a fully-differential correlated doubling sampling readout ... · readout circuit for...

JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.15, NO.3, JUNE, 2015 ISSN(Print) 1598-1657 http://dx.doi.org/10.5573/JSTS.2015.15.3.349 ISSN(Online) 2233-4866

Manuscript received Oct. 27, 2014; accepted Apr. 10, 2015 school of electrical and computer engineering, Ulsan National Institute of Science and Technology, Ulsan, Republic of Korea E-mail : [email protected]

A Fully-Differential Correlated Doubling Sampling Readout Circuit for Mutual-capacitance Touch Screens

Kihyun Kwon, Sung-Woo Kim, Franklin Bien, and Jae Joon Kim

Abstract—A fully-differential touch-screen sensing architecture is presented to improve noise immunity and also support most multi-touch events minimizing the number of amplifiers and their silicon area. A correlated double sampling function is incorporated to reduce DC offset and low-frequency noises, and a stabilizer circuit is also embedded to minimize inherent transient fluctuations. A prototype of the proposed readout circuit was fabricated in a 0.18 mm CMOS process and its differential operation in response to various touch events was experimentally verified. With a 3.3 V supply, the current dissipation was 3.4 mA at normal operation and 140 mA in standby mode. Index Terms—Sensor interface, readout circuit, Touch screen, differential, mutual capacitance

I. INTRODUCTION

Mutual-capacitance touch screens have become popular interfaces for mobile displays and their sensitivity has been increasingly augmented to provide additional functions such as gesture sensing or hovering [1]. Moreover, as mobile devices are continuously becoming thinner and lighter, touch screen panel (TSP) modules are also required to be thinner. According to the touch-sensor location in the display panel, the TSPs are classified as add-on, on-cell, and in-cell [2]. As in-cell TSPs that embed touch sensors in the display glass

become dominant [2], their thin form factor requirement severely increases display noise interference coupled from the display panel [3]. In addition, there exist various environmental noises such as battery-charger noise [4], fluorescent lamp noise and HUM noise [5].

Thus, in order to improve the immunity to these display and environmental noises and also achieve better sensitivity, recent researches have tried to make the touch-sensing topology differential as much as possible and also adopt noise-avoiding techniques such as chopping method [2-7]. But most sensing structures are still pseudo-differential, which is mainly caused by the usage of single-ended charge amplifier and results in limited rejection of common-mode noises. The chopping method has been utilized as another common technique to avoid low-frequency noises by alternating the operating frequency, but it easily generates various unwanted spurs due to its frequency-converting operations and therefore require additional filtering.

In this work, we propose and implement a fully-differential detection architecture by adopting differential charge amplifiers as front ends and also embedding the correlated double sampling (CDS) function [8] to remove DC offset and reduce low-frequency noises instead of the chopping technique. Additionally, a pulse-shape stabilizer function is also included to compensate transient signal fluctuations caused by rectangular TSP pulse signals, thereby minimizing unwanted possible instant detection errors.

Section II shows the architecture of this work. Section III shows the circuit description. Section IV and V show the measurement results and conclusion.

350 KIHYUN KWON et al : A FULLY-DIFFERENTIAL CORRELATED DOUBLING SAMPLING READOUT CIRCUIT FOR …

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(b)

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Fig. 1. TSP Sensing architectures (a) pseudo-differential I, (b) pseudo-differential II, (c) proposed fully-differential.

II. ARCHITECTURE

A conventional pseudo-differential TSP sensing architecture is shown in Fig. 1(a). Generally, TX drivers send out rectangular pulses to the row driving lines of the TSP, and their coupled signals via touch cells’ mutual capacitances are connected to single-ended charge amplifiers where they are capacitively amplified and their amplification gain is mainly decided by the ratio of mutual capacitance between the driving line and the sensing line to feedback capacitance of the charge amplifier. Each cell’s touch event changes its equivalent mutual capacitance and its corresponding charge amplifier output also. Then, two charge-amplifier outputs of adjacent cells in the same row are paired and further amplified in the following differential amplifier, and finally delivered to an analog-to-digital converter. However, this kind of pseudo-differential architecture may not recognize some special touch events when only two adjacent cells are touched because these adjacent cell signals have the same level and they are not sensed differentially. Therefore, some recent works [3, 4, 6] inserted additional differential amplifiers between adjacent touch-cell pairs to cover the whole touch events as shown in Fig. 1(b). This improved pseudo-differential architecture can solve the previous adjacent cell-pair touch problem, but it requires almost twice number of differential amplifiers, resulting in more power consumption and larger silicon area.

This work proposes a fully-differential sensing architecture to solve this adjacent cell-pair touch problem without additional amplifiers and silicon area which is shown in Fig. 1(c). The proposed differential architecture utilizes fully-differential charge amplifiers as front ends, instead of previous single-ended charge amplifiers that make the overall architecture pseudo-differential. The previous adjacent touch problem is solved by placing an input multiplexer at the minus input of the differential charge amplifier to provide two differential-pair compositions of touch cells, that is, an original adjacent pair and another adjacent pair located at its opposite side. Therefore, this input multiplexing enables the detection of three adjacent cells by changing the control signal of M. With M = ‘0’, the same detection as in conventional methods is performed, and M = ‘1’ activates the other cell pair that is shifted to the right by one cell from the

JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.15, NO.3, JUNE, 2015 351

original pair. For load balancing among TSP sensing lines, a dummy multiplexer is amended at the plus input of the differential charge amplifier. Through this input-multiplexed fully-differential sensing architecture, the whole touch events can be effectively detected, reducing the number of amplifiers and their power consumption at least by 67%. When the gain of the charge amplifier is maximized, this differential readout circuit generates +1’s or -1’s only at the boundary of the touch area if the overall gain is maximized. This boundary indication property can be utilized to provide a new function of boundary or area detection. The designed fully-differential amplifier whose details are given in Section III, also includes the correlated double sampling function and the pulse-shape stabilizer to remove DC offset and low-frequency noises and also minimize transient fluctuations from TX pulses.

III. CIRCUIT DESCRIPTION

The proposed fully-differential method is implemented as shown in Fig. 2, where an overall TSP system is composed of a touch-screen panel, a readout circuit, and a microcontroller with a successive approximation register (SAR) analog-to-digital converter (ADC). If a TX driver in Fig. 1 sends a stimulus signal to touch

panels through a TX line, it is delivered to all RX lines via mutual capacitors between the TX and the RX lines. Then, front-end charge amplifiers in the readout circuit work as fully-differential capacitive amplifiers and detect the variation of the mutual capacitance that is mainly caused by touch events. The detected signal is filtered by an active low pass filter to reduce high frequency noises. Finally, the filtered signal is converted to a digital code for digital signal processing in a microcontroller.

A simplified schematic of the differential charge amplifier is also shown in Fig. 2, where CM,N is a mutual capacitance of Nth touch cell responding to a TX driving signal (DTX) and CP,N is its parasitic capacitance. The proposed differential circuit senses three adjacent touch cells through two sequential comparisons of touch-cell pairs. When the MUX control signal of M is ‘0’, two RX inputs of RXN and RXN+1 are capacitively amplified. This corresponds to the operation of conventional differential sensing methods. However, if these two adjacent cells are touched equally, it gives no output which is indistinguishable from that of untouched events. With M = ‘1’ where RXN+2 and RXN+1 are connected to the amplifier inputs, the adjacent cell-pair touch-sensing problem is resolved. If all three cells are equally touched, this amplifier also does not provide any detection, but their detections are provided in adjacent charge

Fig. 2. Fully-differential mutual-capacitance TSP readout circuit.


amplifier’s multiplexed operations. When the stimulus signal DTX is applied to mutual capacitors of CM,N and CM,N+1, their charges of QM,N, QM,N+1 flow into input nodes of the differential amplifier. The difference of QM,N and QM,N+1 which is originated from their mutual capacitance difference is converted to the voltage and amplified through the feedback capacitor CF. However, both touch cells are stimulated with same polarity signal of DTX, which makes unnecessary transient fluctuations in common-mode input voltage of their differential amplifier. At this point, even small difference of their parasitic capacitances in the cell pair (CP,N - CP,N+1) might convert this common-mode fluctuations into differential noises. In order to reduce this input common-node fluctuation and the unintended noises from unbalanced parasitic capacitances, the differential input part is augmented to include a stabilizer circuit that is composed of two capacitors CS and an inverted signal of DTX. If the stabilizer is activated and the CS value is made close to CM, transient fluctuations of VIN,1 and VIN,2 at signal transitions of DTX are absorbed by two stabilizer capacitors of CS, resulting in minimizing the differential input-node fluctuations as seen also in Fig. 2.

A correlated double sampling (CDS) function is also incorporated in Fig. 2 to remove DC offset and low-frequency noises. It utilizes two non-overlapping clocks of Ф1 (Sampling) and Ф2 (Differential comparison), where Ф2 is synchronized to DTX. At Ф1 phase, a DC-offset voltage (VOS) is stored in the feedback capacitor CF. At Ф2 phase, the stored VOS is deducted from output signal, resulting in the DC-offset cancellation of the charge amplifier. Its post-simulations with parasitic effects extracted form a prototype layout were performed and their results are given in Fig. 3. Each up-side graph is a DTX waveform to indicate the operating cycle and each down-side graph presents output waveforms of readout circuit with and without the CDS function. Blue dashed lines represent differential readout operations without CDS and red solid lines describe their offset-cancelled waveforms when the CDS is activated. Fig. 3(a) shows simulation results when both TSP cells are untouched and their differential charge amplifier contains DC offset. If the CDS is not activated, the DC offset appears in the differential output during the sampling cycle and remains also during the comparison cycle. With the CDS function activated, it can be seen that the DC offset is removed

during the comparison cycle. Fig. 3(b) shows simulation results when only one TSP cell is touched. Like the untouched case, the DC offset which remains unremoved during the full period is effectively removed by activating the CDS function.

IV. MEASUREMENT RESULTS

A prototype of the proposed differential readout circuit for mutual-capacitance touch screens was fabricated in a 0.18 mm CMOS process. Fig. 4 shows the chip photography with an area of 2 mm ´ 2 mm, integrating 8-channel readout circuits for the verification of various

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Fig. 3. Post-simulation results without and with the CDS function (a) at untouched event, (b) at single-touch event.


touch events, where each 2-channel differential pair occupy 0.18 mm2. With a 3.3 V supply, the current consumption of each channel was 3.4 mA in the normal operation. In the standby mode where most readout circuits are deactivated except for biasing circuits, the current consumption was only 140 mA. Measured waveforms of the fabricated prototype are given in Fig. 5. For the single-point touch event that is shown in Fig. 5(a), the readout output waveforms show the differential detection operation and its multiplexed output of Δ = ‘0’ confirms that it is a single-point event. Fig. 5(b) shows the measured waveforms when there is a concurrent touch event where a pair of differential TSP cells are touched at the same time. The differential output cannot detect the touch events, but its multiplexed output of Δ =

‘-1’ informs that both cells were touched simultaneously. These results verify the validity of the proposed TSP readout architecture and its fully-differential operation including the input multiplexing.

Fig. 6 shows the measurement environment which consists of a 49 mm x 100 mm mutual capacitance touch screen panel and a test board with the designed readout chip. The signal-to-noise ratio (SNR) of the overall readout architecture was measured by utilizing the definition in [9]. Based on 200 untouched and touched events that were sampled by an ADC, the signal level was measured by the average difference of the touched and the untouched signals and the noise level was given by the rms noise of all sampled signals. The measured SNR of the proposed differential TSP ROIC was 47dB, showing 30dB improvement of the SNR when compared with the previous pseudo-differential work of [7]. The measured waveform for the SNR was shown in Fig. 7.

V. CONCLUSIONS

A fully-differential sensing method for capacitive touch screens was proposed and its silicon prototype was experimentally verified. The proposed method showed

Fully-DifferentialCharge Amplifiers

Active Low-Pass Filters

8-CH Readout Circuitsfor Differential TSP Sensing

Fig. 4. Chip microphotograph.

(a) (b)

Fig. 5. Measured results of fully-differential TSP readout circuit (a) at single-touch event, (b) at multi-touch event.

Fig. 6. Measurement environment to verify TSP readout function.

Fig. 7. Measured signal-to-noise ratio (SNR) of the proposed TSP readout circuit.


that the adjacent cell-pair detection problem in differential sensing circuits could be effectively eliminated with minimal set of differential charge amplifiers, and it was also verified that overall power consumption could be reduced by at least 50%. Additionally, the CDS and the stabilizer functions were successfully embedded in the charge amplifier to reduce DC offset, low-frequency noises, and transient signal fluctuations. This fully-differential work is supposed to contribute to improvement of the overall noise immunity and sensitivity and also advancement of mobile touch sensing technologies.

ACKNOWLEDGMENT

This research was supported by the Center for Flexible OLED Displays funded by Samsung Display Co., Ltd.

REFERENCES

[1] Y. Hu, et al., “3D gesture-sensing system for interactive displays based on extended-range capacitive sensing,” IEEE Int. Solid-State Circuits Conf. Dig. Tech. Papers, pp. 212–213, Feb., 2014.

[2] J.-H. Yang,, et al, “A Noise-Immune High-Speed Readout Circuit for In-Cell Touch Screen Panels,” IEEE Trans. Circuits Syst. I, Reg. Papers., 60, (7), pp. 1800-1809, 2013.

[3] J.-E. Park, D.-H. Lim and D.-K. Jeong, “A reconfigurable 40-to-67 dB SNR, 50-to-6400 Hz frame-rate, column-parallel readout IC for capacitive touch-screen panels,” IEEE J. Solid-State Circuits, vol. 49, no. 10, pp. 2305–2318, Oct., 2014.

[4] J.-H. Yang, et al., “A highly noise-immune touch controller using filtered-delta-integration and a charge-interpolation technique for 10.1-inch capacitive touch-screen panels,” IEEE Int. Solid-State Circuits Conf. Dig. Tech. Papers, pp. 390–391, Feb., 2013.

[5] H. Shin, et al., “A 55dB SNR with 240Hz frame scan rate mutual capacitor 30x24 touch-screen panel read-out IC using code-division multiple sensing technique,” IEEE Int. Solid-State Circuits Conf. Dig. Tech. Papers, pp. 388–389, Feb., 2013.

[6] D.-H., Lim, J.-E. Park, and D.-K. Jeong, “A low-

noise differential front-end and its controller for capacitive touch screen panels,” Proc. 38th Eur. Solid-State Circuits Conf., pp. 237–240, Sept., 2012.

[7] I.-S. Yang and O.-K. Kwon, “A touch controller using differential sensing method for on-cell capacitive touch screen panel systems,” IEEE Trans. Consumer Electron., 57, (3), pp. 1027–1032, May, 2011.

[8] C. C. Enz and G. C. Temes “Circuit technique for reducing the effects of op-amp imperfections: autozeroing, correlated double sampling, and chopper stabilization,” Proc. of the IEEE, vol. 84, no. 11, pp. 1584–1614, Nov., 1996.

[9] K. Lim, K.-S Jung, C.-S Jang, J.-S. Baek and I,-B. Kang, “A Fast and Energy Efficient Single-Chip Touch Controller for Tablet Touch Applications,” Journal of Display Technology, vol. 9, no. 7, pp. 520–526, 2013.

Kihyun Kwon was born in Daegu, Korea, in 1990. He received the B.S. degree in the Department of Electrical and Computer Engineering from Ulsan National Institute of Science and Technology (UNIST), Korea, in 2014. He is currently pursuing the

M.S. degree in the Department of Electrical and Computer Engineering from Ulsan National Institute of Science and Technology (UNIST), Korea. His research interests include data converters, automotive circuits and sensor readout circuits.

Sung Woo Kim was born in Jinhae, Korea, in 1987. He has received his B.S degree in 2012 from Department of Electronic Engineering, Dankook University, Juk-jeon, Korea. He is currently pursuing the Ph.D. degree in the Department of Electrical and

Computer Engineering from Ulsan National Institute of Science and Technology (UNIST), Korea. His research interests include CMOS readout circuits for bio/environmental sensors and data conversion.


Franklin Bien received the B.S. degree in electronics engineering from Yonsei University, Seoul, Republic of Korea in 1977, the M.S degree in electrical and computer engineering from Georgia Institute of Technology, Atlanta, GA, USA in

2000, and the Ph.D degree in electrical and computer engineering from Georgia Institute of Technology, Atlanta, GA, USA in 2006. He is currently an Associate Professor in the School of Electrical and Computer Engineering at Ulsan National Institute of Science and Technology (UNIST), Ulsan, Republic of Korea. Prior to joining UNIST in 2009, Dr. Bien was with Staccato Communications in San Diego, CA as a Senior IC Design Engineer working on analog/mixed-signal IC and RF front-end circuits for Ultra-Wideband (UWB) products such as Wireless-USB in 65-nm CMOS technologies. Prior to working at Staccato, he was with Agilent Technologies and Quellan Inc., developing transceiver ICs for enterprise segments that improve the speed and reach of communication channels in consumer, broadcast, enterprise and computing markets. His current research interests include circuits for wireless power transfer technologies, analog/RF IC design for consumer electronics, vehicular electronics, and biomedical applications.

Jae Joon Kim received the B.S. degree in electronic engineering from Hanyang University, Seoul, Korea, in 1996 and the M.S. and Ph.D. degrees in electrical engineering from the Korea Advanced Institute of Science and Technology, Daejeon, Korea, in

1998 and 2003, respectively. From 2000 to 2001, he was with Berkana Wireless Inc., San Jose, CA (now merged into Qualcomm Inc.), where he was involved in designing wireless transceivers. From 2003 to 2005, he was with Hynix Semiconductor, Seoul, working on wireless transceivers and smart-card controllers. From 2005 to 2011, he was a Deputy Director with the Korean government, Ministry of Information and Communications and also Ministry of Trade, Industry & Energy. From 2009 to 2011, he was also with Georgia Institute of Technology, Atlanta, GA as a research engineer II. Since 2011, he has been an Assistant Professor with Ulsan National Institute of Science and Technology, Ulsan, Korea. His research interests include integrated circuits for various sensor systems, wireless transceivers, consumer electronics, biomedical appliances, and automotive electronics.

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