4,237,421
`[11]
`United States Patent 19
`Dec. 2, 1980
`[45]
`Waldron
`coIIA
`
`[54] SINGLE-ELECTRODE CAPACITANCE
`TOUCHPAD SENSOR SYSTEMS
`
`[75]
`
`Inventor: Wesley K. Waldron, Scotia, N.Y.
`
`[73] Assignee: General Electric Company,
`Schenectady, N.Y.
`
`OTHER PUBLICATIONS
`IBM Tech.Disclre. Bltin., Toch-Sensing Circuit, J. A.
`Williams vol. 17, No. 1, 6/74 pp. 166 & 167.
`Primary Examiner—Stanley D. Miller, Jr.
`Assistant Examiner—B. P. Davis
`Attorney, Agent, or Firm—Geoffrey H. Krauss; Marvin
`Snyder; James C. Davis
`[57]
`ABSTRACT
`Each of an array of capacitive touchpad sensors has a
`single touchpad electrode fabricated upon a substrate
`and accessible to user personnel, with series coupling
`and shunt capacitance being provided respectively be-
`tween a driving generator and the touchpad electrode,
`and the touchpad electrode and an array ground. Each
`capacitive touchpadsensor operates with a sense ampli-
`fier to provide a high density sensor array requiring
`References Cited
`relatively low driving voltage amplitudes and may be
`U.S. PATENT DOCUMENTS
`utilized with driven shields and a normalization net-
`3,846,780
`11/1974—Gilcher .....sssessssessesecsenescensoee 340/562
`work to provide reliable capacitance sénsing with re-
`
`3,846,791
`11/1974
`Foster ...
`a 307/308
`duced sensitivity to contamination of the surface of the
`
`3,893,035
`
`W/A9TS=Higgens............ .. 307/307
`sensor array.
`
`4,016,490
`4/1977 Weckenmannetal.
`200/DIG. 1
`4,107,555
`8/1978 Haas et all.
`
`-..c.sesesesseserseeeeteneres 328/5
`20 Claims, 8 Drawing Figures
`
` 1/1979—Waldron oo... seesssesssecsnereseneee 307/3084,136,291
`
`[21] Appl. No.: 952,705
`
`[22] Filed:
`
`Oct. 19, 1978
`
`Int. C13 ..seeesesseeesnen HO03K 3/26; GO8B 13/26
`[51]
`[52] US. Ch. cesssccsstssescscenesssssseseeesnsen 328/5; 307/116;
`307/308; 340/562
`[58] Field of Search ..........-.-se 328/5; 307/308, 116;
`331/65; 340/552, 365 C; 202/DIG.1
`
`[56]
`
`
`
`APPLE 1011
`
`1
`
`APPLE 1011
`
`

`

`U.S. Patent
`
`Dec. 2, 1980
`
`Sheet 1 of 4
`
`4,237,421
`
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`Sheet 2 of 4
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`4,237,421
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`Dec. 2, 1980
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`

`

`U.S. Patent
`
`Dec. 2, 1980
`
`Sheet 3 of 4
`
`4,237,421
`
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`

`U.S. Patent
`
`Dec.2, 1980
`
`Sheet 4 of 4
`
`4,237,421
`
`Fig. 8
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`

`1
`
`4,237,421
`
`SINGLE-ELECTRODE CAPACITANCE
`TOUCHPAD SENSOR SYSTEMS
`
`BACKGROUNDOF THE INVENTION
`
`5
`
`Thepresent invention relates to user-actuated sensors
`and, more particularly, to a novel single electrode ca-
`pacitance touchpad sensor andrelated system electron-
`ics.
`
`Conventional capacitance touchpad devicestypically
`require three separate electrodes, fabricated upon at
`least one surface of a supporting substrate. Typically,
`the touchpad electrode is fabricated upon the opposite
`side of a relatively thick dielectric substrate from the
`remaining two electrodes and may require an area of
`about one square inch. Even with relatively large
`touchpad electrode areas, the active capacitance of the
`sensor is typically less than 5 picofarads, whereby ge-
`ometry variations, mask registration or parasitic capaci-
`tance problems will typically result in device capaci-
`tance variations of up to 2 picofarads, and create a de-
`vice of extreme.sensitivity to design process variations.
`Thus, the conventional touchpad sensor devices, being
`of a relatively high impedance design, exhibit signifi-
`cant performance differences between sensors, even
`whenseveral sensors are arrayed upon the same sub-
`strate and fabricated simultaneously. It is known to
`alleviate parasitic capacitance coupling problems by
`utilizing relatively large device. areas and relatively
`large amplitude of driving voltages. It is desirable to
`provide arrays of capacitive touchpad sensors having
`both a reduced sensor area, to provide for greater sen-
`sor density in the array, and with reduced requirements
`for the amplitude of the drive voltages, to provide
`greater safety to user personnel (contacting the touch
`electrodes upon which at least a portion of the drive
`voltage appears) and to reduce driving and sensing
`circuit complexity and attendant cost.
`BRIEF SUMMARY OF THE INVENTION
`
`In accordance with the invention, a novel single-elec-
`trode capacitance touchpad sensor, for use in touchpad
`sensor arrays, comprises a single electrode fabricated
`upon the surface of a substrate and forming a touchpad
`capable ofbeing contacted by user personnel to activate
`the sensor. The differential input of a sense amplifier is
`connected between the touchpad electrode and the
`electrical ground bus of the array, with a coupling ca-
`pacitor being utilized between a pulse generator driving
`the sensor and the touch electrode. The input capaci-
`tance of the sense amplifier and other parasitic capaci-
`tance appear in shunt from the touch electrode to sys-
`tem ground. The touchpad electrode is a circuit node
`formed of any conductive or semiconductive material
`deposited upon the dielectric substrate. Contacting the
`touch electrode introduces the body-to-ground capaci-
`tance into the electrode node. The user capacitanceis
`greater than the coupling capacitance (from the sensor
`driver) which is in turn greater than the input capaci-
`tance of the sense amplifier, to provide a positive node
`capacitance change when the nodeis touched.
`In presently preferred embodiments, each of the
`novel single-electrode touchpads,
`in an array of both
`spatially-multiplexed
`and
`non-spatially-multiplexed
`types,
`is surrounded by a shield electrode, with the
`shield electrode being driven to minimize the leakage
`current effect induced in each sensorofthe array, as by
`touchpad electrode:surface contamination and the like
`
`2
`external influences. Level-clamping circuitry is utilized
`to assure reliable establishment of the “no-touch” out-
`put of the sensor to be greater than a reference condi-
`tion, with the sensor output in the “touch” condition
`being reliablyless than the.reference, whereby the sen-
`sor outputis established only by the presence or absence
`of the body-to-ground capacitance of a user contacting
`the associated touchpad electrode.
`In another preferred embodiment, each of a multiplic-
`ity of sensors contains an additional impedance between
`the touchpad element and the sense amplifier, to able
`time-multiplex operation.
`Accordingly, it is an object of the present invention
`to provide novel single electrode capacitive touchpad
`sensor and arrays thereof, for reliably ascertaining the
`presence or absence of a “touch” condition.
`This and other objects of the present invention will
`become apparent upon consideration of the following
`detailed description,.
`taken in conjunction with the
`drawings.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 is a schematic diagram of one of my novel
`single-electrode capacitive touchpad sensors, illustrat-
`ing the principles of operation thereof,
`©
`FIG. 2 is a schematic diagram of an array of the
`single-electrode capacitive touchpad sensors of FIG.1;
`FIG. 3 is a front view of a substrate containing an
`array of touchpad electrodes: for a non-multiplexed
`touchpad array, and illustrating the positioning of a
`shield therefore;
`FIG. 4 is a front view of a substrate containing an
`array of touchpadelectrodesfora spatially-multiplexed
`array of single-electrode capacitive touchpad sensors;
`FIG.5 is a schematic diagram of a capacitive touch-
`pad sensor having a level-clamping network;
`FIG.6 is a pair of coordinated graphsillustrating the
`driving and response waveformsfor the sensor of FIG.
`5;
`
`FIG.7 is a schematic diagram of a capacitive touch-
`pad sensor having a level-clamping network: and an
`active network for driving a sensor array shieldto mini-
`mize effects of surface contamination; and
`FIG.8 is a schematic block diagram of a time-mullti-
`plexed array of a plurality of capacitive touchpad sen-
`sors.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`Referring initially to FIG. 1, a capacitive touchpad
`sensor 10 includes a touchpad electrode 11, typically
`fabricated upon a surface of a dielectric substrate (illus-
`trated in FIGS. 3 and 4 herein-below) to be accessible
`for contact by user personnel. A single lead 11¢ from
`touchpad11 is utilized to couple the touchpad to one
`input lead 13a of a differential-input sense amplifier 12,
`having its remaining input lead 13d coupled to a source
`14 of a reference potential E,2: The remaining terminal
`of the reference source 14 is coupled to a common bus
`15 of the sensor. Sense amplifier 12 has an input capaci-
`tance between inputs 13@ and 13b, whereby a capaci-
`tance Cp appears in shunt with the sense amplifier-
`source combination,
`that
`is, capacitance C, appears
`between touchpad electrode lead 11¢ and sensor com-
`mon bus 15. One_lead of a coupling capacity C1 is con-
`nected tothe junction between touch electrode lead 114
`and sense amplifier input 13a, with the remaining lead of
`
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`

`4,237,421
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`15
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`w0
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`35
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`3
`4
`capacitor C; being connected: to a sensor input terminal
`ation circuitry of known type is coupled between input
`17. A sensor input common terminal 18 is connected to
`terminals 17’ and 18’ to simultaneously drive all of the
`common bus 15.
`.
`-
`sensors with the unipolar pulse train, having a peak
`A waveform generator (not shown), as known to.the
`amplitude Eg. Each of the individual sensors 10a-10x in
`art, is coupled between sensor input terminals 17.and 18
`the array includes, in addition to its coupling capacitor
`to provide a driving waveform. .In my.preferred em-
`Cyi-C1,, and the touchpad TP;-TP, associated there-
`bodiment, this driving waveform is a train of substan-
`with, an associated sense amplifier 12a-12n, having an
`tially rectangular, unipolar pulses having a resting level
`input lead coupled to the junction between the coupling
`of essentially zero volts and a peak level of Eg volts. A
`capacitor and touchpad electrode of the associated sen-
`division of the driving pulse voltage amplitude occursat
`sor. The input capacitance of each of sense amplifiers
`the input of sense amplifier 12, for comparison against
`12a-12n appears as a shunt capacitance Cy1-Cpy, cou-
`the reference voltage of reference source 14, whereby
`pled between the system ground bus and the junction
`the magnitude Eo of the potential at the sense amplifier
`beween the associated touchpad electrode and coupling
`output 13¢c is indicative of the contacting of touchpad
`capacitor. The remaining inputs.of all of sense amplifi-
`ers 12a-12n are advantageously connected together to a
`electrode 11 by user personnel(indicated by closure of
`imaginary switch S, placing the user personnel body-to-
`single reference potential input 19, at which a voltage of
`ground capacitance Cr between electrical ground and
`magnitude E,.r is impressed. The. operation of each
`the junction of coupling capacitance Cy and shunt ca-
`sensor 10a-10n in the array is identical to the herein-
`above described operationofthe single sensor. It should
`pacitance C,) or the non-contacting of touchpad elec-
`trode 11 by user personnel(indicated by the open condi-
`be understood that the array may have the touchpad
`tion of imaginary switch S, whereby body-to-ground
`electrodes TP|-TP,, arranged in linear or two-dimen-
`capacitance Cris removed from the sensorcircuit).
`sional fashion, and may be of any practical size, being
`Touchpad electrode 11 is typically formed by metal-
`limited only by the ability of the driving circuitry, cou-
`lizing an area uponadielectric substrate with a quantity
`pled between input terminals.17' and 18’, to form pulses
`of a conductor or semiconductor material to form a
`across the total capacitance of the array to be driven.
`circuit node thereat. Proper operation of the sensor
`The requirements on the driving circuitry complexity
`requires that the coupling capacitance C) is very much
`are reducedbythe relatively large detectable difference
`larger than the shunt capacitance C, and that the body-
`in voltages appearing at the sense amplifier inputs in the
`to-ground capacitance Cris very much larger than the
`“touch” and “‘no-touch” conditions. Ilustratively, with
`coupling capacitance. Typically,
`the body-to-ground
`the aforementioned values of shunt, coupling and body-
`capacitance Cris about 70 picofarads, and the equiva-
`to-ground capacitances, and with a pulse amplitude Ez
`lent input capacitance of the sense amplifier appearing
`of about five volts (as available from TTL logic inte-
`as shunt capacitance Cpis about 5 picofarads, whereby
`grated circuits and the like), the sense amplifier input
`voltage E; in the “no-touch” condition is about four
`a coupling capacitance C, of about 22 picofarads may be
`utilized.
`‘
`volts while the magnitude E, of the sense amplifier
`In operation,
`in the ‘‘no-touch” condition, capaci-
`input voltage in the “touch” condition is about one volt.
`tance Cris not present in the circuit and the relatively
`It will be seen that large relative differences in the sense
`narrow pulses of the driving waveform are coupled to
`amplifier input voltage are present and easily discern-
`sense amplifier input 12a by the capacitive divider
`able by the sense amplifier, even for substantial percent-
`formed of coupling capacitance C; and shunt capaci-
`age changes in the values of coupling, shunt and body-
`tance Cy. The “no-touch” voltage amplitude E; at sense
`to-ground capacitance. Thus, reliable operation of the
`amplifier input
`lead 13a is approximately equal
`to
`individual sensors in an array is facilitated and process-
`ing variation dependence is reduced.
`(C\Ea/(Cp+C}). The magnitude E, of ‘reference
`source 14 is selected to be less than E; for the no-touch
`Referring now to FIG.3, a non-spatially-multiplexed
`condition, whereby the magnitude Eo of the voltage at
`array of touchpad sensorsis fabricated uponadielectric
`sense amplifier output 13c, is of a first condition, e.g. a
`substrate 25. The touchpad electrodes TP1,1-TP3,3 are
`fabricated of a conductive or semiconductive material
`non-zero voltage. In the “touch” condition, where ca-
`upon a surface 25a of the substrate, with the substrate
`pacitance Cy appears in parallel with shunt capacitance
`Cp, the magnitude E2 of the voltage at sense amplifier
`being subsequently positioned for exposure to the exter-
`input 13a is determined by the voltage divider formed
`nal environment, whereby user personnel may contact
`by the coupling capacitance and the paralleled C7 and
`one or more of the touchpad electrodes. In the illus-
`Cp capacitances,
`and is
`approximately equal.
`to
`trated embodiment, the nine touchpad electrodes are
`(Ea(C)))/(Cp+C1+C7). The value of C; or Cpmay be
`arranged in a 3X3 array, with each touchpad being of
`adjusted as required to assure that the magnitude E, of
`substantially square shape. and separated from there-
`the voltage at sense amplifier input 13a, in the touch
`maining touchpads. It should be understood that the
`condition, is less than the magnitude E,e of the refer-
`shape of the individual touchpadsis a matter of design,
`ence source, whereby the magnitude Eo at sense ampli-
`as is the areaof each thereof and the spacing therebe-
`fier output 13c assumesa value different from the mag-
`tween. Advantageously, as the individual touchpadsare
`nitude thereof in the “no-touch” condition, e.g. Eo is
`not required to have a capacitive value with respect to
`approximately zero volts.
`:
`other elements of the sensor array and system, in the
`Referring now to FIG.2, an array of a plurality N of
`“no-touch” condition (with the only capacitance being
`sensors 10 is formed by connecting together the cou-
`added to the system externally by a contact with a
`pling capacitor input lead, ie. that lead of coupling
`touchpad); the touchpad area can be reduced and the
`capacitor Ci, Cy2,..., Cim furthest from the associ-
`array sensor density can be increased.
`ated touchpad electrode TP), TP2,..., TP. The con-
`Each touchpadis in connection with a conductive or’
`nected coupling capacitor input leads are connected to
`semiconductive lead 27a-27i, which may be fabricated
`an array input terminal 17’, while an array common bus
`at the.same time as the associated touchpadis deposited
`15’ is coupled to anotherinput terminal 18’. Pulse gener-
`upon the substrate surface, and maybefabricated of the
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`5
`same material as the touchpad electrode. Advanta-
`geously, all of the touchpads and their associated leads
`are fabricated in one simultaneous operation, such as by
`silk screening a film of a conductive material upon the
`substrate surface 25a. As will be seen, each touchpad
`electrode lead 25¢-25iis spatially separated from every
`other Jead and from al! touchpads other than the touch-
`pad to which the lead is connected. Each lead termi-
`nates in a single pad, aperture or other formation,
`knownto the printed circuitry and thelike arts, to allow
`connection of each lead to the associated coupling ca-
`pacitor and an associated sense amplifier input. In the
`itlustrated example, with nine independent touchpads,
`nine coupling capacitor-sense amplifier combinations
`are required. In very large arrays of M touchpad sen-
`sors, a total of M connections to the touchpad-elec-
`trode-bearing substrate (one connection to each of the
`M leads thereon)is required.
`The touchpad-electrode-bearing substrate may ad-
`vantageously be fabricated with the deposition of a
`shield electrode 29 covering a major portion of the area
`remaining after fabrication of the touchpads and associ-
`ated leads, but being insulated therefrom. The shield
`provides a certain degree of isolation between each
`touch pad-lead group and may, as described hereinbe-
`jow, be coupled to the sensorcircuitry to reduce effects
`of surface contamination and like, which contamination
`may be brought about by deposit of skin oil and other
`contaminants from the contacting portion of the user
`personnel onto the substrate surface and contactible
`surface areas of the touchpad electrodes. A separate
`lead 29a is provided for the shield.
`Referring now to FIG.4, a very large array of capac-
`itive sensors has a multiplicity of touchpad electrodes
`fabricated upon the surface 32a of a dielectric substrate
`32, and is arranged for spatial-multiplexing to reduce
`the total number of leads to which connection must be
`made at the substrate. The spatially-multiplexed array
`of touchpad electrodes is fabricated as a two-dimen-
`sional matrix having J columns and K rows, with an
`additional column of K touchpads for enabling a row-
`select function. Thus, an array having M individual
`selectable functions, where M=JxK, has a touchpad
`array of Kx (J=1) touchpads. Illustratively, for an
`array wherein M=9, J=3 and K=3,a total of 12 touch-
`padsare utilized. The three touchpads TP.,, TPs,,, and
`TP,, are not spatially multiplexed, due to their use as
`row selectors, and are each arranged one above the
`other on the substrate surface, with individual leads 35a,
`35d and 35c being integrally connected to the associated
`row-select touchpad and separate from all other touch-
`pads and interconnection leads. The remaining plurality
`(JK) of touchpads are arranged with J touchpads in
`each of the K rows and with connective leadsintegrally
`formed between each of the touchpads in a column and
`extending beyond oneof the end touchpad electrodesin
`the column to form a lead for all of the K touchpads in
`that column to the remainder of the sensor circuitry
`associated with each column. Thus, touchpads TPi,1,
`TP2,; and TP3; of column 1 of the array, are intercon-
`nected by lead portion 37a (between touchpad elec-
`trode TP),; and TP21) and lead portion 376 (between
`touchpad electrode TP2,; and TP3,1) with an additional
`lead portion 37c extending from an edge of touchpad
`electrode TP3,1 opposite the remainder of the touchpad
`electrodes in that column, to provide a lead for connec-
`tion to the single coupling capacitor and single sense
`amplifier for the touchpad column 1 sensor. Similarly,
`
`— 5
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`4,237,421
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`6
`electrode interconnective and lead portions 38a, 38)
`and 38c are utilized to interconnect
`the touchpads
`TP1,2, TP2,2, and TP3,2, forming column 2, while inter-
`connective and lead portions 39a, 395 and 39c are uti-
`lized to connect touchpad electrodes TPi,3, TP2,3, and
`TP3,3 to form the J=3 columnfurthest from the column
`of row-select touchpad electrodes. It should be under-
`stood that the spatially-multiplexed touchpad sensor
`array maybe utilized with or withouta shield electrode,
`and the spatially-multiplexed array of FIG. 4 is, for
`purposesofillustration, shown without the presence of
`the shield.
`Thespatially-multiplexed array of sensors is operated
`by user personnel initially contacting that one of row-
`select touchpads TP,,-TP,to designate that one of
`rows 1-K in which the desired touchpadis found.Iius-
`tratively, if the input function requires that data associ-
`ated with touchpad TP3,2 be input to a system to which
`the touchpad sensor array is coupled, the row-select
`touchpad TP,,; is initially contacted to select the K=3
`(bottom) row, and then the individual touchpad elec-
`trode along that row e.g. touchpad electrode TP3, (in
`column 2), is contacted to complete the input function.
`It should be understood that suitable storage means,
`such as logic latches and the like, and logic gating
`means (not shown), such as are knownto the art, are
`utilized to: store the output of one ofthe sense amplifi-
`ers associated with the row-select portion of the array;
`store the output of one of the sense amplifiers coupled
`to a particular column (with one sense amplifier being
`coupled to each individual one of the columnleads 37c,
`38c or 39c); and logically detect the presence ofa partic-
`ular column and row signal from the storage means to
`determine which element of the matrix has been en-
`tered. It should also be understood that the use of non-
`spatially-multiplexed row-select sensors with a spatial-
`ly-multiplexed column-connected sensor array allows
`the number of connections to substrate 32 to be mini-
`mized relative to the non-spatially-multiplexed sensor
`array of FIG.3, e.g. for a nine-element array, the spa-
`tially-multiplexed embodiment (FIG. 4) requires only
`six contacts, while the non-spatially-multiplexed em-
`bodiment requires nine contacts. It will be seen that
`even greater reduction of substrate connections occur
`for large arrays, e.g. for a ten by ten array (100 ele-
`ments), the non-spatially-multiplexed array requires 100
`individual connections, while the spatially-multiplexed
`array requires only 20 connections.
`Referring now to FIGS.5 and 6, reliable operation
`with relatively low drive voltages is further assured by
`incorporating a noval level clamping networkinto each
`sensor 50 of an array. It should be understood that the
`touchpad electrode TP, shown for the level-clamped
`capacitor touchpad sensor 50, may be a single touchpad
`electrode, in a non-spatially-multiplexed array, or may
`be any one of a plurality of touchpad electrode areas
`connected together to form one column (or row) of a
`spatially-muitiplexed array. The clamped sensor 50
`contains, in addition to touchpad. electrode TP (andits
`imaginary switch S representing the contact or non-
`contact of the body-to-ground capacitance Cr thereto),
`a sense amplifier 52 having one input 53a thereof cou-
`pled to 2 common node 51’ and having a remaining
`input 535 coupled to a source of reference potential of
`magnitude Eye. A capacitance Cp is connected between
`the touchpad-sense amplifier input junction and a sensor
`ground bus 55; the shunt capacitance Cp may be the
`equivalent input capacitance of the sensor amplifier,
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`either alone or in parallel with an additional physical
`capacitance.
`A pair of electrical resistances Ri and Roare series
`connected between a sensor input terminal 57 and a
`common terminal 58, coupled to a commonbus 55. A
`driving waveform, comprised of a train of unipolar
`pulses having a resting amplitude of approximately zero
`volts and a peak amplitude of Eg; volts, with a predeter-
`mined pulse width Ty, is applied between input termi-
`nals 57 and 58. Thé values of resistances Ri and R2 are
`selected to provide a Thevenin equivalent source hav-
`ing an output voltage Eg. and having a very low source
`impedance equal to the parallel combination of Ry and
`Ro. Anotherelectrical resistance R3 is coupled from the
`junction of resistance R; and R2 to the touchpad elec-
`trode TP. Coupling capacitor C, is connected between
`sensor driven input terminal 57 and node 51; node 51 is
`also connected to the anode of a diode Dj, having its ©
`cathode coupled to the junction of resistance R3 and the
`touchpad electrode TP. A resistance Ry of relatively
`high value is coupled across diode Dj.
`In operation, the Thevenin equivalent voltage Ego is
`predeterminately selected, by choice of suitable values
`for resistances R; and Ro, to be greater than the refer-
`ence voltage E,r applied to sense amplifier lead 535.
`Commencing with the rising edge 60a of the input pulse
`(FIG. 6, waveform A) the voltage E;, (between sense
`amplifier input lead 532 and sensor ground) instanta-
`neously rises to a value Ej1=Da(Egi—Vp1)+ Voi,
`where Ky is
`the
`“no-touch”
`capacitance
`ratio
`(CiA(C1+C,)) and Vp}is the “on” voltage across diode
`D}. This instantaneous rise of the sense amplifier input
`waveform (FIG. 6, waveform B) is shown as edge 62.
`The amplitude E,,1 is selected to be greater than the
`steady state Thevenin equivalent voltage Eg2, which is
`itself greater than the reference voltage.
`The output voltage of sense amplifier 52 is latched at
`a time t, following the leading edge 60a of drive pulse
`Egl when an enable signal arrives on line 54. The time
`te should be about one time constant 7 of the clamped
`sensor circuit in the “no-touch” condition, and should
`also be muchless than the time constantof the circuit in
`the “touch” condition, in which condition the circuit
`includes resistance tending to charge capacitor C1 such
`that Ej, asymptotically approaches Ego. The drive pulse
`width t, of Eg; must therefore be greater than te. Thus,
`the input voltage Ey, is greater than the Thevenin volt-
`age Eg) and always greater than the reference voltage
`Eve to assure that the amplifier output 53c is “off” and
`the no-touch condition is reliably indicated by the state
`of the sense amplifier output voltage Ep.
`In the “touch” condition, the sense amplifier input
`voltage Ey, assumes an initial value of Ejy.2=K-
`AEgi—Vpi)+Vn1 where Kr=(Ci/(Ci+Cp+C,).
`The touch constant Kvis selected such that the voltage
`Ejn2 is always less than both the Thevenin voltage Eg2
`and the reference voltage Eyes, prior to enablement of
`the sense amplifier latch, whereby the differential sense
`amplifier input voltage (EryK—Ejn2) is reliably de-
`tected by the sense amplifier to enable another condi-
`tion of output voltage E,, indicative of the “touch”
`condition, at sense amplifier output 53c.
`Diode D; prevents the sensor node voltage E,, from
`charging to the reference level Ego prior to enablement
`of sense amplifier 52 and the comparatoraction thereof
`during touch conditions. Resistance Ry acts to discharge
`coupling capacitor Ci during the interval between driv-
`ing pulses, i.e. when the input pulse level is essentially
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`equal to zero volts; resistance R4 is selected such that
`the time constants ofresistance R4 and coupling capaci-
`tor Cy is much greater than the driving voltage pulse
`width ty, but much less than the cycle time of the drive
`signal Es}, whose duty cycleis relatively low, e.g. a
`duty cycle of about 0.001.
`Referring now to FIG. 7, minimization of surface
`contamination effects and cross-talk with an array of
`touchpads is provided by isolating each touchpad and
`lead thereof by the aforementioned conductive shield,
`preferably formed ofa thin film of a conductive mate-
`rial, such as indium tin oxide and the like. Advanta-
`geously, the shield has a very low circuit impedance to
`provide the desired reduction in cross-talk, i.e. touch-
`pad-to-touchpad coupling. In the event of surface con-
`tamination, the shield current I, is a non-zero value,
`generated by the
`contamination current
`flowing
`through the low circuit impedance. The shield contami-
`nation current is conducted to sensor circuit 50’ via
`shield lead 29a.
`Sensor circuit 50’, wherein like reference designa-
`tionsare utilized, with respect to FIG.5, for like func-
`tions, also includes a diode D2 havingits anode coupled
`to the junction of resistance Rj and R3, and havingits
`cathode coupled to resistance R2. A PNP transister Q
`has its base electrode coupled to the junction between
`the cathode of diode D2 and resistance R2; the emitter
`electrode of transistor Q is connected to shield lead 29a
`‘and is also connected to sensorcircuit input terminal 57
`via an emitter resistance R51, while the collector of
`transistor Q is connected to the anode of a detector
`diode D3 and is also connected to common bus 55 via a
`series circuit including a collector resistance Rg and a
`diode Daz. The cathode of the detector diode is coupled
`to a diode detector load consisting of a capacitor C2 and
`a resistance R7, both of which have their remaining
`terminals coupled to common bus 55. The inverting (—)
`input 70a of a second sense amplifier (comparator) 72 is
`coupled to the junction between the detector diode
`cathode, capacitor C2 and resistor R7. The non-invert-
`ing input 706 of the sense amplifier is connected to a
`source of a second reference voltage E,.j2. The sense
`amplifier output 70c provides a signal Ex, indicative of
`the comparison of shield contamination current against
`a reference voltage (E;¢7).
`In operation, prior to the first driving pulse of ampli-
`tude Egi, the potential difference between input termi-
`nals 57 and 58 is essentially zero and transistor Q is
`cut-off, whereby a current of substantially zero magni-
`tude flows through collector resistance Rg and diode
`D4. The collector resistance voltage drop is thus ap-
`proximately zero and the detector capacitance Cz is
`charged to substantially zero volts. The detector load
`capacitance voltage appears at sense amplifier input 702
`and is less than the magnitude E,p of the reference
`voltage at the remaining comparator input lead 705,
`whereby the sense amplifier output 70¢ is in a first con-
`dition indicative of shield current being less than the
`limit established by the magnitude E,ep.
`Upon occurrence ofthe first driving pulse, the volt-
`age at input terminal 57 rises to a magnitude Eg) and the
`base electrode of transistor Q receives a voltage of
`magnitude (((Egi—Eg2)Ra(R1 +R2))—R2), where Eq is
`the voltage drop across diode D2. The value ofresis-
`tance R5 is selected such that the emitterbase diode of
`the transistor is forward biased and emitter current is
`injected into the emitter electrode of the transistor. The
`magnitude of the total emitter electrode current is the
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`sum ofthe shield current (received via lead 29a) and the
`current injected by resistance Rs, due to the difference
`in potential
`thereacross. The summed currents flow
`from the transistor collector electrode and through
`resistance Rg and diode D4 to ground wherebya voltage
`drop exists thereacross, which voltage is the sum of a
`steady voltage due to the current injected by action of
`emitter resistance R5 and a voltage dueto the injection
`of the shield current. As increasing values of shield
`currentare injected into the emitter of the transister, the
`voltage at the anode of detector D3rises. The increase
`in the voltage further forward-biases detector diode D3
`and causes additional current to be injected into capaci-
`tance Cy to increase the voltage thereacross, at the in-
`verting input 70¢ of the shield sense amplifier. During
`the time intervals between driving pulses, the voltage
`across collector resistance R¢ falls substantially to zero,
`whereby the cathode of detector cathode D3 is at a
`higher potential than the anodethereof, and the diode is
`cut-off. Detector load resistance R7 is of a reasonably
`high resistance value, whereby the decay of the voltage
`across capacitance C2is relatively slight between driv-
`ing pulses, but still decays to zero if the