United States Patent
`[45]
`Dec. 2, 1980
`Waldron
`___________________________.__.___-—————————————-—-
`
`[19]
`
`[11]
`
`4,237,421
`
`[54] SINGLE-ELECTRODE CAPACITANCE
`TOUCHPAD SENSOR SYSTEMS
`
`[75]
`
`Inventor: Wesley K. Waldron, Scotia, N.Y.
`
`[73] Assignee: General Electric Company,
`Schenectady, N.Y.
`
`[21] Appl. No.: 952,705
`
`[22] Filed:
`
`Oct. 19, 1978
`
`Int. Cl.3 ........................ H03K 3/26; GOBB 13/26
`[51]
`[52] U.S. Cl. ........................................ 328/5; 307/ 116;
`307/308; 340/562
`[58] Field of Search .................... 328/5; 307/308, 116;
`331/65; 340/552, 365 C; 202/DIG. l
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`3,846,780
`3,846,791
`3,893,035
`4,016,490
`4,107,555
`4,136,291
`
`11/1974 Gilcher ................................ 340/562
`
`307/308
`11/1974
`Foster
`.. 307/307
`
`7/1975 Higgens ............
`.
`
`4/1977 Weckenmann et a1.
`ZOO/DIG. 1
`
`8/ 1978 Haas et a1. ................... 328/5
`1/1979 Waldron .............................. 307/308
`
`OTHER PUBLICATIONS
`
`IBM Tech. Disclre. Blttn., Tech-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
`
`ABSTRACT
`[57]
`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 touchpad sensor operates with a sense ampli-
`fier to provide a high density sensor array requiring
`relatively low driving voltage amplitudes and may be
`utilized with driven shields and a normalization net-
`work to provide reliable capacitance sensing with re-
`duced sensitivity to contamination of the surface of the
`sensor array.
`
`20 Claims, 8 Drawing Figures
`
`
`
`ref
`
`APPLE 1011
`
`1
`
`APPLE 1011
`
`

`

`US. Patent
`
`Dec. 2, 1980
`
`Sheet 1 of4
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`4,237,421
`
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`US. Patent
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`Dec. 2, 1980
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`US. Patent
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`Dec. 2, 1980
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`Sheet 3 of4
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`U.S. Patent
`
`Dec. 2, 1980
`
`Sheet4 of4
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`4,237,421
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`1
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`4,237,421
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`SINGLE-ELECTRODE CAPACITANCE
`TOUCHPAD SENSOR SYSTEMS
`
`BACKGROUND OF THE INVENTION
`
`5
`
`10
`
`20
`
`The present invention relates to user-actuated sensors
`and, more particularly, to a noVel single electrode ca-
`pacitance touchpad sensor and related system electron-
`ics.
`Conventional capacitance touchpad devices typically
`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 15
`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 extremesensitivity 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
`when several 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
`
`25
`
`30
`
`35
`
`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 of being 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 capacitance is
`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 node is 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 sensor of the array, as by
`touchpad electrode surface contamination and the like
`
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`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 reliably less than the reference, whereby the sen—
`sor output is 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 frOut 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 touchpad electrodes for a 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 graphs illustrating the
`driving and response waveforms for 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 shield to mini-
`mize effects of surface contamination; and
`FIG. 8 is a schematic block diagram of a time-multi-
`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 11a from
`touchpad 11 is utilized to couple the touchpad to one
`input lead 13a of a differential-input sense amplifier 12,
`having its remaining input lead 13b coupled to a source
`14 of a reference potential Eref. 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 13a 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 110 and sensor com-
`mon bus 15. One lead of a coupling capacity C1 is con-
`nected to the junction between touch electrode lead 11a
`and sense amplifier input 13a, with the remaining lead of
`
`6
`
`

`

`3
`capacitor C1 being connected to a sensor input terminal
`17 A sensor input common terminal 18 is connected to
`common bus 15.
`A waveform generator (not shown). as known to the
`art, is coupled between sensor input terminals 17 and 18
`to provide a driving waveform. .In my, preferred em-
`bodiment, this driving waveform is a train of substan-
`tially rectangular, unipolar pulses having a resting level
`of essentially zero volts and a peak level of Ed volts. A
`division of the driving pulse voltage amplitude occurs at
`the input of sense amplifier 12, for comparison against
`the reference voltage of reference source 14, whereby
`the magnitude E0 of the potential at the sense amplifier
`output 13c is indicative of the contacting of touchpad
`electrode 11 by user personnel (indicated by closure of
`imaginary switch S, placing the user personnel body-to-
`ground capacitance CT between electrical ground and
`the junction of coupling capacitance C; and shunt ca-
`pacitance GP) or the non-contacting of touchpad elec-
`trode 11 by user personnel (indicated by the open condi-
`tion of imaginary switch S, whereby body-to-ground
`capacitance CTis removed from the sensor circuit).
`Touchpad electrode 11 is typically formed by metal-
`lizing an area upon a dielectric substrate with a quantity
`of a conductor or semiconductor material to form a
`circuit node thereat. Proper operation of the sensor
`requires that the coupling capacitance C1 is very much
`larger than the shunt capacitance C1, and that the body-
`to-ground capacitance Cris very much larger than the
`coupling capacitance. Typically,
`the body-to-ground
`capacitance Cris about 70 picofarads, and the equiva-
`lent input capacitance of the sense amplifier appearing
`as shunt capacitance Cp is about 5 picofarads, whereby
`a coupling capacitance 01 of about 22 picofarads may be
`utilized.
`in the “no-touch” condition, capaci-
`In operation,
`tance Cris not present in the circuit and the relatively
`narrow pulses of the driving waveform are coupled to
`sense amplifier input 12a by the capacitive divider
`formed of coupling capacitance C] and shunt capaci-
`tance Cp. The “no-touch” voltage amplitude E1 at sense
`amplifier input
`lead 13a is approximately equal
`to
`(C1E4)/(Cp+C1). The magnitude Ere] of reference
`source 14 is selected to be less than E1 for the no-touch
`condition, whereby the magnitude E0 of the voltage at
`sense amplifier output 130, is of a first condition, e.g. a
`non-zero voltage. In the “touch” condition, where ca-
`pacitance Crappears in parallel with shunt capacitance
`Cp, the magnitude E2 of the voltage at sense amplifier
`input 13a is determined by the voltage divider formed
`by the coupling capacitance and the paralleled CT and
`Cp capacitances,
`and is
`approximately equal
`to
`(Ed(C1))/(CP+C1+CT). The value of C1 or Cp may be
`adjusted as required to assure that the magnitude E2 of
`the voltage at sense amplifier input 13a, in the touch
`condition, is less than the magnitude Ergf of the refer-
`ence source, whereby the magnitude E) at sense ampli-
`fier output 13c assumes a value different from the mag-
`nitude thereof in the “no-touch” condition, e.g. E0 is
`approximately zero volts.
`Referring now to FIG. 2, an array of a plurality N of
`sensors 10is formed by connecting together the cou-
`pling capacitor input lead,
`i.e. that lead of coupling
`capacitor C11, C12,
`.
`.
`.
`, C1". furthest from the associ-
`ated touchpad electrode TP1, TP2, .
`.
`.
`, TPn. The con-
`nected coupling capacitor input leads are connected to
`an array input terminal 17’, while an array common bus
`15’ is coupled to another input terminal 18'. Pulse gener-
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`ation circuitry of known type is coupled between input
`terminals 17’ and 18’ to simultaneously drive all of the
`sensors with the unipolar pulse train, having a peak
`amplitude Ed. Each of the individual sensors 100—1011in
`the array includes, in addition to its coupling capacitor
`C11—C1n, and the touchpad VTPl—TPn associated there-
`with, an associated sense amplifier 1241—1211, having an
`input lead coupled to the junction between the coupling
`capacitor and touchpad electrode of the associated sen-
`sor. The input capacitance of each of sense amplifiers
`12a—12n appears as a shunt capacitance Cpl—CW, cou-
`pled between the system ground bus and the junction
`beween the associated touchpad electrode and Coupling
`capacitor. The remaining inputsof all of sense amplifi-
`ers 12a—12n are advantageously connected together to a
`single reference potential input 19, at which a voltage of
`magnitude Eref is impressed. The. operation of each
`sensor 10a—10n in the array is identical to the herein-
`above described operation of the single sensor. It should
`be understood that the array may have the touchpad
`electrodes TPl—TPn arranged in linear or two-dimen-
`sional fashion, and may be of any practical size, being
`limited only by the ability of the driving circuitry, cou-
`pled between input terminals 17' and 18', to form pulses
`across the total capacitance of the array to be driven.
`The requirements on the driving circuitry complexity
`are reduced by the relatively large detectable difference
`in voltages appearing at the sense amplifier inputs in the
`“touch” and “no-touch” conditions. Illustratively,_with
`the aforementioned values of shunt, coupling and body—
`to-ground capacitances, and with a pulse amplitude E1
`'of about five volts (as available from TTL logic inte-
`grated circuits and the like), the sense amplifier input
`voltage E1 in the “no-touch” condition is about four
`volts while the magnitude E2 of the sense amplifier
`input voltage in the “touch” condition is about one volt.
`It will be seen that large relative differences in the sense
`amplifier input voltage are present and easily discem-
`able by the sense amplifier, even for substantial percent-
`age changes in the values of coupling, shunt and body-
`to-ground capacitance. Thus, reliable operation of the
`individual sensors in an array is facilitated and process-
`ing variation dependence is reduced.
`Referring now to FIG. 3, a non-spatially-multiplexed
`array of touchpad sensors is fabricated upon a dielectric ‘
`substrate 25. The touchpad electrodes TP1,1—TP3,3 are
`fabricated of a conductive or semiconductive material
`upon a surface 25a of the substrate, with the substrate
`being subsequently positioned for exposure to the exter-
`nal environment, whereby user personnel may contact
`one or more of the touchpad electrodes. In the illus-
`trated embodiment, the nine touchpad electrodes are
`arranged in a 3x3 array, with each touchpad being of
`substantially square shape and separated from the re-
`maining touchpads. It should be understood that the
`shape of the individual touchpads is a matter of design,
`as is the area' of each thereof and the spacing therebe-
`tween. Advantageously, as the individual touchpads are
`not required to have a capacitive value with respect to
`other elements of the sensor array and system, in the
`“no-touch” condition (with the only capacitance being
`added to the system externally by a contact with a
`touchpad); the touchpad area can be reduced and the
`array sensor density can be increased.
`Each touchpad is in connection with a conductive or ‘
`semiconductive lead 2711—271, which may be fabricated
`at the same time as the associated touchpad is deposited
`upon the substrate surface, and may be fabricated of the
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`7
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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 25a—25i is spatially separated from every
`other lead and from all touchpads other than the touch-
`pad to which the lead is connected Each lead termi-
`nates in a single pad, aperture or other formation,
`known to the printed circuitry and the like arts, to allow
`connection of each lead to the associated coupling ca-
`pacitor and an associated sense amplifier input. In the
`illustrated 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-
`trade-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-
`low, be coupled to the sensor circuitry 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 I 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=J XK, 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-
`pads are utilized. The three touchpads TP,”, TPbJ, 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,
`35b and 35c being integrally connected to the associated
`row-select touchpad and separate from all other touch-
`pads and interconnection leads. The remaining plurality
`(J XK) of touchpads are arranged with J touchpads in
`each of the K rows and with connective leads integrally
`formed between each of the touchpads in a column and
`extending beyond one of the end touchpad electrodes in
`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 TP1,1,
`TP2,1 and TP3,1 of column 1 of the array, are intercon-
`nected by lead portion 370 (between touchpad elec-
`trode TPM and TP,”) and lead portion 37b (between
`touchpad electrode TP2,1 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,
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`4,237,421
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`6
`electrode interconnective and lead portions 38a, 38b
`and 38c are utilized to interconnect
`the touchpads
`TPLZ, TP2,2, and TP3,2, forming column 2, while inter-
`connective and lead portions 39a, 39b and 396 are uti-
`lized to connect touchpad electrodes TP1,3, TP2,3, and
`TP3,3 to form the J: 3 column furthest from the column
`of row-select touchpad electrodes. It should be under~
`stood that the spatially-multiplexed touchpad sensor
`array may be utilized with or without a shield electrode,
`and the spatially-multiplexed array of FIG. 4 is, for
`purposes of illustration, shown without the presence of
`the shield.
`The spatially-multiplexed array of sensors is operated
`by user personnel initially contacting that one of row-
`select touchpads THU—Th). to designate that one of
`rows 1—K in which the desired touchpad is found. Illus-
`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 TPc, is initially contacted to select the K=3
`(bottom) row, and then the individual touchpad elec-
`trode along that row e.g. touchpad electrode TE3,2 (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 known to the art, are
`utilized to: store the output of one of the 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 column leads 37c,
`38c or 39c); and logically detect the presence of a 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-rnultiplexed 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-multipleried 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 network into 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-rnultiplexed array, or may
`be any one of a plurality of touchpad electrode areas
`connected together to form one column (or row) of a
`spatially-multiplexed array. The clamped sensor 50
`contains, in addition to touchpad electrode TP (and its
`imaginary switch S representing the contact or non-
`contact of the body-to-ground capacitance CTthereto),
`a sense amplifier 52 having one input 53a thereof cou-
`pled to a common node 51' and having a remaining
`input 53b coupled to a source of reference potential of
`magnitude Eref- A capacitance Cp is connected between
`the touchpad-sense amplifier input junction and a sensor
`ground bus 55; the shunt capacitance C]: may be the
`equivalent input capacitance of the sensor amplifier,
`
`8
`
`

`

`4,237,421
`
`5
`
`10
`
`15
`
`7
`either alone or in parallel with an additional physical
`capacitance.
`A pair of electrical resistances R1 and R2 are series
`connected between a sensor input terminal 57 and a
`common terminal 58, coupled to a common bus 55. A
`driving waveform, comprised of a train of unipolar
`pulses having a resting amplitude of approximately zero
`volts and a peak amplitude of Egl volts, with a predeter—
`mined pulse width Tw, is applied between input termi-
`nals 57 and 58. The values of resistances R1 and R2 are
`selected to provide a Thevenin equivalent source hav-
`ing an output voltage Eg2 and having a very low source
`impedance equal to the parallel combination of R1 and
`R2. Another electrical resistance R3 is coupled from the
`junction of resistance R1 and R2 to the touchpad elec-
`trode TP. Coupling capacitor C1 is connected between
`sensor driven input terminal 57 and node 5]; node 51 is
`also connected to the anode of a diode D1, having its '
`cathode coupled to the junction of resistance R3 and the
`touchpad electrode TP. A resistance R4 of relatively
`high value is coupled across diode D1.
`In operation, the Thevenin equivalent voltage Eg2 is
`predeterminately selected, by choice of suitable values
`for resistances R1 and R2, to be greater than the refer-
`ence voltage Eref applied to sense amplifier lead 53b.
`Commencing with the rising edge 60a of the input pulse
`(FIG. 6, waveform A) the voltage E1), (between sense
`amplifier input lead 53:: and sensor ground) instanta-
`neously rises to a value Em1=DN(Eg1——VD1)+V01,
`where KN is
`the
`“no-touch”
`capacitance
`ratio
`(C1/(C1+Cp)) and V3] is the “on” voltage across diode
`D1. This instantaneous rise of the sense amplifier input
`waveform (FIG. 6, waveform B) is shown as edge 62.
`The amplitude Em] 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 600 of drive pulse
`Egl when an enable signal arrives on line 54. The time
`tc should be about one time constant T of the clamped
`sensor circuit in the “no-touch” condition, and should
`also be much less than the time constant of the circuit in
`the “touch” condition, in which condition the circuit
`includes resistance tending to charge capacitor C1 such
`that Em asymptotically approaches Eg2. The drive pulse
`width tw of E31 must therefore be greater than to. Thus,
`the input voltage E;,, is greater than the Thevenin volt—
`age Eg2 and always greater than the reference voltage
`Eref; to assure that the amplifier output 53c is “oft” and
`the no-touch condition is reliably indicated by the state
`of the sense amplifier output voltage E0.
`In the “touch” condition, the sense amplifier input
`voltage Em assumes an initial value of Em2=K-
`7(Egi—V01)+VD1 where KT=(C1/(C1+CP+CI))-
`The touch constant Kris selected such that the voltage
`E1172 is always less than both the Thevenin voltage Eg2
`and the reference voltage Emf, prior to enablement of
`the sense amplifier "latch, whereby the differential sense
`amplifier input voltage (ErefK—Emz) is reliably de-
`tected by the sense amplifier to enable another condi~
`tion of output voltage E0, indicative of the “touch”
`condition, at sense amplifier output 53c.
`Diode D1 prevents the sensor node voltage Em from
`charging to the reference level Eg2 prior to enablement
`of sense amplifier 52 and the comparator action thereof
`during touch conditions. Resistance R4acts to discharge
`coupling capacitor C1 during the interval between driv-
`ing pulses, i.e. when the input pulse level is essentially
`
`8
`equal to zero volts; resistance R4 is selected such that
`the time constants of resistance R4 and coupling capaci-
`tor C1 is much greater than the driving voltage pulse
`width tw, but much less than the cycle time of the drive
`signal Eg1, whose duty cycle is relatively low, e.g. a
`duty cycle of about 0001.
`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 of a 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 290,
`Sensor circuit 50’, wherein like reference designa-
`tions are utilized, with respect to FIG. 5, for like func-
`tions, also includes a diode D2 having its anode coupled
`to the junction of resistance R1 and R3, and having its
`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 290
`and is also connected to sensor circuit 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 R5 and a
`diode D42. 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 700 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 70b of the sense amplifier is connected to a
`source of a second reference voltage Ere/2. The sense
`amplifier output 70c provides a signal Ex, indicative of
`the comparison of shield contamination current against
`a reference voltage (Erefl).
`In operation, prior to the first driving pulse of ampli-
`tude Eg1, 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 R5 and diode
`D4. The collector resistance voltage drop is thus ap-
`proximately zero and the detector capacitance C2 is
`charged to substantially zero volts. The detector load
`capacitance voltage appears at sense amplifier input 70a
`and is less than the magnitude Eref2 of the reference
`voltage at the remaining comparator input lead 701),
`whereby the sense amplifier output 70c is in a first con—
`dition indicative of shield current being less than the
`limit established by the magnitude Erefl.
`Upon occurrence of the first driving pulse, the volt—
`age at input terminal 57 rises to a magnitude Eg1 and the
`base electrode of transistor Q receives a voltage of
`magnitude (((EgI—Ed2)R2(R1+R2))“R2), where Ed: is
`the voltage drop across diode D2. The value of resis-
`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
`
`'
`
`20
`
`25
`
`30
`
`35
`
`4o
`
`45
`
`50
`
`55
`
`6O
`
`6.5
`
`9
`
`

`

`4,237,421
`
`9
`sum of the shield current (received via lead 290) and the
`current injected by resistance R5, due to the difference
`in potential
`thereacross. The summed currents flow
`from the transistor collector electrode and through
`resistance R6 and diode D4 to ground whereby a 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 due to the injection
`of the shield current. As increasing values of shield
`current are injected into the emitter of the transister, the
`voltage at the anode of detector D3 rises. The increase
`in the voltage further forward-biases detector diode D3
`and causes additional current to be injected into capaci-
`tance C2 to increase the