figure 2.12 Charge Sensing Circuit Timing Diagram To sum up, the above capacitance-to-voltage and capacitance-to-time capacitive touch sensing schemes are both good works. However, reference [12] requires large off-chip integrating capacitors and power hungry ADC, and reference [13] needs a 16

27 CMOS current reference circuit for the current generator which adds to the circuit complexity. The proposed circuit shows an improvement from these aspects. 2.4 Sigma-delta Modulators in Capacitance Sensing Circuits Sigma-delta modulation is a method for encoding analog signals into digital signals as found in an ADC. Sigma-delta modulator structures can be very useful in capacitive sensing circuits, since the output waveform changes with the variations in the capacitance used as a sampling or summing element of the sigma-delta modulator. The conventional sigma-delta modulator has the circuit structure shown in Figure It includes the comparator and the integrator realized with four switches controlled by non-overlapping clocks Φ 1 and Φ 2, sampling capacitor C IN and integration capacitor C INT. The comparator output is used as the feedback signal that is subtracted from the input V IN. Figure 2.13 Conventional Sigma-delta Modulator Structure Basically the sigma-delta modulator operates as follows. In Φ 1, C IN samples the input voltage V IN. In Φ 2, the charge stored in C IN during Φ 1 is transferred to C INT. As time goes by, when the integrator (C INT and the amplifier) output voltage exceeds the comparator threshold voltage V TH, the comparator output changes its state from L to H, thus the circuit feedback changes its state, which reduces the integrator output in the 17

28 next clock cycle. The comparator output is a pulse stream. C IN variation leads to charge variation in Φ 1, so the charge transferred to C INT during Φ 2 is different based on different C IN ; this results in the difference in integrator output increment value after each clock cycle. Therefore, the time required for the integrator output to reach the comparator threshold value is different, and so is the comparator output pulse length. Thus the time or the pulse numbers could be measured to detect the panel capacitance variation assuming the V IN is a constant voltage. The sigma-delta modulator structure is widely used in CDC (Capacitance-to- Digital Converter) circuits. The basic concept of the CDC is to generate a digital output, where the output value within a certain time interval is determined by the value of the capacitor. In reference [4], the capacitive sensor circuit using sigma-delta techniques implements a differential circuit structure, where only the integrator block is shown in Figure The reference and feedback circuitry and the comparator have been omitted for clarity. This structure has two input sensing capacitors C IN1 and C IN2, and two integrating capacitors that have the same value of C INT. By alternating the connection of Φ 1 and Φ 2 switches to V REF+ and V REF-, the sampling and integrating occurs alternately. In this case, the output can be described as (C -C ) V -V =- (V -V ). (2.3) IN1 IN2 OUT+ OUT- REF+ REF- CINT As a result, the output is proportional to the difference between the two input capacitors (C IN1 - C IN2 ); if one capacitor is fixed as a reference capacitor, the other input capacitor can be used as the sensing capacitor. Thus the output pattern depends on the value of the input capacitors. 18

29 Figure 2.14 Differential Sigma-delta Modulator-Based CDC [4] In reference [5], a single-ended sigma-delta modulator-based capacitance detector is proposed as shown in Figure Similar to work [4], this circuit also uses two input sensing capacitors. However, this work only has one integrating capacitor to charge from the two input capacitors. In this circuit, the comparator output goes into a flip-flop whose output is V OUT. Switches S 1, S 2 and S 3 are controlled by V OUT and the flip-flop s clock through some logic gates as shown in Figure Table 2.1 shows the switch position depending on V OUT level and clock edge. This circuit works in the following way. Assuming the initial output level of the flip-flop is set to H, for the first clock cycle, S 2 is set fixed to position 2. When the clock level is H, S 1 and S 3 are in position 1, and when the clock level is L, S 1 and S 3 are in position 2. The equivalent circuits for V OUT = H is shown in Figure 2.16 (a). As a result, at the end of this clock cycle, the integrator output is increased by V REF (C IN1 /C INT ). This increase makes the integrator output positive, thus the comparator and flip-flop output will become L. Whenever V OUT becomes L, at the next control clock cycle, S 1 will be set fixed to position 2. When the clock level is H, S 2 will be at position 1 and S 3 will be at position 2; when 19

30 the clock level is L, S 2 will be at position 2 and S 3 will be at position 1. The equivalent circuits for V OUT = L is shown in Figure 2.16 (b). For this case, the integrator output will be decreased by V REF (C IN2 /C INT ). If C IN1 = C IN2, the integrator output will deduct the same amount of voltage which is added from last clock cycle, since V REF (C IN1 /C INT ) = V REF (C IN2 /C INT ). Thus the integrator output will become negative, making the comparator output H again. As time goes by, there will be equal number of H s and L s at the flip-flop output. However, if C IN1 > C IN2, after one V OUT = H and one V OUT = L controlled clock cycle, the integrator output decreases less than it increases in the previous clock cycle, so V OUT remains L for another clock cycle. This allows another -V REF (C IN2 /C INT ) added to the comparator input. V OUT remains low until the integrator output decreases to below zero. Therefore there will be more L s than H s at V OUT for C IN1 > C IN2. On the contrary, if C IN1 < C IN2, there will be fewer L s than H s at V OUT. Thus, by fixing the value of one input capacitor as a reference, and detecting the number of the output H s and L s, the input capacitance variation can be detected. Figure 2.15 Single-ended Sigma-delta Modulator-Based CDC [5] 20

31 Table 1.1 Switch S 1, S 2 and S 3 Position with Comparator Output Level and Switch Control Clock Edge Clock V OUT = H V OUT = L H L S 1 - Pos. 1 S 2 - Pos. 2 S 3 - Pos. 1 S 1 - Pos. 2 S 2 - Pos. 2 S 3 - Pos. 2 S 1 - Pos. 2 S 2 - Pos. 1 S 3 - Pos. 2 S 1 - Pos. 2 S 2 - Pos. 2 S 3 - Pos. 1 Figure 2.16 Equivalent Circuit Depending on V OUT Level (a) Equivalent Circuit for V COMP = H ; (b) Equivalent Circuit for V OUT = L In this Chapter, the background information and the reviews of some conventional and recent circuit structures related to touch sensor readout circuits are presented, as well as the sigma-delta modulator-based capacitance sensors. Just as the conventional touch sensor readout circuits, the proposed readout circuit in this paper converts the panel capacitance change into some more easily detectable information. The proposed touch 21

32 sensor readout circuit implements the sigma-delta modulator technology. However, compared to the sigma-delta modulator structures introduced in Section 2.4, the proposed readout circuit uses a passive sigma-delta modulator structure. The reduction of capacitor numbers and the elimination of the active amplifier give advantages in terms of reduced power consumption, silicon area and circuit complexity compared to other touch sensor readout circuits. In the remaining chapters, the design and performance of the proposed touch sensor readout circuit will be presented. 22

33 CHAPTER III PROPOSED TOUCH SENSOR READOUT CIRCUIT 3.1 Sensor Readout Circuit Model The touch sensor readout circuit detects the touched location by scanning the capacitance value at each panel location. Generally, the capacitance value at a given panel location is converted into a voltage level or pulse width, and the touched location can be figured out by checking the capacitance variation pattern throughout the touch screen panel. Figure 3.1 shows the proposed touch sensor readout circuit, which can detect the touch screen panel condition by converting the panel capacitance into pulse numbers. The readout circuit includes the first order passive sigma-delta modulator and the panel capacitance selector (S X1 ~ S XX and S Y1 ~ S YY ). The panel capacitance selector will be discussed in Chapter 5 in more detail. V in represents the DC voltage applied to the sigmadelta modulator, which is connected to V DD for normal CDC operation. The passive sigma-delta modulator consists of the sampling capacitor C s that can be adjusted to 0.25 pf, 0.5 pf, 0.75 pf, 1 pf, 1.25 pf or 1.5 pf, several switches and a comparator, where each panel capacitance C P is used as the summing capacitor. The adjustable C s is to adapt the proposed readout circuit with different types of capacitive touch panels. 23

34 Figure 3.1 Proposed Touch Sensor Readout Circuit The operation timing of the sensor readout circuit including the control clock waveforms are shown in Figure 3.2, where the START signal indicates the beginning of the frame. Each panel capacitance is connected to the readout circuit one at a time by the switches S X1 ~ S Xi and S Y1 ~ S Yj. Once the panel capacitance is connected to node V x, the reset signal RST will discharge the panel capacitance. After the reset, the passive sigmadelta modulator will repeat the charge sampling and charge summing operation, which will generate different output pulse patterns depending on the value of the touch panel capacitance. In this case, the panel condition (touched or untouched) can be detected by simply counting the number of output pulses within the channel detection period. 24

35 Figure 3.2 Proposed Readout Circuit Operation Timing The operation of the passive sigma-delta modulator is divided into two phases. In the charge sampling phase (Φ 1 = H), input voltage V in is sampled in the sampling capacitor C s. During the charge summing phase (Φ 2 = H), C s and C P are connected together, where charge sharing between C s and C P will occur. As a result, the node voltage V x will be set to a certain level depending on the size of C s and C P that will be compared with the comparator threshold level V TH, in order to generate the sigma-delta modulator output level that is either H or L. The equivalent z-domain model of the passive sigma-delta modulator for the proposed touch sensor readout circuit is derived using the following charge equations. For the n-th operation cycle, the charge stored in C s and C P is given by Q Cs (n) = C s [V in (n) - V o (n )], (3.1) 25

36 Q Cp (n) CP V x (n 1/ 2). (3.2) For the (n + 1/2)-th operation cycle, due to the charge sharing between C s and C P, the following relationship can be reached [Cs C P ] V x (n 1/ 2) Q Cs (n ) Q Cp (n). (3.3) Furthermore, using the following relationship V (n) V (n 1/ 2), (3.4a) x x V (n + 1) = V (n + 1/2), x x (3.4b) V (n) = V (n - 1/2), o o (3.4c) with eq. (3.1) and eq. (3.2), the node voltage V x can be rewritten as V (n +1) = G [V (n) - V (n)] + G V (n), (3.5) x 1 in o 2 x where coefficients G 1 and G 2 are given as C s G 1 =, C s + C P (3.6a) C P G 2 =. (3.6b) C s + C P 26

37 As a result, the z-domain expression of node voltage V x is given by V (z) = z G [V (z) - V (z)] + z G V (z), (3.7) -1-1 x 1 in o 2 x where based on eq. (3.7), the equivalent z-domain representation of the first order passive sigma-delta modulator based touch sensor readout circuit is shown in Figure 3.3. In this case, the sigma-delta modulator output pattern will depend on the value of the coefficients G 1 and G 2. By using the touch panel capacitance as the summing capacitor of the passive sigma-delta modulator, any change in panel capacitance due to a touch causes a change in the coefficients G 1 and G 2, which will change the output pattern of the sigmadelta modulator. In addition, since the output format of the sigma-delta modulator is digital, the sensor readout block does not require an ADC; simple pulse counting logic is enough, so the silicon area and the power consumption of the touch sensor readout circuitry is reduced over other approaches. Figure 3.3 Passive Sigma-delta Modulator Z-domain Model 3.2 Passive Sigma-delta Modulator Based Capacitance-to-Digital Converter The basic concept of the proposed CDC is to generate an output pulse sequence, where the number of pulses within a certain time interval (detection period) is determined 27

38 by the value of C P, since C s will be set to a fixed capacitance. As a result, the value of C P can be estimated from the number of pulses within the detection period. Figure 3.4 describes the operation of the proposed CDC, where M is the total number of operation cycles in one detection period. Within each cycle n, there are two phases, charge sampling (Φ 1 = H) and charge summing (Φ 2 = H). In the first cycle (n = 1), the operation of the CDC starts with discharging C P (activated by the RST signal), which will make V x (1) = 0 V and sigma-delta modulator output V o = L. Assuming V in is a constant DC voltage, from the second cycle (n = 2), the level of V x will gradually increase due to the charge sampling and summing operation of the sigma-delta modulator until it reaches the threshold level of the comparator V TH. In this case, the node voltage V x after each cycle is given by C C, (3.8) n s P k V x (n) = (Vin V L) (1 <n < m) CP k=1 C s + CP where m denotes the operation cycle when V x exceeds V TH. In addition, the increment of V x for each cycle is given as C C ΔV (n) = (V V ) (1 < n < m). (3.9) n-1 s P x in L n (C s+ C P) Once V x (n) exceeds V TH (for n m), the sigma-delta modulator output will become H. In the next cycle n = (m + 1), the V x (n) level will be less than V TH, and for the following cycle n = (m + 2), the V x (n) level will be greater than V TH again due to the negative feedback operation of the sigma-delta modulator. As a result, the sigma-delta modulator will generate an output pulse sequence for the remaining (M - m) cycles. The total 28

39 number of output pulses within (M m) cycles can be written as N pulse = (M- m) / 2. (3.10) Figure 3.4 Proposed CDC Operation Assuming the sigma-delta modulator has a constant input, once the integrator output reaches the threshold level of the comparator V TH, the comparator output will become H, which will subtract a certain amount of feedback voltage from the input in the next clock cycle, thus making the comparator output L again. However, in the next clock cycle, the feedback voltage will be added to the input, thus making the comparator output to H. This operation will repeat and the comparator output will be alternating between H and L for each clock cycle, and this will be always valid for the first order passive sigma-delta modulator regardless of the value of G 1 and G 2. However, the value of N pulse will be mainly determined by the relative difference between G 2 and G 1. That is for G 2 >> G 1 (C P >> C s ), N pulse will be a small integer, since the step size ΔV x (n) will be small, 29

40 and it will take more cycles for V x to reach V TH. On the other hand, if G 2 G 1, yet G 2 > G 1 (C P C s, C P > C s ), N pulse will be a large integer, since ΔV x (n) will be large, which enables V x to reach V TH faster. Therefore, N pulse is inverse-proportional to the value of C P. Considering the general capacitive (projected capacitance type) touch panels, the value of C P can vary anywhere from 10 pf to 50 pf depending on the panel condition (touched or untouched). Therefore, the touch sensor readout circuit should effectively detect the capacitance variation within this range. The range of detectable panel capacitances depends on the value of the sampling capacitance C s. Figure 3.5 illustrates why, by showing time domain waveforms for the voltage output for proposed CDC, operating with M = 33 and C s = 0.75 pf, for two different panel capacitance values, 10 pf and 20 pf. For the larger panel capacitance, it takes a longer time for the voltage to rise to the threshold, and so fewer pulses are generated. The range of detectable panel capacitances is limited because panel capacitances larger than a certain size cause there to be no output pulses. The value of the sampling capacitance C s also determines the CDC resolution, or smallest panel capacitance change that can be detected. Figure 3.6 shows the results of a simulation experiment to explore the trade-off between CDC resolution and range. It determines how the number of output pulses N pulse varies with the panel capacitance C P for different C s values. The results are obtained as follows. The sampling capacitor C s is set to 0.25 pf, 0.5 pf, 0.75 pf, 1.0 pf and 1.5 pf in turn, and for each C s value, the output pulse numbers are recorded while varying the panel capacitance C P. Pulse numbers verses panel capacitance are then plotted for different C s. For this case, the 30

41 sigma-delta modulator input V in = VDD (3.3 V), the comparator threshold V TH = V DD /2, and the total operation cycles per touch pattern M is set to 33. As shown, N pulse decreases linearly with C P, with a slope that is steeper for smaller values of C s ; thus, the CDC s range of detectable panel capacitances is inversely proportional to the sampling capacitance. For example, for the proposed CDC using M = 33, with Cs = 0.25 pf, the CDC resolution is about 0.5 pf, but panel capacitances C P in only a relatively small range of up to 11 pf can be detected. By using a larger sampling capacitor of C s = 1.5 pf, the CDC resolution is reduced to 4 pf, but the range of detectable panel capacitances is increased to 70 pf. Figure 3.5 CDC Output Waveforms with C s = 0.75pF (a) C P = 10pF. (b) C P = 20pF 31

42 Figure 3.6 Proposed CDC, Pulse Number Versus Panel Capacitance with Different Sampling Capacitor Values (M = 33) The advantage of the proposed CDC is the relative small size sampling capacitor C s compared to the panel capacitance C P and the usage of the supply voltage V DD for the sigma-delta modulator input; because using a small C s saves the silicon area of the IC, and using the supply voltage for the readout circuit input simplifies the circuit. 3.3 CDC System Level Analysis In order to investigate the system performance of the proposed CDC including trade-offs between CDC resolution and detection range, a system level analysis of the CDC has been performed. For this analysis, the total number of operation cycles M was varied; the values 33, 65, 129, and 257 were used. The reason why these values are selected for M is because 1 cycle is for the reset, and the remainders of 32, 64, 128 and 256 cycles can make the resolution of the CDC 4, 5, 6 and 7 bits, respectively; since N pulse can range 32

43 from 0 to (M-1)/2, the resolution is log 2 [(M-1)/2]. In addition, the detectable capacitance range is determined by the value of the sampling capacitor C s. For the analysis, five different values were used. Based on Figure 3.5, Table 3.1 shows the C P detection range for the different combinations of C s value and M. As C s and M increase, the capacitance range is extended; however the detection time is increased while increasing M. The resolution is determined by the C s value, due to the different step size of ΔV x. Table 3.2 shows the C P resolution with different C s values, and this result applies for all M values. It is shown that the resolution improves by using smaller C s values, however this will reduce the capacitance detection range as illustrated in Table 3.1. Table 3.2 C P Detection Range with C s and M. (Unit: pf) M C s [ pf ] Table 3.3 C P Resolution with C s (M = 33) C s [ pf ] Resolution [pf] To sum up, there are trade-offs between detection time, detection range and CDC resolution. The detection range gets wider with the sacrifice of the detection time and detection resolution. The best values of M and C s could be selected based on the system requirements that the touch screen readout circuit will be implemented on. 33

44 CHAPTER IV EFFECT OF NON-IDEALITIES So far, the proposed touch sensor readout circuit has been presented with ideal components and under ideal conditions. However, in reality, there are many non-ideal conditions which may affect the system performance. Some major non-idealities and the effects will be discussed in this chapter. The major non-idealities that can affect the performance of the proposed passive sigma-delta modulator based touch sensor readout circuit include parasitic capacitance, capacitor error, and comparator offset. In order to figure out the tolerable limit of the readout circuit, the effect of each non-ideality on circuit performance is analyzed. Furthermore, simulations are performed to investigate the effect of each non-ideality. Figure 4.1 shows the proposed CDC with the non-idealities, where C px, C py, and C pz represent the parasitic capacitance at nodes x, y and z, respectively, and ε denotes the capacitor error and V os denotes the comparator offset. 34

45 Figure 4.1 Proposed CDC with Non-idealities 4.1 Parasitic Capacitance The different parasitic capacitance at each node is due to switch S 1 ~ S 4 parasitic, top and bottom plate parasitic of C s, the reset switch and the panel capacitance selection switch parasitic. In order to analyze the effect of parasitic capacitance, the passive sigmadelta modulator coefficients G 1 and G 2 are re-derived in the presence of C px, C py, and C pz. During the sampling phase (where S 1 and S 2 are closed), the voltage across the parasitic C px, C py, and C pz is V x (n - 1/2), V in (n), and V o (n - 1/2), respectively, where V x (n - 1/2) and V o (n - 1/2) represent the previous phase V x node and the sigma-delta modulator output voltage, respectively. In the summing phase (when S 3 and S 4 are closed), charge sharing among the capacitors will occur, however, since both sides of C pz are connected to GND, the effect of parasitic C pz is negligible. Considering the initial charge stored in each capacitor, the total change for the summing phase can be written as Q T (n +1/2) = Cs V Cs (n) + Cpy V in (n) + [C p +C px ] V x (n-1/2), (4.1) 35

46 where the voltage across Cs is given by V Cs (n) = [V in (n) - V o (n-1/2)], (4.2) Furthermore, assuming V o (n - 1/2) = V o (n), V x (n - 1/2) = V x (n), and V x (n + 1/2) = V x (n + 1), the node voltage V x can be expressed as Q (n +1/2) T V x (n +1) =, (4.3) C s + C p + C px + C py and re-arranging eq. (4.3) after taking the z-transform, the z-domain expression for V x including the parasitic capacitance is given as V (z) = z G V (z) - z G V (z) + z G V (z), (4.4) x p1 in p2 o p3 x where C + C s py G p1 =, (4.5a) C s + C p + C px + C py C s G p2 =, (4.5b) C s + C p + C px + C py C + C p px G p3 = C s + C p + C px + C. (4.5c) py 36

47 Figure 4.2 Z-domain Block Diagram of the Passive Sigma-delta Modulator with Parasitic Capacitance As a result, the z-domain block diagram of the passive sigma-delta modulator with the parasitic is shown in Figure 4.2, where the reversed Z shaped block between V x and V o represents a comparator. Without the parasitic capacitance, G p1 and G p2 are identical to G 1 given in eq. (3.6a), and G p3 is identical to G 2 given in eq. (3.6b). It is clear that parasitic capacitance C px and C py can affect the performance of the CDC by changing the coefficients. In order to further investigate the effect of C px and C py, the expression for ΔV x (n) including the parasitic capacitance is obtained using eq. (3.9) and Figure 4.1. In Figure 4.1, noticing the CDC output voltage V o = L for n < m, which is equivalent to connecting C s to GND, C px will be connected in parallel with C p, and C py will be connected in parallel with C s, during the interval n < m. Therefore, the expression for ΔV x (n) with the parasitic capacitance can be derived by adding C px and C py to C p and C s, respectively. That is (C +C ) (C +C ) ΔV (n) = V (1 < n < m). (4.6) n-1 s py p px x,p in n (C s + C p+c px +C py) 37

48 Furthermore, to see the effect of the parasitic capacitance, the ratio between ΔV x (n) and ΔV x,p(n) is obtained, which is n n-1 ΔV x (n) C C s px +C py C p = 1+ ΔV x,p (n) C s +C py C s +C p C p +C px. (4.7) As shown, the ratio between ΔV x (n) and ΔV x,p (n) will be very close to 1, since C px, C py << C s, Cp, and n > 1. This implies the parasitic capacitance will not change the output pattern of the CDC much. The actual ratio V x (n)/ V x,p(n) for the entire range of the panel capacitance (10 pf to 50 pf) considering the worst case parasitic capacitance of the CMOS 0.35 µm technology where C px is estimated to be 200 ff and C py to be 50 ff, is from to The result is as expected from eq. (4.7), which verifies the robustness of the proposed CDC to parasitic capacitance. Table 4.1 shows N pulse with C p for different C px and C py values, where C s is set to 0.75 pf, and all other non-idealities are neglected. The results are obtained from circuit simulations by adding a C px of different values shown in the table to C P of the nominal value and C py to C S. The values for C px and C py are set considering the actual circuit where C px will be greater than C py. As shown, N pulse does not change much for the worst case C px = 200 ff and C py = 50 ff, and slightly changes until C px reaches 500 ff and C py reaches125 ff. The last column where C px = 500 ff and C py = 125 ff is to show the limit of parasitic capacitance when the circuit output pulse patterns begins to slightly shift. 38

49 Table 4.4 N pulse with C p for Different C px and C py C p C px = 100f C px = 200f C px = 500f (pf) Ideal C py = 25f C py = 50f C py = 125f Capacitor Error The sampling capacitor C s is an important component that determines N pulse, depending on the value of the panel capacitance C p. The CDC performance is analyzed by applying a ±10% variation in C s from the nominal value. A 10% variation is considered the worst case capacitor error (difference between the absolute and actual value of C s ) because of the given technology [14]. 39

50 Table 4.5 N pulse with C p for ±10% C s Error C p (pf) C s = 0.825f ( = +10%) Ideal C s = 0.675f ( = -10%) Table 4.2 shows N pulse with C p for ±10% C s error, where the nominal value of C s is set to 0.75 pf. The results show a slight shift in the N pulse with C p, however the basic N pulse pattern with C p does not significantly change. The panel capacitance difference between the untouched case and touched case is above 5 pf for most projected touch screen panels, which will leave a big enough gap between pulse numbers for the two cases. In other words, for example, if a touch screen panel has the untouched panel capacitance above 21 pf and the touched below 15 pf, from which the pulse numbers will be detected as below 6 and above 9 with nominal C s, by setting a threshold of 7.5 between untouched and touched cases, even with the worst case capacitor error, a 15 pf panel capacitance will have 8 pulses instead of 9. But 8 is still above the threshold of 7.5, 40

51 thus it still will be detected as the touched case. This implies the proposed CDC can tolerate the worst case C s error for the given technology. 4.3 Comparator Offset Table 4.6 N pulse with C p for Comparator Offset V os C p (pf) Ideal 50mV - 50mV V os 100mV -100mV Comparator offset is another non-ideality that can degrade the CDC performance. Comparator offset acts as a constant DC voltage added at the input of the comparator. Table 4.3 shows N pulse with C p for different offset values, where C s is set to 0.75 pf. Results show the output pattern of the CDC remains almost the same with offset up to ±50mV, however for offset of 100mV, a shift in the output pattern is observed. Results 41

52 show the positive offset has the similar effect as the positive capacitor C s mismatch + s where the output pattern shifts to the same direction, and the negative offset has the similar effect as s. This is because positive offset and positive capacitor C s mismatch both reduce the effective V TH level of the comparator, and vice versa. 4.4 Supply Sensitivity Power line interference (50 Hz or 60 Hz) and SMPS noise (up to 300 khz) can intrude into the touch panel by adding ripples on the supply voltage [15]. To investigate the supply sensitivity of the proposed CDC through circuit level simulations, a ripple is added to the supply voltage V DD, and the pulse numbers are measured with different ripples. The ripple is a sinewave ripple with amplitudes between 0 to 100 mv at the frequencies of 1 khz, 10 khz, 50 khz, 100 khz and 500 khz. Table 4.7 N pulse with Supply Ripple of Different Amplitude and Frequency (C P =20 pf) Amp. (mv) Frequency 1 khz 10 khz 50 khz 100 khz 500 khz Table 4.4 shows N pulse with supply ripple for C s = 0.75 pf, C p = 20 pf (this is assuming the un-touched panel condition) and M = 33. The ideal N pulse without supply 42

53 ripple is 6. Results show the proposed CDC can tolerate supply ripple up to 50 mv for most of the frequency ranges. However, for ripple amplitude larger than 50 mv, the CDC performance is most critically affected when the ripple frequency is near the detection frequency f s /M, where fs is the sampling rate of the CDC (set to 1 MHz). This is the worst case where the CDC experiences the maximum supply voltage fluctuation within the detection period. Not much degradation is observed for other frequency ranges for the untouched case to be detected as touched or vice versa. With the worst case ripple frequency, the 6 pulse output under nominal conditions will be detected as 8 pulses, as explained in Section 4.2, the un-touched panel can be detected as touched. However, as shown in reference [16] and [17] due to the advance in CMOS switching regulator technology of using a Switching Noise Robust Charge-pump, the supply ripples can be easily suppressed within the 50 mv range [16], [17]. 43

54 CHAPTER V CIRCUIT IMPLEMENTATION This chapter presents the implementation of each part of the proposed touch sensor readout circuit. The design details of the passive sigma-delta modulator, the comparator and the panel capacitance selector are presented. Some simulation results of the comparator are also discussed in this chapter as the comparator is an important and independent component which may affect the overall readout circuit performance. 5.1 Passive Sigma-delta Modulator Figure 5.1 Transistor Level Circuit of the Proposed Passive Sigma-delta Modulator 44

55 The key component of the proposed touch readout sensor circuit is the passive sigma-delta modulator based CDC which consists of the sampling capacitor C s, the touch panel capacitor C p (used as the summing capacitor), switches, and the comparator. The transistor level passive sigma-delta modulator is shown in Figure 5.1. The sampling capacitor that is adjustable from 0.25 pf to 1.5 pf is realized with inter-digitized layout using a unit capacitor of 125 ff to improve the matching, where Figure 5.2 just shows C s of 0.75 pf for simplicity. The four sampling switches S 1 ~S 4 are realized using CMOS transmission gates with PMOS (W/L) p = (4µ/0.35µ) and NMOS (W/L) n = (2µ/0.35µ), where the maximum on-resistance is set to 800 Ω. Considering the maximum C p of 50 pf and operation clock period T s of 1 µs (fs = 1 MHz), this leads to the RC time constant τ = 40 ns, which is less than T s /2. This enables sufficient time for the V x node voltage to settle within half of the sampling period. The reset switch S 5 that discharges C s and C p is realized using an NMOS with (W/L) n = (20µ/0.35µ). 5.2 Comparator The details of the comparator which is between V x and V o from Figure 5.1 are discussed in this section. Figure 5.2 shows the comparator, where the pre-amp and the positive feedback is effectively combined in the input stage [18]. The comparator output is valid when clock V CK is H, and enters the reset mode when V CK is L. As a result, clock Φ 1 is used for V CK, since the comparator has to operate when V x node is completely settled at the end of the charge summing phase. In order to design the comparator with minimum power consumption, the comparator bias current I Bias was first set to 10 µa with V CK clock frequency of 1 MHz which is the nominal operation frequency. Then I Bias 45

56 was reduced until the comparator malfunctioned. Based on this experiment, the minimum I Bias that still enables the comparator to properly function was 1 µa. Thus, by reducing I Bias to 1 µa, the total power consumption of the comparator was reduced to 14.5 µw without degrading the comparator performance. Figure 5.2 Comparator Circuit Simulations of the comparator have been completed in the Cadence Virtuoso to show the comparator s performance. The input offset and propagation delay are obtained from circuit simulations. Figure 5.3 shows the comparator input-offset results from Monte-Carlo simulations of 200 runs. The widths of the top four PMOS are generated from an online Monte-Carlo sequence generator for 200 sets, and each set of widths are used in the comparator circuit to run a simulation for a corresponding input-offset. Each input-offset voltage is obtained in simulation by holding the inverting input at 1.65V and applying a slow ramp from 20mV below to 20mV above the inverting voltage for 1ms to the non- 46

57 inverting input. The input-offset is the differential input voltage at the time when the comparator output is 1.65V. The 200 simulation results of the input-offset voltage are then summarized into the bar chart in Figure 5.3, from which the average deviation of 100μV and a standard deviation of 3.9mV are obtained, and they are both far away from the 50mV limit that the CDC output pattern just begins to shift as in Table 4.3. These results show that the comparator can produce the required comparator input-offset accuracy for the readout circuit. Figure 5.3 Comparator Offset Monte-Carlo Simulation Result In addition, the propagation delay of the comparator is also measured in Monte Carlo simulations. Each simulation is done by holding the inverting input at 1.65V and stepping the non-inverting input voltage from 10mV (worst case input-offset) below to 10mV above the inverting input for a rising edge step response. The step input is a 20mV step with a rise time of 1ns. The results show that the worst case propagation delay is 1.86ns, which is quite within the time that the comparator clock V CK remains H (0.5μs) in one cycle. Therefore, the CDC clock phase Φ 1 will allow sufficient time for the comparator output to settle. 47

58 5.3 Panel Capacitance (Channel) Selector This block generates the control signals for the touch panel channel selection switches that connect each panel capacitance to the readout circuit. In order to cover the entire 10.4 touch panel with 21 rows and 29 columns, each row selection switch is sequentially activated while the column selection switch is on. This operation is repeated with the next column switch until all the columns are scanned. The on-period of each row selection signals is equal to the detection period M, and the duration of the column selection signal is 29 M, where M can be adjusted to 33, 65, 129, and 257 clock cycles. However, to indicate the starting of the frame, the START signal will be generated when the first column and the first row switch is selected. In addition, the RST signal will be generated at the beginning of each row selection, to discharge the panel capacitance. The channel selection, START, and RST signals are realized with counters and digital logic. 48

59 CHAPTER VI EXPERIMENTAL RESULTS The proposed touch sensor readout circuit is implemented using CMOS 0.35 µm technology (4-metal, 2-poly) with supply voltage of 3.3 V. Figure 6.1(a) shows the chip micrograph, where common centroid and inter digitized layout techniques are used with dummy devices to improve the matching. In addition, analog and digital power lines are separated in order to eliminate the digital switching noise, and major analog signals such as the integrator output V x is shielded. The core area of the proposed circuit is 0.1 mm 2. Figure 6.1(b) shows the test setup including the test board and the touch panel. The test board is a 4 layer PCB, where the top and bottom plates are used for power supplies, clocks, CDC input/output signals with separate analog and digital domains. In addition, the intermediate layers are used for row and column panel capacitance selection lines. The comparator threshold voltage V TH is externally supplied from a DC power supply through a low noise amplifier (AD797, Analog Device), and the reference clock is provided from a clock generator. The touch panel is a 10.4 projected mutual capacitance type. 49

60 Figure 6.3 Chip Micrograph (a) and Measurement Setup (b) The basic characteristics of the passive sigma-delta modulator including frequency spectrum, decimated output Differential Non-Linearity (DNL) and Integral Non-Linearity (INL) plots, and offset error are measured. For this case in order to evaluate the sigma-delta modulator itself, M is set to 65, C s is set to 0.75 pf and C p is fixed to 1 pf, which makes the output pulse number 31 in one detection period of 65 clock cycles. By varying the input voltage V in from 0 to 3.3 V, the output pulse number will change linearly with V in, therefore the sigma-delta modulator is equivalent to a 5-bit ADC. Figure 6.2 shows the frequency spectrum of the passive sigma-delta modulator. A 6 db sine wave with frequency of khz is used as the input voltage. Although the 2nd and 3rd harmonic tones are present, the spectrum shows a first order noise shaping behavior. 50

61 Figure 6.4 Passive Sigma-delta Modulator Output Spectrum Figure 6.3 shows the decimated output DNL and INL plot. For an ideal n-bit ADC the output is divided into 2 n uniform steps, and each step has the same fixed width. Any deviation from the ideal step width is defined as the DNL, and DNL errors accumulate to produce a total INL. To measure the DNL and INL, the input voltage range from 0 to 3.3V is divided into 32 equal levels with each level of 103mV corresponding to the 0 to 31 output pulse numbers, and each 103mV is again split into 4 intervals of equal width with each interval representing 0.25LSB. The pulse numbers are measured for each interval in each 103mV, the difference between the measured step width and ideal for one input level is the DNL for the corresponding digital output value as shown in Figure 6.3. The INL of each digital output value is obtained by accumulating the DNL values before that point. The results show that the maximum DNL and INL are 0.5 LSB and LSB, respectively. 51

62 Figure 6.5 DNL and INL Plot of Decimated Sigma-delta Modulator Output Figure 6.6 Input-output Transfer Characteristic of Decimated Sigma-delta Modulator Furthermore, the offset error of the decimated sigma-delta modulator output is obtained. The offset error of an ADC is defined as the difference between the actual input value and the ideal input value of the starting points of the input-output transfer characteristic. To measure the offset error, the input voltages at which the digital output code increments by 1 are measured, and marked in the input-output coordinate as the transition points. By drawing a straight line which makes the transition points distributed 52

63 evenly along the line, the actual input-output characteristic is obtained. Figure 6.4 shows the measured and ideal input-output transfer characteristic, and only lower levels are shown for clarity. The offset error is measured to be 3.82 mv. Figure 6.7 Three Different Touch Locations for the Panel The operation of the sensor readout circuit is verified by monitoring the output waveform with different panel conditions (touched and un-touched) at different panel locations. Furthermore, in order to accurately capture the output waveform corresponding to each touch panel location, the RST and START signals are applied to the data acquisition unit (NI, DAQ USB 6361) that generates a trigger signal for the oscilloscope. The DAQ is pre-programmed to generate the trigger signal which enables the oscilloscope to capture the output waveform corresponding to arbitrary panel locations by counting the number of RST signals synchronized with the START signal. Figure 6.5 shows the three different panel locations (edge, middle and corner), where the panel is touched and untouched with a finger. The CDC operation is verified with different panel locations, since the panel capacitance variation can be different depending on the location [19]. Figure 6.6(a), (b), and (c) show the proposed CDC output waveform for the panel 53

64 edge, middle, and corner with touched and untouched condition. In this case, the sampling capacitor C s is set to 1 pf with M = 33 for all cases. The yellow and blue waveforms represent the RST signal and the CDC output, respectively. The N pluse for edge, middle, and corner (un-touched : touched) are (8 : 11), (9 : 12), and (9 : 11), respectively. Based on Figure 3.5, this indicates the panel capacitance including the pad parasitic (1 pf to 2pF) and the PCB parasitic is around 22 pf to 27 pf for the un-touched condition, and changes to around 12 pf to 18 pf for the touched condition. The panel edge shows slightly larger capacitance than the middle or corner. However, the above results are for a specific touch panel. Other touch panels have different panel capacitance values, which may require different M and C s values to accurately detect the panel condition. In addition, the touch sensor readout circuit total power is obtained from measuring the average currents from the analog and digital power supplies to the circuit using a multi-meter. The total power of the proposed touch sensor readout circuit is 65 µw, which is the product of the supply voltage of 3.3 V and the measured total current of 19.6 µa. 25 µw is consumed in the passive sigma-delta modulator and 40 µw is consumed in the panel capacitance selection block. 54

65 Figure 6.8 CDC Output Waveforms Corresponding to Each Touch Location (a) Edge (b) Middle (c) Corner The SNR of the proposed CDC is 31.4 db, where the SNR is obtained using the touch strength and RMS touch noise collected from 100 different touch panel locations that are evenly distributed. The SNR measurement is based on the following equations [3], 55