A High Noise Immunity, Channel Finger Touch Sensing IC Using OFDM and Frequency Translation Technique

Transcription

1 sensors Artcle A Hgh Nose Immunty, Channel Fnger Touch Sensng IC Usng OFDM and Frequency Translaton Technque SangYun Km 1, Behnam Samadpoor Rkan 1, YoungGun Pu 1, Sang-Sun Yoo 2, Mnjae Lee 3, Keum Cheol Hwang 1 ID, Youngoo Yang 1 and Kang-Yoon Lee 1, * 1 College of Informaton and Communcaton Engneerng, Sungkyunkwan Unversty, Suwon 16419, Korea; ksy0501@skku.edu (S.Y.K.); behnam@skku.edu (B.S.R.); hara1015@naver.com (Y.G.P.); khwang@skku.edu (K.C.H.); yang09@skku.edu (Y.Y.) 2 Department of Smart Automoble, Pyeongtaek Unversty, Pyeongtaek 17869, Korea; rapter@kast.ac.kr 3 School of Informaton and Communcatons, Gwangju Insttute of Scence and Technology, Gwangju 61005, Korea; mnjae@gst.ac.kr * Correspondence: klee@skku.edu; Tel.: Receved: 5 Aprl 2018; Accepted: 17 May 2018; Publshed: 21 May 2018 Abstract: In ths paper, a hgh nose mmunty, channel fnger touch sensng IC for an orthogonal frequency dvson multplexng (OFDM) touch sensng scheme s presented. In order to ncrease the sgnal-to-nose rato (SNR), the OFDM sensng scheme s proposed. The transmtter (TX) transmts the orthogonal sgnal to each channels of the panel. The recever (RX) detects the magntude of the orthogonal frequency to be transmtted from the TX. Due to the orthogonal characterstcs, t s robust to narrowband nterference and nose. Therefore, the SNR can be mproved. In order to reduce the nose effect of low frequences, a mxer and hgh-pass flter are proposed as well. After the nose s fltered, the touch SNR attaned s 60 db, from 20 db before the nose s fltered. The advantage of the proposed OFDM sensng scheme s ts ablty to detect channels of the panel smultaneously wth the use of multple carrers. To satsfy the lnearty of the sgnal n the OFDM system, a hgh-lnearty mxer and a ral-to-ral amplfer n the TX drver are desgned. The proposed desgn s mplemented n 90 nm CMOS process. The SNR s approxmately 60 db. The area s 13.6 mm 2, and the power consumpton s 62.4 mw. Keywords: analog front-end; capactve touch panel; orthogonal frequency dvson multplexng; low pass flter; varable gan amplfer; mxer 1. Introducton Recently, as the demand for the varous touch applcatons has grown, the nterest of touch technology has ncreased [1 8]. The touch system s composed of the touch panel and the touch sensng IC. In general, to detect the touch pont, many types of touch panels, such as resstve, capactve, and surface acoustc wave, are used to detect the touch pont. Among many types of touch panels, the capactve touch panel s adopted due to ts advantages, such as the hgh optcal qualty, mult-touch capablty, and so on [9]. In the area of the capactve touch, the projected and mutual capactance of the panel s used mostly. In ths paper, the mutual capactance touch panel s adopted to employ mult-touch and to reduce the ghost pont [10]. In recent touch systems, tme dvson has been used to process the touch sgnal [11 13]. In order to ncrease very small touch sgnals and reduce the nose, ntegrators are mplemented n the recever. By ntegratng the touch sgnal n capactors, the touched small sgnal s amplfed. In addton, to decrease the nose, the large capactors of ntegrators are used because the ntegrators also have the Sensors 2018, 18, 1652; do: /s

2 Sensors 2018, 18, of 17 role of a low-pass flter. Ths structure has a lmt to ncrease the SNR n the analog front-end (AFE) and spends tme to scan all of the channels n the touch panel. Addtonally, the chp becomes large due to capactors n the ntegrators. In ths paper, to overcome the lmt of the AFE, a frequency dvson sensng scheme s used. In general, when usng multple carrers, frequency dvson multplexng (FDM) s wdely used. To avod the dstorton of each frequency, the frequency band of the FDM s wde, whereas the frequency band of orthogonal frequency dvson multplexng (OFDM) s small due to the orthogonal characterstc [14]. As the bandwdth of the OFDM s small, desgnng the AFE for the OFDM s easer than for the FDM. In addton, the varous noses are rejected by orthogonalty of frequences and therefore the OFDM touch sensng scheme can acqure a hgh SNR n ths system. By usng the OFDM sensng scheme, t s possble to detect each channel of the panel at the same tme. Therefore, usng the multple-carrer sensng scheme, the scan rate can be ncreased. In addton, the proposed system can acqure tme to average out nose by sensng one channel several tmes. Fgure 1 shows the conceptual dagram of the fnger touch system. The transmtter (TX) of the AFE generates the OFDM pattern to the touch panel. The recever (RX) of the AFE detects the receved touch sgnal from the touch panel. When the touch s generated on the panel, the capactor of the touch pont s vared and the AFE senses the vared touch sgnal. After the sgnal s compared at each channel, the touch pont s detected. Fgure 1. Fnger touch system. The suggested applcaton s a capactve touch panel and ths panel s composed of 28 TX channels and 16 RX channels. Ths paper proposes the touch soluton for the capactve touch panel applcaton. The rest of ths paper s organzed as follows: Secton 2 descrbes the system descrpton; Secton 3 elaborates the desgn of the AFE archtecture, ncludng the buldng sub blocks; the AFE and ts expermental results are dscussed n Secton 4, followed by conclusons n Secton System Descrpton Fgure 2 shows the crcut model of the touch system. In the touch system, many knds of noses are generated besdes the touch screen panel (TSP). Among the varety of noses, the charger nose and dsplay nose are domnant. Therefore, n order to mprove the touch SNR n the read-out IC, t s mportant to reject them. The charger nose s the nose that s generated from the battery charger. Ths nose s physcally coupled nto the TSP when there are touches. Due to ths nose, performance factors, such as accuracy, can be degraded and the possblty of false or phantom touch can be ncreased. The typcal characterstcs of the charger, besdes the TSP, are the frequency and ampltude of the charger nose, whch are about 34.8 khz and 2 Vpp, respectvely. In electronc devces, the dsplay nose can be conducted to the TSP drectly because the TSP s fabrcated on the dsplay. Thus, the dsplay nose can also degrade the touch performance. Ths nose s almost perodc from 10 khz to 30 khz and the ampltude s random from 500 mvpp to 3 Vpp [14].

3 Sensors 2018, 18, of 17 In ths paper, the touch sensng scheme and structure are proposed so that ambent noses, such as charger and dsplay noses, are mnmzed. Fgure 2. The crcut model and nose sources of the touch system. Fgure 3 shows the characterstcs of the panel model. The panel model has attenuatons of 45 db of 1 MHz and 50 db at 2 MHz. The archtecture of the AFE s determned by the characterstcs of the panel n order to acqure an optmum sgnal processng spot. The attenuaton of ths panel has the characterstcs of a bandpass flter. The flat band of the panel s from 100 khz to 1 MHz. Therefore, to prevent the touch sgnal from decreasng n terms of the touch panel, the flat band of the panel s used. Fgure 3. The characterstcs of the panel model. Fgure 4 shows the sgnal and ambent nose, such as the charger and dsplay noses, n the frequency doman. Snce the noses le n the low frequency, the TX wth the multple carrers translates the baseband sgnals to hgh frequency to reduce the effect of the nose. Before the mxer down-converts the receved sgnals to baseband sgnals, the hgh pass flter (HPF) n front of the RX can reduce the nose as shown n Fgure 4a. After the noses are reduced by the HPF, the mxer of RX converts the receved sgnal down to the baseband by elmnatng the carrers, whle the noses n the low frequency are converted nto a hgh frequency. Therefore, the down-converted sgnal can be passed and the noses can be fltered by the LPF, as shown n Fgure 4b. Through ths mechansm, the noses are reduced and the touch sgnal can be acqured.

4 Sensors 2018, 18, of 17 Fgure 4. Spectrum of the touch sgnal and noses (a) before down-converson and (b) after down-converson. Fgure 5 shows the block dagram of the capactve touch panel AFE. The AFE s composed of the TX and RX scan unt. The TX scan unt conssts of dgtal-to-analog converter (DAC), TX drver, and up-converson mxer for drvng the touch panel. The RX scan unt s composed of the HPF, sngle-to-dfferental converter (StoD), varable gan amplfer (VGA), low-pass flter (LPF) and analog-to-dgtal Converter (ADC). The TX scan unt generates the OFDM code through the DAC and ths sgnal s converted to hgh frequency by the mxer. The mxed sgnal s transmtted to the capactve touch panel by the TX drver. The mxer s used for the OFDM s mult-carrer to avod charger nose. The RX scan unt receves the sgnal to be converted to hgh frequency. The RX scan unt receves the sgnal that s transmtted through the touch panel. In general, because the nose characterstc of the dfferental sgnal processng has a lower nose characterstc than that of sngle sgnal processng, the StoD s used to convert the sngle sgnal to a dfferental sgnal. To decrease the charge nose and low-frequency nose, the HPF s located n front of the RX scan unt. To pass the HPF, the bandwdth of the HPF s desgned n consderaton of the up-converson frequency. After the sgnal from the HPF s changed from the sngle form to dfferental form, the mxer and the LPF converts the hgh to low frequency to recover the OFDM s touch sgnal. The ADC converts the touch sgnal n order to modulate the OFDM. Contrary to the general AFE, the lnearty and output dynamc range are mportant n the OFDM s touch sensng scheme. Snce the OFDM has multple carrers, the lnearty of the mxer and dynamc range of the amplfer are mportant. The mxer s desgned as a hgh P1dB to obtan a hgh lnearty. To reduce the current consumpton and to prevent the dstorted sgnal, the amplfer s desgned as a ral to ral amplfer, whch mproves the dynamc range and a class AB amplfer s used. The AFE s composed of the 6th TX and the 16th scan unt to prevent bulky chp sze and t can scan all of the touch panel channels by swtchng the MUX.

5 Sensors 2018, 18, of 17 Fgure 5. Block dagram of proposed analog front-end for the capactve touch panel. Fgure 6 shows the Walsh code that s used. The formal Walsh functons are used n ths system [15] and the channel s detected through the usng the Walsh code wth an orthogonal characterstc [16] n the OFDM system. In the Walsh functon, as shown n Equaton (1), when the Walsh code length s selected, the PAPR (peak to average power rato) and orthogonalty are consdered. The shorter the code length, the more the PAPR s reduced [17]. The longer the code length, the more the orthogonalty s mproved. To acqure the orthogonalty and low PAPR, the Walsh code length s optmzed to 16. In the 16th channel s code length, 16 knds of the orthogonal sgnal can be obtaned. In ths system, only 6sx knds of orthogonal are selected because of the sxth channel s TX desgn. Fgure 6. The used Walsh code n the OFDM system. Fgure 7 shows the conceptual dagram of the RX. After the down converson, the ADC converts the analog sgnal nto dgtal. At each channel, the receved sgnal from the TSP s reduced by the capactance of the TSP and the nose s added from the TSP. The dscrete cosne transform (DCT) block changes the tme to the frequency doman to process the orthogonal frequency. The channel estmaton can fnd the locaton of the channel from whch the sgnal s generated. The channel estmaton uses the orthogonalty of the frequency. The h j (t) and z j (t) represent the characterstcs of the th row and the jth column and the nose of the jth column from the TSP, respectvely.

6 Sensors 2018, 18, of 17 Fgure 7. Conceptual dagram of the recever. The y j (t) s the total sgnal to be receved from the jth column channel of the TSP. The receved sgnal s refned by the DCT block and channel estmaton. In the OFDM demodulator, the requested frequency can be acqured by the orthogonalty and the nose s rejected by the DCT and the channel estmaton. Fgure 8 shows the DCT block n the dgtal unt. In the dgtal unt, the pont to be touched s detected by the OFDM scheme. In the AFE, the sgnal s transmtted to the dgtal unt usng the orthogonal frequency. The dgtal unt loads the Walsh code at each frequency and the DCT block demodulates each code. The characterstcs of the Walsh code means that f a dfferent code s multpled wth the receved code, the summaton of each element n the Walsh code s zero. On the other hand, t s not zero f the same code s multpled. Fgure 8. Block dagram of DCT. Wal ( 2 m 1 + k 1, t ) = xm(t), k [ m = 1, 2,..., l = 1,..., 2 m 1 ] { xm+1 2k 1 (t) = xm(2t), k 0 t < 0.5 ( 1) k+1 xm(2t k 1), 0.5 t < 1 x k m+1 (t) = { xm(2t), k 0 t < 0.5 ( 1) k xm(2t k 1), 0.5 t < 1 (1) As wth the use of orthogonalty, the noses of several frequences n the panel are rejected and, therefore, the SNR s also mproved.

7 Sensors 2018, 18, of 17 The OFDM scheme uses the DCT. The DCT block receves the sgnal y j = (y j [0],..., y j [N 1]) from the ADC. Y j [0], Y j [1],..., Y j [5] are only acqured through the DCT block because ths OFDM system only uses sx orthogonal frequences as the subcarrer. The DCT block uses Equaton (2), as follows: Y j [k] = N 1 ( ) 2π y j [n] cos N kn, k = 0, 1,..., N 1 (2) n=0 Ĥ j = Y e f f j X e f f / X e f f 2 = ( Y j [0], Y j [1],..., Y j [4] ) (X [0], X [1],..., X [4])/ X e f f 2 ( ) = H 1j X e f f H 6j X e f f 6 + Z e f f j (X [0], X [1],..., X [4])/ ( ) X e f f = H j 2 + Z e f f j X e f f / X e f f 2 = H j + Z e f f j X e f f / ( ) f = j, X e f f = 0 by orthogonalty X e f f j 2 X e f f 2 X e f f Fgure 9 shows the concept of the channel estmaton. The channel estmaton can detect the channel by usng the orthogonalty of the frequency. Snce each carrer comprses orthogonal characterstcs, ths system can fnd the channel by usng the nner product calculaton from Equaton (3). Equaton (2) presents the channel estmaton equaton. The ndex X e f f = (X [0],..., X [5]) to relate wth the orthogonal frequency s ncluded n the channel estmaton. Wth the nner product of ths ndex, the Ĥ j s generated, and ths system can recognze the channel of the touch panel. (3) Fgure 9. The concept of the channel estmaton. 3. Proposed Analog Front-End Archtecture for a Capactve Touch Panel 3.1. Transmtter Fgure 10 shows the transmtter archtecture of the touch panel. It s composed of the DAC, StoD, mxer, and TX drver. The DAC s used to convert the OFDM code to an analog sgnal. The converted OFDM analog sgnal s converted from a low to a hgh frequency by usng the mxer. Due to the characterstc of the capactance of the touch panel, the OFDM analog sgnal s transmtted to the touch panel by the TX drver. In the desgned transmtter, n order to reduce the nose and to avod the charger nose, dfferental sgnal processng n the desgned transmtter s adopted and an up-converson mxer s proposed. Fgure 10. Archtecture of transmtter.

8 Sensors 2018, 18, of Transmtter Mxer Fgure 11 presents the transmtter mxer. The characterstc of the OFDM sensng scheme must be consdered n ths transmtter mxer. In order to acqure a hgh SNR, a large sgnal has to be transmtted to the touch panel. For ths reason, the AFE must have the ablty to drve a hgh dynamc range. In addton, when the sgnal s transmtted to the touch panel, the lnearty has to be guaranteed. The lnearty and removal of the PAPR dsadvantage are mportant n terms of the OFDM sensng scheme. If the lnearty s broken n ths system, t s dffcult for the OFDM modulator to detect the touch pont because the carrers are dstorted by the AFE. Therefore, the P1dB of the mxer s mportant n ths desgn. Fgure 11. Block dagram of transmtter mxer. To acqure lnearty, the passve-type mxng stage has been selected. Snce the gan of the passve mxer s low, the gan stage s desgned n front of the passve mxer to ncrease the gan. Fgure 12 presents the P1dB smulaton result. The proposed mxer guarantees the lnearty untl 5.39 dbm because the DAC generates a 1.2 Vpp OFDM sgnal. Fgure 12. P1dB smulaton of transmtter mxer Transmtter Drver Fgure 13 shows the transmtter drver that drves the capactve touch panel. It s composed of the AB type amplfer and ressters. Ths drver delvers the sgnal as followng the rato of R 1 and R 2. Especally, the proposed drver conssts of hgher voltage metal oxde slcon feld effect transstor (MOSFET) than that of the other desgned block. Snce the TX sgnal s reduced when t passes through the touch panel, t s necessary to ncrease the TX sgnal. For the acquston of a hgh SNR, only ths block s desgned as a hgh-voltage MOSFET.

9 Sensors 2018, 18, of 17 Fgure 13. Transmtter drver. The amplfer to be used n transmtter drver s class AB type amplfer. In ths TX drver, the lnearty n the OFDM sensng scheme, current consumpton n the AFE and the panel drvng ablty s consdered. In addton, n order to acqure the accuracy and to reduce the hgh gan, a PMOS nput type amplfer s adopted. The characterstcs of the class A type s that the sgnal dstorton s lowest and the current consumpton that s greater than those of the other classes of amplfers. In the case of the class B type amplfer, the current consumpton s low. However, because of the push-pull type amplfer, crossover dstorton s generated and the lnearty s not good. In order to explot both class A and B advantages, the class AB type amplfer s used n the transmtter drver. As ths type amplfer operates at a conducton angle between 180 and 360, the crossover dstorton s mnmzed and the lnear characterstc s mproved. In addton, the current consumpton s reduced blow that of the class A type [18]. Therefore, through the adopton of the class AB type amplfer, the lnearty of the OFDM sensng scheme, the mnmzed current consumpton of the AFE, and the touch panel drvng ablty are obtaned Recever Fgure 14 shows the recever archtecture of the touch panel. It s composed of a HPF, StoD, mxer, LPF, and VGA. To ncrease the SNR and reduce the effect of the low-frequency nose, the HPF and the dfferental sgnal processng AFE are adopted. As the transmtter delvers the upconverson frequency to the touch panel, the recever mxer converts the frequency from hgh to low frequency by the recever mxer so that the OFDM sensng scheme can be used n the demodulator. In general, because the sgnal that s passed from the touch panel s too small, the VGA s needed to ncrease the sgnal. The desgned 2 db and 1 db steps of the VGA compensates the gan msmatch at each AFE channel. After the VGA ncreases the analog sgnal, the ADC processes the analog-to-dgtal converson to delver the sgnal to the OFDM demodulator.

10 Sensors 2018, 18, of Recever Mxer Fgure 14. Archtecture of the recever. Fgure 15 shows the recever mxer that s used n the recever. To acqure the transmtter-mxer-derved lnearty, the passve type mxer s used. Due to the mxer nput, the TX and RX mxer structures are dfferent. The transmtter mxer that s close to 0 db VGA s attached n front of the mxer. Snce the nput of the transmtter mxer s too large, the amplfcaton of the sgnal s not needed and the P1dB s too hgh. If the TX mxer s used n the recever, the sgnal of the recever cannot be amplfed. Therefore, the recever mxer needs that the Gm cell can ncrease the sgnal and drve the current. Fgure 15. Block dagram of the recever mxer. Fgure 16 presents the Gm cell that s used n the recever. The cross-coupled Gm cell s adopted to cancel the thrd-order harmonc frequency. Therefore, t can acqure a hgh lnearty and a current gan s obtaned through gan boostng. Fgure 16. Archtecture of the Gm cell.

11 Sensors 2018, 18, of 17 Fgure 17 shows the P1dB smulaton result of the RX mxer. The desgned RX mxer guarantees the lnearty untl 5.03 dbm. Fgure 17. P1dB smulaton of the recever mxer Flter Fgure 18 shows the archtecture of the HPF n the recever. To reduce low-frequency charger nose, ths block s the frst of the AFE that s connected to the touch panel. The desgned HPF s desgned as fourth-order Butterworth, the touch sgnal s not affected, and the cut-off s approxmately 300 khz. Fgure 18. Block dagram of the hgh-pass flter. Fgure 19 shows the archtecture of the LPF n the recever. The role of the LPF n the RX elmnaton of the LO sgnal that remans n the touch sgnal. Snce the down converson frequency s under 300 khz, the sxth-order Butterworth type s adopted and the cut off s desgned at about 300 khz. Fgure 19. Block dagram of the low-pass flter. Fgure 20 shows the flter tunng block for the HPF and LPF. If the resstor and the capactor are vared, the bandwdth of the HPF and LPF are changed. In order to mnmze the capactor and resster varatons followng the process, ths block s desgned to compensate the msmatch of the capactor and the resstor. Ths block can also detect the process varaton by usng the replca flter. Accordng to the value of the replca flter, ths block changes the ressters and the capactors of the HPF and the LPF. After the flter-tunng block calbrates the capactor and the resster, the characterstcs of each AFE channel must be constant.

12 Sensors 2018, 18, of 17 Fgure 20. Block dagram of flter tunng. 4. Expermental Results Fgure 21 shows the chp mcrophotograph for the touch panel. To mnmze the dfference between the TX and the RX, symmetry s mplemented. The chp area s 4.84 mm 4.2 mm n the 90 nm CMOS process. Fgure 22 shows the measurement test board for the AFE. It s composed of a mutual capactance touch panel and fabrcated chp for the verfcaton of the performance of the AFE. The measured touch panel sze s 5.1 nches. Fgure 21. Chp mcrophotograph. Fgure 22. Measurement envronment. Fgure 23 shows the smulaton results of the transmtter. Each transmtted sgnal from TX(0) to TX(5) n Fgure 5. Each transmtted sgnal s added to the touch panel and the added sgnal s receved n the RX. The frequency of characterstc of the sgnal from RX(0) to RX(15) s the same as those of

13 Sensors 2018, 18, of 17 each RX channel of the touch panel, wth the magntude representng the only dfference. By usng the orthogonalty of each Walsh functon, the touched locaton can be detected effcently. Fgure 24 shows the transent dfferental outputs (VO-VOB) of VGA2 n the RX AFE 16th channel as shown n Fgure 5. The 6th TX channel transmts the OFDM sgnal to the 16th RX AFE. Each carrer of the OFDM s combned n the touch panel and the 16th RX AFE receves the combned sgnal. The combned sgnals have a dfferent ampltude as followng the characterstc of the touch panel. Snce each output of the VGA2 n the RX AFE has a dfferent ampltude, ths system can sense the poston of the touch by usng the order of the magntude at each channel. If a touch s generated, the poston s detected because the order of the magntude from RX(0) to RX(15) s changed. Fgure 23. Transent smulaton results of the 16th AFE channel. Fgure 24. Transent smulaton outputs of the VGA2 n the 16th RX AFE. Fgure 25 shows the measurement results of VGA2 s dfferental outputs (VO-VOB) n Fgure 5 when there are touch and no touch, respectvely. When the touch s not appled on the touch panel, the VGA2 s dfferental outputs of RX(15) n the recever s 2.24 Vpp, whle the VGA2 s dfferental outputs of RX(15) s 2.16 Vpp when the touch s appled. The dfference between the touch and no touch s approxmately 0.1 V. Due to the generated touch, t can change the magntude order, as shown n Fgure 24. Fgure 26 shows the pen touch mage before and after the nose flterng, respectvely. The X and Y axes represent the channel of the touch panel. By usng the output code, the touch spot can be detected. Fgure 26a shows the touch mage when the nose s not fltered. Ths case represents that the touch

14 Sensors 2018, 18, of 17 sgnal s passed wthout the HPF, LPF, and the mxer. Snce the nose s not rejected and the touch sgnal s not upconverted, the nose s drectly presented and the touch sgnal s reduced followng the touch panel characterstc n Fgure 3. Addtonally, because the touch sgnal s not up converted, t s reduced by the characterstcs of the panel, as shown n Fgure 3. The SNR acqured s about 20 db due to the nose and decreased touch sgnal. Fgure 26b shows the touch mage after flterng the nose. The nose can be rejected because the HPF, LPF, and the mxer are used. In addton, the characterstc of the panel s not affected because the sgnal s upconverted by the mxer. Fgure 25. Measurement of (a) touch and (b) no touch OFDM sgnal. Fgure 26. (a) Touch mage before nose flterng, and (b) touch mage after nose flterng. Fgure 27 shows the no touch and touch mages n the software, respectvely. By usng the touch software, we can check the proposed desgn detects the poston. Also, the sgnal of 6 recever channel as shown n Fgure 5 s presented n ths program. We can perceve the touch pont through the varaton of the magntude. If the touch s not generated, the software shows the same color, as shown n Fgure 27a, whereas f the touch-pont s generated, the color s changed, as shown n Fgure 27b. The desgned system can fnd the touch poston exactly. Fgure 27. Cont.

15 Sensors 2018, 18, of 17 Fgure 27. (a) No touch and (b) touch mage n the software program. The SNR for the touch panel applcaton s calculated from Equaton (4) [19], as follows: SNR = 20 log M N M T σ M N = mean value when not touched M T = mean value when touched σ = standard devaton when touched (4) The X axs and the Y axs represent the number of samples and the output code, respectvely. To acqure the SNR, the mean values of the touch and no touch are calculated. In addton, the standard devaton of nose n touch s calculated. By subtractng the average of no touch from the average of touch, and then dvdng ths result by the standard devaton of the nose of touch, the SNR s acqured. Table 1 shows the crcut performance summary of the proposed AFE for the touch panel. The applcaton of ths work s for a channel capactve touch panel. The power consumpton s lower than those presented n [12,20,21]. The chp area s smaller than the reported results [12,21,22]. To obtan a hgh SNR and scan rate, the OFDM sensng scheme s proposed and the SNR of ths chp attans up to 60 db. Contrary to the conventonal structure for the touch panel n [12,20 22], the mxer, VGA, and HPF are proposed n the AFE. The AFE of [12,20 22] s composed of ntegrators, but ths paper proposes the mxer and VGA nstead of ntegrators. Therefore, the touch sgnal processng tme n the AFE s faster and hgher than n [12,20 22]. Therefore, the proposed structure has a SNR of 60 db and a frame rate of 200 Hz to scan the channel capactve touch panel. Table 1. Performance of the analog front end for the touch panel. [12] [22] [20] [21] Ths Work Touch Panel Type Mutual Capactve Mutual Capactve Mutual Capactve Mutual Capactve Mutual Capactve Touch Panel Channel Technology CMOS BCD CMOS CMOS CMOS 180 nm 180 nm 350 nm 130 nm 90 nm SNR 39 db 49 db 27.5 db 45.5 db 60 db Frame Rate 240 Hz 120 Hz 175 Hz 977 Hz 200 Hz Power Consumpton 52.8 mw 30 mw 76 mw mw 62.4 mw Area 14.8 mm mm mm mm mm 2 5. Conclusons In ths paper, the AFE for a capactve touch panel s presented. A hgh nose mmunty, channel fnger touch sensng IC for an OFDM touch sensng scheme s presented for AFE. In order to ncrease the SNR, the OFDM sensng scheme s proposed. The TX transmts the orthogonal sgnal to each channel of the panel. The RX detects the magntude of the orthogonal frequency to be transmtted from the TX. Due to the orthogonal characterstcs, t can be robust to the narrowband nterference and nose. Therefore, the SNR can be mproved. To reduce the nose effect of low frequences,

16 Sensors 2018, 18, of 17 a mxer, and a hgh pass flter are also proposed. After the nose s fltered, the touch SNR attaned s 60 db compared to 20 db before the nose s fltered. The advantage of the proposed OFDM sensng scheme s ts ablty to detect channels of the panel smultaneously wth the use of mult-carrers. To satsfy a lnearty of the sgnal n the OFDM system, a hgh lnearty mxer and a ral-to-ral amplfer n the TX drver are desgned. The proposed desgn s mplemented n the 90 nm CMOS process. The SNR s approxmately 60 db. The area s 13.6 mm 2, and the power consumpton s 62.4 mw. Author Contrbutons: The authors completed ths work under the supervson of Kang-Yoon Lee. SangYun Km proposed the dea of AFE scan unt for the capactve touch panel. Behnam Samadpoor Rkan and YoungGun Pu helped n the setup and smulaton of OFDM system. Sang-Sun Yoo and Mnjae Lee helped n wrtng the paper. Keum Cheol Hwang and Youngoo Yang helped n desgnng the top archtecture. Acknowledgments: Ths research was supported by Basc Scence Research Program through the Natonal Research Foundaton of Korea (NRF) funded by the Mnstry of Scence, ICT & Future Plannng (NRF-2017R1A2B ). Conflcts of Interest: The authors declare no conflcts of nterest. References 1. Wang, T.-M.; Ker, M.-D. Desgn and mplementaton of readout crcut on glass substrate for touch panel applcatons. IEEE J. Dsp. Technol. 2010, 6, [CrossRef] 2. Lee, J. Pcture-Based Address Power Savng Method for Hgh Resoluton Plasma Dsplay Panel (PDP). IEEE Trans. Ind. Electron. 2008, 55, [CrossRef] 3. Won, J.; Ryu, H.; Delbruck, T.; Lee, J.; Hu, J. Proxmty Sensng Based on a Dynamc Vson Sensor for Moble Devces. IEEE Trans. Ind. Electron. 2015, 62, [CrossRef] 4. Lm, K.; Jung, K.-S.; Jang, C.-S.; Baek, J.-S.; Kang, I.-B. A fast and energy effcent sngle-chp touch controller for tablet touch applcatons. IEEE J. Dsp. Technol. 2013, 9, [CrossRef] 5. Wang, J.S.; Hsu, Y.L.; Lu, J.N. Poston Estmaton and Smooth Trackng wth a Fuzzy-Logc-Based Adaptve Strong Trackng Kalman Flter for Capactve Touch Panels. IEEE Trans. Ind. Electron. 2015, 62, Darren, L.; Clfton, F.; Rcardo, J.; Steven, S.; Danel, W. Hgh Rate, Low-Latency Mult-Touch Sensng wth Smultaneous Orthogonal Multplexng. In Proceedngs of the 27th annual ACM symposum on User nterface software and technology (UIST 14), Honolulu, HI, USA, 5 8 October 2014; ACM: New York, NY, USA; pp Jonathan, W.; Jefferson, Y. Capactve Touch Sensor Havng Correlaton wth a Recever. U.S. Patent 20,120,013,564, 19 January Darren, L.; Clfton, F.; Danel, W.; Steven, L. Frequency Converson n a Touch Sensor. U.S. Patent 9,710,116, 18 July Park, J.; Lm, D.; Jeong, D. A Reconfgurable 40-to-67 db SNR, 50-to-6400 Hz Frame-Rate, Column-Parallel Readout IC for Capactve Touch-Screen Panels. IEEE J. Sold-State Crcuts 2014, 49, [CrossRef] 10. Touch Technology Bref ; 3M Company: Maplewood, MN, USA, Mura, N.; Dosho, S.; Takaya, S.; Fujmoto, D.; Kryama, T.; Tezuka, H.; Mk, T.; Yanagawa, H.; Nagata, M. A 1mm-ptch channel 322 Hz frame-rate multtouch dstrbuton sensor wth two-step dual-mode capactance scan. In Proceedngs of the IEEE Internatonal Sold-State Crcuts Conference (ISSCC), San Francsco, CA, USA, 9 13 February 2014; pp Shn, H.; Ko, S.; Jang, H.; Yun, I.; Lee, K. A 55dB SNR wth 240Hz frame scan rate mutual capactor touch-screen panel read-out IC usng code-dvson multple sensng technque. In Proceedngs of the IEEE Internatonal Sold- State Crcuts Conference (ISSCC), San Francsco, CA, USA, February 2013; pp Petr, S.; Olmos, A.; Berens, M.; Boas, A.V.; Goes, M. A Fully Integrated Touch Screen Controller Based on 12b 825kS/s SAR ADC. In Proceedngs of the IEEE Argentne School of Mcro-Nanoelectroncs, Technology and Applcaton, San Carlos de Barloche, Argentna, 1 2 October 2009; pp Nose Wars Projected Capactance Strkes Back; Cypress Semconductor Corp.: San Jose, CA, USA, Yarlagadda, R.K.; Hershey, J.E. Hadamard Matrx Analyss and Synthess: Wth Applcatons to Communcatons and Sgnal/Image Processng; Kluwer: Phlp Drve Norwell, MA, USA, 1997.

17 Sensors 2018, 18, of Rchard, D.; Van, N.J.; Prasad, R. OFDM for Wreless Multmeda Communcatons; Artech House, Inc.: Norwood, MA, USA, Gannett, F.; Lottc, V.; Stupa, I. PAPR Analytcal Characterzaton and Reduced-PAPR Code Allocaton Strategy for MC-CDMA Transmssons. IEEE Trans. Wrel. Commun. 2010, 10, [CrossRef] 18. Crpps, S.C. In RF Power Amplfers for Wreless Communcatons; Artech House: Norwood, MA, USA, Touch Sensors Desgn Gude; ATMEL Corporaton: San Jose, CA, USA, Lee, J.; Yeo, D.; Kwon, H.; Km, B.; Sm, J.; Park, H. An LCD-VCOM-Nose Reslent Mutual-Capactve Touch-Sensor IC Chp wth a Low-Voltage Drvng Sgnal. IEEE Sens. J. 2015, 15, [CrossRef] 21. An, J.; Han, S.; Park, K.; Km, J.; Ye, J.; Lee, S.; Jeong, J.; Km, J.; Baek, K.; Chung, K.; et al. Mult-Way Interactve Capactve Touch System wth Palm Rejecton of Actve Stylus for 86 Touch Screen Panels. In Proceedngs of the 2018 IEEE Internatonal Sold- State Crcuts Conference (ISSCC), San Francsco, CA, USA, February 2018; pp Park, C.; Park, S.; Km, K.; Park, S.; Park, J.; Huh, Y.; Kang, B.; Cho, G.-H. A Pen-Pressure-Senstve Capactve Touch System Usng Electrcally Coupled Resonance Pen. In Proceedngs of the 2015 IEEE Internatonal Sold-State Crcuts Conference (ISSCC), San Francsco, CA, USA, February 2015; pp by the authors. Lcensee MDPI, Basel, Swtzerland. Ths artcle s an open access artcle dstrbuted under the terms and condtons of the Creatve Commons Attrbuton (CC BY) lcense (