Download An Excessive Current Subtraction Technique to Improve

JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.16, NO.3, JUNE, 2016
http://dx.doi.org/10.5573/JSTS.2016.16.3.375
ISSN(Print) 1598-1657
ISSN(Online) 2233-4866
An Excessive Current Subtraction Technique to
Improve Dynamic Range for Touch Screen Panel
Applications
Sanghyun Heo, Hyunggun Ma, and Franklin Bien
Abstract—A current subtraction technique with
parallel operation system is proposed to remove
excessive current in touch screen application. The
proposed current subtraction remove the current
which go into the input node of charge amplifier. The
value of subtraction current is same with current
when touch screen is not touched. As a result, charge
amplifier output is only proportional to variation of
mutual capacitor, which make dynamic rage is
increased. Also, Transmitter (Tx) driving signal and
subtraction driving signal are out of phase each other.
Thus, noise generated in Tx is cancelled. The
proposed IC is implemented in a mixed-mode 0.18-um
CMOS process. Overall system is designed for touch
screen panel (TSP) with 16 driving lines and 8 sensing
lines. 5-V supply voltages are used in the proposed
circuits. For multiple Tx driving signal, Walsh codes
are used and signal frequency is 300 khz. By using
proposed technique, dynamic rage is improved 36 dB.
Index Terms—Touch screen, high voltage, parallel
operation, current subtraction
I. INTRODUCTION
Multiple driving signal methods are used to satisfy
high signal-to-noise ratio (SNR) in touch screen
applications [1-3]. Code-division multiple sensing
(CDMS) [3] read-out IC, which transmit multiple coded
Manuscript received Mar. 14, 2015; accepted Jan. 21, 2016
Ulsan National Institute of Science and Technology, Ulsan, Korea
E-mail : [email protected]
signal to driving lines simultaneously, is one of the multi
touch sensing methods. CDMS read-out IC uses multiple
signal, it has higher SNR compared to time-interleaved
sensing method. Also, comparing with time-interleaved
sensing method on same SNR, CDMS method has higher
speed. By using high speed property, CDMS can
compensate decreased speed in large load system. In
CDMS read-out IC, each sensing channel faces multiple
driving channels. Multiple driving signals which
simultaneously go into driving channels meets at sensing
channel and make large current on each sensing line as
shown in Fig. 1. Large current issue reduces the dynamic
range [1], which is difference between touched and
untouched voltage in touch screen system. Sensitivity
and accuracy are related to dynamic range. To ensure
high sensitivity and accuracy, wide dynamic range is
necessary.
In this paper, a current subtraction method utilizing
capacitance and out-phase driving voltage is proposed to
cancel the excessive current. By cancelling the current
which is generated when touch screen is not touched,
dynamic range is effectively increased.
II. PROPOSED CURRENT SUBTRACTION
TECHNIQUE
Concept of proposed current subtraction circuit has
been depicted in Fig. 2. By using CDMS read-out IC for
TSP, a large number of Tx voltage signals go through
TSP at the same time. In this circuit, Walsh code is used
as multiple driving signals. High value in Walsh code is
expressed with in-phase sinusoidal signal and low value
376
SANGHYUN HEO et al : AN EXCESSIVE CURRENT SUBTRACTION TECHNIQUE TO IMPROVE DYNAMIC RANGE FOR …
Fig. 1. Excessive current which is generated in multiple driving
signaling.
subtraction capacitance. A plurality of stimulated current
from multiple in-phased Tx voltage signal is mixed at
input of charge amplifier. Since polarity between the
current from Tx input voltage signal and the current from
subtraction circuit is opposite, the mixed current is
countervailed. By using this subtraction circuit, excessive
current caused by parallel driving method is eliminated.
Futhermore, there are lots of harmonic and switch
noise in Tx signal [6]. In this process, noise generated in
Tx is cancelled since Tx driving signal and subtraction
driving signal are out of phase each other.
Voltage output is expressed by mutual capacitance and
impedance of feedback circuits. Shown in Eq. (1),
voltage output is proportional to summation of mixed
current. If summation current is zero, untouched mutual
capacitance can be omitted by subtraction capacitance.
As a result, only Δ-mutual capacitance is measured.
Continuous-mode charge amplifier plays role of bandpass filter. It is possible to reduce low frequency noise.
In Eq. (1), Coffset-k is capacitor difference between mutual
capacitor in TSP and subtraction capacitor. In case,
subtraction capacitor and mutual capacitor are different,
subtraction offset is occurred which is caused by
difference of mutual capacitor and subtraction capacitor.
Even though subtraction offset is occurred, these value is
smaller than offset such as un-touched mutual
capacitance. Since subtraction offset is small, subtraction
offset cannot make output voltage saturated. Also, these
subtraction offset is easily removed by digital calibration
since subtraction offset value is always same.
Vout = I ´ Z f
N
= å ( sCs - kVTx - k + sCsubV `Tx - k ) ´ Z f
k =1
Fig. 2. Proposed Continuous-mode charge amplifier utilizing
current subtraction technique.
N
= å ( sCs - kVTx - k + sCsubV `Tx - k ) ´
k =1
is expressed with out-phase sinusoid signal [4, 5].
Current-subtraction circuit is placed at input side of
analog-front end. Current subtraction circuit consists of
variable capacitor array and voltage source, which is outphase Tx voltage signal. Each capacitance in capacitor
array is same with mutual capacitor when untouched.
Fig. 2 shows that continuous-mode charge amplifier
with current subtraction technique. Subtraction current,
which is generated by subtraction capacitor, is
determined by out-phased Tx voltage signal and
=
Rf
( R f / sC f )
R f + 1/ sC f
N
1 + sR f C f
å (sC
V
s - k Tx - k
+ sCsubV `Tx - k )
k =1
N
å (C
=
V
s - k Tx - k
Cf
N
å (C
offset - k
=
+ CsubV `Tx - k )
k =1
k =1
Cf
(if sR f C f >> 1)
+VCs - k )
(Coffset - k = Cs - kVTx - k + CsubV `Tx - k )
(1)
JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.16, NO.3, JUNE, 2016
6
Output [V]
5
Un-touch with CF=15pF
Touch with CF =15pF
15mV
4
3
2 Static Value
1
0
5.0µ
=1.8V
6.0µ
7.0µ
8.0µ
Time [s]
9.0µ
10.0µ
Fig. 3. Simulation result of conventional read-out IC.
5
Un-Touch with CF=0.4pF
Touch with CF=0.4pF
4
Output [V]
Previous work, current subtraction technique is used in
discrete mode [1]. To implement current subtraction
technique in discrete mode, current mirror and switches
are used to make subtraction current. The advantage of
current subtraction technique in discrete mode is that
additional capacitors are not used. However, it is
necessary to take additional subtraction time in discretemode subtraction technique. Also it is hard to control
subtraction current as channel length modulation of
current mirror and lots of switching noise.
In this paper, current subtraction technique in
continuous mode is proposed. The advantage of proposed
current subtraction technique in continuous mode,
current subtraction is conducted continuously. It means
there are no additional current subtraction time. Also,
charge amplifier with feedback resistor and capacitor is
operated as band-pass filter in continuous mode, which
means noise component of which frequency is close to
input frequency is attenuated by band-pass filter. Thus, it
is possible to get the advantage of current subtraction
technique and continuous-mode charge amplifier as
band-pass filter.
377
990mV
3
2
1
III. SYSTEM PERFORMANCE
Mutual capacitance becomes smaller when touch
screen is touched by 10%. In this simulation, subtraction
capacitance is same with sensing capacitance by 1.2 pF.
If mutual capacitance is touched, the value of mutual
capacitance is changed amount of 1.1 pF. It can be
accomplished by controlling subtraction capacitance
value. In order to check system performance, 8bit
parallel operation system is used [2]. Walsh code is used
as orthogonal code signal.
Even though Walsh code is the perfect orthogonal
code signal, it has driving energy concentration phase [7].
In that phase, excessive current make output voltage
saturated. Current subtraction technique remove
excessive current in driving energy concentration phase
so that it is possible to use Walsh code.
Fig. 3 shows the charge amplifier output voltage
without current-subtraction technique. Current variation
is very large between 0 to VDD (Feedback capacitance is
15pF), however, the difference of output voltage when
touch screen touched or not is so small that dynamic
range of charge amplifier is 15 mV. Fig. 4 shows the
0
-4.0µ
-2.0µ
0.0
2.0µ
4.0µ
Time [s]
Fig. 4. Measured result of proposed read-out IC.
charge amplifier output voltage without currentsubtraction technique. Since proposed read-out IC
remove static value, it is possible to reduce feedback
capacitor value. Cf is chosen as 0.4 pF, in that case
dynamic range is 990 mV. By utilizing the current
subtraction circuit, it is possible to reduce feedback
capacitance and make dynamic range increased. Result
shows that 36 dB improvement of dynamic range.
By utilizing the current subtraction circuit, it is
possible to reduce feedback capacitance and make
dynamic range increased. Thus, feedback capacitance is
decided as 0.4 pF by applying current subtraction circuit
and dynamic range is improved from 15 mV to 1210 mV.
Result shows that 36 dB improvement of dynamic range.
Fig. 5 show that chip photograph of TX 16 and RX 8
channel and Fig. 6 show that test board TX and RX.
378
SANGHYUN HEO et al : AN EXCESSIVE CURRENT SUBTRACTION TECHNIQUE TO IMPROVE DYNAMIC RANGE FOR …
TX Total 16 Channel
RX Total 8 Channel
[2]
TX 2 Channel
RX 1 Channel
Fig. 5. Chip photograph of TX 16 and RX 8 channel.
[3]
External Voltage
External Voltage
TX Test Chip
RX Test Chip
[4]
Control Signal Port
Control Signal Port
[5]
Fig. 6. Test board TX and RX.
[6]
V. CONCLUSIONS
A current-subtraction technique is presented for
CDMS type read-out IC in this paper. By applying these
circuits to charge amplifier for TSP, it can increase
dynamic range by 36 dB and reduce current variation
which is generated in CDMS read-out IC. This proposed
read-out IC is implemented in a 0.18-um mixed-mode
CMOS process.
[7]
with current subtraction technique," European Solid
State Circuits Conference (ESSCIRC), ESSCIRC
2014 - 40th, vol., no., pp.327,330, 22-26 Sept. 2014
H-C. 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,” ISSCC Dig. Tech Papers, Feb
2012, pp 388-389
William R. Krenik, Anand Dabak, “TouchSensitive Interafac And Method Using Orthogonal
Signaling”, U.S Patent 20120056841. Mar. 8,2012
Christoph Horst Krah, et al. “Multiple
Simultaneous Frequency Detection”, Apple Inc,
U.S Patent 20080309625. Dec. 18, 2008
Steven P. Hotelling, et al. “Simultaneous Sensing
Arrangement”, Apple Inc. U.S Patent 20100328265,
Dec. 30, 2010
Ko, Seunghoon, et al. "A 70dB SNR capacitive
touch screen panel readout IC using capacitor-less
trans-impedance amplifier and coded Orthogonal
Frequency-Division Multiple Sensing scheme."
VLSI Circuits (VLSIC), 2013 Symposium on. IEEE,
2013.
Ma, Hyunggun, et al. "Algorithm for improving
SNR using high voltage and differential Manchester
code for capacitive touch screen panel." Electronics
Letters 50.24 (2014): 1813-1815.
ACKNOWLEDGMENTS
This work was supported by the Basic Science
Research Program through the National Research
Foundation of Korea (NRF) funded by the Ministry of
Science, ICT and future Planning under Grant NRF2014R1A2A2A0100412 and by the MSIP (Ministry of
Science, ICT and Future Planning), Korea, under the
ITRC (Information Technology Research entre) support
program (IITP-2015-H8501-15-1010) supervised by the
IITP (Institute for Information & communications
Technology Promotion).
REFERENCES
[1]
Sanghyun Heo, et al "Dynamic range enhanced
readout circuit for a capacitive touch screen panel
Sang Hyun Heo received the B.S
degree in electrical and electronic
engineering from the Pusan National
University, Pusan, Korea, in 2012.
He is currently working on the
Combined Master-Doctoral program
in electrical Engineering at Ulsan
National Institute of Science and Technology (UNIST),
Ulsan, Republic of Korea. . His current research interests
are Readout IC for flexible touch screen panel and
fingerprint touch screen panel and Analog Mixed Signal
and IC.
JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.16, NO.3, JUNE, 2016
Hyunggun Ma received the B.S
degree in electrical engineering from
Gyeongsang National University,
Jinju, Gyeongsang nam-do, Republic
of Korea, in 2013. He is currently
working on the Combined MasterDoctoral program in electrical
Engineering at Ulsan National Institute of Science and
Technology (UNIST), Ulsan, Republic of Korea. His
current research interests are Readout IC for flexible
touch screen panel and fingerprint touch screen panel,
CMOS RF circuit for biomedical wireless communication and wireless power transfer technologies
especially focused on transceiver IC using high voltage
BCDMOS process.
379
Franklin Bien (M04-) 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/mixedsignal 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.