United States Patent
`
`[191
`
`Redmayne
`
`[11]
`
`[45]
`
`Patent Number:
`
`Date of Patent:
`
`5,650,597
`
`Jul. 22, 1997
`
`US005650597A
`
`[54] CAPACITIVE TOUCH SENSOR
`
`[75]
`
`Inventor: Derek V. Redmayne. Delta. Canada
`
`[73] Assignee: Dynapro Systems, Inc.. New
`Westminster. Canada
`
`[21]
`
`[22]
`
`[5 1]
`[52]
`[53]
`
`[56]
`
`Appl. No.:
`
`375,592
`
`Filed:
`
`Jan. 20, 1995
`
`Int. Cl.5 ..................................................... G08C 21/00
`US. Cl. ................................ 178/19; 345/174; 341/33
`Field of Search .................................. 178/18. 19. 20;
`345/173. 174; 364/7091. 709.11; 341/5.
`33
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`.
`
`3,482,241
`4,186,392
`4,233,522
`4,281,323
`4,293,734
`4,455,452
`4,476,463
`4,550,221
`4,707,845
`4,740,781
`4,743,895
`4,806,709
`4,831,566
`4,853,498
`
`12/1969 Johnson.
`1/1980 Holz.
`11/1980 Grummer et a1.
`7/1981 Burnett et a1.
`.
`10/1981 Pepper, Jr.
`.
`6/1984 Schuyler.
`10/1984 Ng et a1.
`.
`10/1985 Mabusth.
`11/1987 Krein et a1.
`4/1988 Brown.
`5/1988 Alexander .
`2/1989 Evans.
`5/1989 Mattews et al.
`8/1989 Meadows et a1.
`
`.
`
`4,916,699
`4,922,061
`5,041,820
`5,033,118
`5,153,572
`5,231,381
`5,237,879
`5,270,711
`5,305,017
`5,418,551
`5,488,204
`5,491,706
`
`4/1990
`5/1990
`8/1991
`1/1992
`10/1992
`7/1993
`8/1993
`12/1993
`4/1994
`5/1995
`1/1996
`2/1996
`
`.
`
`Ohashi .
`Meadows et al. .
`Kazama .
`Kazama .
`Caldwell et a1.
`Duwaer .
`Specter .
`Knapp .
`Gerpheide .
`Ise ............................................. 178/20
`Mead et al.
`178/18
`
`Tagawa et a1.
`........................... 178/19
`
`Primary Examiner—Curtis Kuntz
`Assistant Examiner—Paul Loomis
`
`Attorney, Agent, or Firm—Foley & Lardner
`
`[57]
`
`ABSTRACT
`
`The present invention relates to a capacitive touch sensor
`including a plurality of horizontal sensor bars arranged in a
`single direction. The touch sensor preferably receives dif-
`ferential sensor signals from the sensor array to reduce
`proximity effects and noise associated with conventional
`capacitive touch sensors. The touch sensor also utilizes an
`isolation circuit or floating interface to reduce the effects of
`external interference and increase the accuracy of touch
`sensing and localization. The bars are preferably comprised
`of indium tin oxide oriented in the machine direction of roll
`
`for superior linearity. The touch sensor may be utilized with
`display screens having thick dielectrics and also eliminates
`the need for a rear guard layer.
`
`.......................... 178/20
`.
`
`22 Claims, 13 Drawing Sheets
`
`
`
`1306
`130E
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`120E
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`130
`
`1
`
`APPLE 1012
`
`1
`
`APPLE 1012
`
`

`

`US. Patent
`
`Jul. 22, 1997
`
`Sheet 1 0f 13
`
`5,650,597
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`

`US. Patent
`
`Jul. 22, 1997
`
`Sheet 2 of 13
`
`5,650,597
`
`FIG. 2
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`

`

`US. Patent
`
`Jul. 22, 1997
`
`Sheet 3 of 13
`
`5,650,597
`
`FIG. 3
`
`400
`
` DETECTION
`
`PHASE
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`
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`4
`
`

`

`US. Patent
`
`Jul. 22,
`
`1997
`
`Sheet 4 of 13
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`5,650,597
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`

`US. Patent
`
`Jul. 22, 1997
`
`Sheet 5 of 13
`
`5,650,597
`
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`US. Patent
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`Ju1.22, 1997
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`U.S.' Patent
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`US. Patent
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`Ju1.22, 1997
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`Sheet 8 of 13
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`US. Patent
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`Jul. 22, 1997
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`Sheet 9 of 13
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`5,650,597
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`US. Patent
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`Jul. 22, 1997
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`Sheet 11 of 13
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`US. Patent
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`Jul. 22, 1997
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`US. Patent
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`Jul. 22, 1997
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`Sheet 13 of 13
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`1
`CAPACITIVE TOUCH SENSOR
`
`FIELD OF THE INVENTION
`
`The present invention relates to touch sensitive devices or
`touch sensors. More particularly,
`the present
`invention
`relates to a capacitive sensor for providing a control signal
`indicative of where the sensor was touched by a fingertip.
`utensil. or other body.
`BACKGROUND OF THE INVENTION
`
`Capacitive touch sensors generally include a sensor array
`configured as a matrix of sensor bars arranged in horizontal
`and vertical directions. Each sensor bar is coupled to a
`control circuit. The control circuit measures capacitive load-
`ing on the array to determine the position or location of the
`touch on the matrix. The control circuit measures capacitive
`loading by providing a drive signal to each sensor bar and
`receiving a sensor signal from each sensor bar. The control
`circuit analyzes the sensor signals to measure the capacitive
`loading on the matrix. The measurement of capacitive
`loading on the horizontal sensor bars allows the capacitive
`touch sensor to determine the vertical location of the touch.
`and the measurement of capacitive loading on the vertical
`sensor bars allows the capacitive touch sensor to determine
`the horizontal location of the touch.
`
`Sensor arrays including sensor bars disposed in both
`vertical and horizontal directions have certain drawbacks.
`For example. a sensor array having a matrix of sensor bars
`generally requires a large number of layers which are
`expensive to manufacture. Also. each bar (or set of bars)
`requires separate sense. drive. and switching circuitry within
`the control circuit. Further. interleaving sensor bars in two
`directions (e.g.. horizontal and vertical) increases the cost of
`the sensor array and detrirnentally affects the optical perfor—
`mance of the display. Therefore. there is a need for a sensor
`array having a reduced number of sensor bars which does
`not utilize an interleaved matrix of sensor bars.
`
`Conventional single sheet capacitive touch sensors are
`problematic because they are susceptible to body or prox-
`imity effects which can significantly decrease the accuracy
`of the touch localization when used with thick dielectrics.
`Proximity effects cause the sensor array to be prone to
`“false" or otherwise inaccurate touch signals or sensor
`signals. For example. a large conductive body proximate the
`sensor array may cause the capacitive touch sensor to
`generate a touch signal (a signal indicating that the array has
`been touched) when a hand or other object nears the sensor
`array. The capacitive nature of the large conductive body
`aflects the capacitive loading of the sensor array and may
`even appear as a touch to the capacitive touch sensor. Large
`conductive bodies may be hands. forearms. or other objects
`which can affect the capacitive sensing of the sensor array
`even though the object is not in contact with (up to several
`feet away from) the sensor array.
`Proximity effects are also associated with a display. A
`high dielectric constant in the substrate results in capacitive
`loading from conductive bodies to the rear of the sensor
`array and also results in proximity effects. Heretofore.
`capacitive touch sensors have thin dielectrics such as a 0.001
`inch or less layer of silicon dioxide. The thin face plate or
`layer is less afiected by the proximity effects. However. thin
`face plates are prone to scratching and wear.
`Conventional capacitive touch sensors are also disadvan-
`tageous if they are used with thick dielectrics because they
`are susceptible to electromagnetic noise from electronic
`components associated with the display or other devices
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`external the sensor and subject to proximity effects. Capaci—
`tive sensors often employ a rear guard layer to prevent
`electric and magnetic interference generated by the display
`or other electrical components from affecting the measure-
`ment of capacitive loading. The rear guard layer is generally
`a_ transparent conductor which is placed on the rear surface
`of the sensor. Rear guard layers are expensive and often
`degrade the optical performance of the display. especially
`the performance of flat screen displays.
`Thus. there is a need for a capacitive touch sensor having
`a sensor array which is easy to manufacture and low cost.
`Preferably. the sensor array has a reduced number of sensor
`bars. Also. there is a need for a capacitive touch sensor
`which does not require a rear guard layer and can be utilized
`with a screen or window having a thick face layer. Further.
`there is a need for a capacitive touch sensor which is less
`susceptible to proximity effects and external electromag-
`netic noise. Additionally. there is a need for a capacitive
`sensor topology which is usable with thick dielectrics and
`which exhibits stability.
`
`SUMMARY OF THE INVENTION
`
`The present invention relates to a capacitive sensor screen
`for sensing a touch. The touch screen includes a plurality of
`conducting bars. and a control circuit coupled to the plurality
`of conducting bars. The plurality of conducting bars are
`configured in an arrangement wherein each bar is substan-
`tially parallel to a first axis and substantially perpendicular
`to a second axis. The control circuit is coupled to the
`plurality of conducting bars and provides an excitation
`signal to the bars. The control circuit receives a capacitive
`sense signal from the conductive bars and provides a control
`signal indicative of the position of the touch on the arrange-
`ment. The control signal indicates the position along the first
`axis and the position along the second axis.
`The present invention also relates to a method of locating
`the position of a touch on a touch sensor including an array
`of bars disposed in parallel to a first axis and a control
`circuit. The control circuit provides excitation signals to a
`first side of the array. receives sense signals at a second side
`of the array. provides excitation signals to the second side of
`the array. and receives sense signals at the first side of the
`array. The method includes the steps of:
`determining a closest bar of the array of bars and response
`to the sense signals. the closest bar being a bar nearest
`the touch; determining a first position of the touch
`along a second axis perpendicular to the first axis by
`determining a bar location of the closest bar in the array
`of bars; and determining a second position of the touch
`along the first axis by analyzing at least one of the said
`sense signals received on the first side of the array and
`at least one of the sense signals received on the second
`side of the array.
`The present invention also relates to an improved capaci-
`tive touch screen for use with the display which provides
`images on its face. The touch screen is situated proximate
`the face of the display and includes a control circuit for
`generating a position signal indicative of the vertical and
`horizontal location of a touch. The improvement is a capaci-
`tive sensor array consisting of bars extending only in a
`horizontal direction.
`The present invention further relates to a touch sensor
`having a sensor array. The touch sensor includes a power
`supply. an excitation driver for providing an excitation
`signal. a control circuit. and a floating interface circuit. The
`excitation driver and control circuit are coupled to the power
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`3
`supply. The floating interface circuit is coupled to the power
`supply. the sensor array. the excitation driver and the control
`circuit. The floating interface circuit receives a power signal
`referenced to the excitation signal. The excitation driver
`provides the excitation signal to the sensor array through the
`floating interface and the control circuit receives a sense
`signal from the sensor array through the floating interface
`circuit.
`The present invention even further relates to an improved
`capacitive touch sensor system including a capacitive sensor
`array and a control circuit. The control circuit provides an
`excitation signal and receives a ditfcrential signal. The
`control circuit generates a control signal indicative of a
`position of a touch in response to the differential signal. The
`improvement includes an interface circuit referenced to the
`excitation signal. The interface circuit is coupled between
`the control circuit and the capacitive sensor array. The
`control circuit is referenced to ground.
`In one aspect of the present invention. the capacitive
`touch screen may sense a touch through an isolated signal
`plane which is driven by the excitation waveform. The
`isolated signal plane is a floating interface which can include
`a preamplifier. electrostatic discharge (BSD) protection. and
`signal processing circuits. The use of a floating isolation
`circuit reduces the effects of external interference. thereby
`increasing the accuracy of touch sensing and localization.
`In another aspect of the present invention. the sensor array
`associated with the capacitive touch screen includes an array
`of bars extending in a single direction. The bars are prefer—
`ably comprised of indium tin oxide (ITO) oriented in the
`machine direction of roll coated ITO for superior linearity.
`In yet another aspect of the present invention. the capaci-
`tive touch screen utilizes dilferential sensing signals to
`detect low level signals. The use of differential signals
`reduces the proximity effects and thereby allows the use of
`laminated screens or screens having thick dielectrics. Dif-
`ferential signals also eliminate the need for a rear guard layer
`in some instances.
`In still another aspect of the present invention. a control
`circuit associated with a touch screen having horizontal bars
`may interpolate between sensing bars to determine a more
`accurate vertical position. The control circuit also senses the
`horizontal position by driving excitation signals on one side
`of the array and receiving sensor signals from the other side
`of the array. The sensor signals are advantageously devel-
`oped across the elements in order to determine vertical and
`horizontal coordinates.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`The invention will hereafter be described with reference
`to the accompanied drawings. wherein like numerals denote
`like elements. and:
`FIG. 1 schematically illustrates a capacitive touch sensor
`in block diagram form in accordance with an exemplary
`embodiment of the present invention;
`FIG. 2 schematically illustrates a sensor array for the
`capacitive touch screen illustrated in FIG. 1;
`FIG. 3 is a flow chart showing the operation of the
`capacitive touch sensor illustrated in FIG. 1;
`FIGS. 4A—I are detailed electrical schematics of a control
`circuit for a sensor similar to the capacitive touch sensor
`illustrated in FIG. 1 in accordance with another exemplary
`embodiment of the present invention; and
`FIG. 5 is a timing diagram illustrating the generation of
`the excitation and sensor signals produced by the capacitive
`touch sensor illustrated in FIG. 1.
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`DETAILED DESCRIPTION OF THE
`PREFERRED EXEMPLARY EMBODIMENT
`
`FIG. 1 is a schematic block diagram of a touch screen or
`capacitive touch sensor 15 including a capacitive sensor
`array 20 which is operatively associated with a display 22.
`(Display 22 is shown physically associated with array 20.
`but is not a part of sensor 15.) Capacitive touch sensor 15
`may be used with a variety of display devices. such as a
`CRT. LCD. projector. printed overlay. printed underlay or
`other arrangement which provides display information from
`which a user makes a selection. Alternatively. capacitive
`touch sensor 15 may be a hand held tablet or other device
`located proximate a display. such as a billboard. sign. or
`menu. The user may touch sensor 15 to select data or
`otherwise provide information.
`Array 20 is preferably mounted within a sandwich of
`dielectric layers (e.g.. approximately 0.040 inches under a
`front surface) or on a rear surface of a window (not shown)
`which is located in front of display 22. The window is
`typically held by a grounded metal bezel surrounding the
`periphery of the window. A guard layer (not shown) can be
`applied to the rear surface of the window over array 20 to
`shield array 20 from electromagnetic interference.
`Capacitive touch sensor 15 includes a control circuit 25
`coupled to sensor array 20 via an electromagnetic link 100.
`Control circuit 25 includes a pre-amp or isolation interface
`28 and a fixed circuit 27. Fixed circuit 27 includes a power
`supply 30. an oscillator 32. a processor 34. a level shifting
`circuit 36. a timing circuit 38. a re—timing and direct access
`circuit 40. a waveform generator 42. an excitation driver 44.
`a synchronous rectifier 46. a level shifting Circuit 48. a direct
`current (DC) amplifier 50. an analog—to—digital (A/D) con—
`verter 52. and a register 54. Fixed circuit 27 is coupled to
`interface 28 via a conductor 82. a conductor 81. a conductor
`61. a conductor 92 and a conductor 94.
`Fixed circuit 27 is powered by power supply 30 which
`provides a VCC voltage level at a conductor 60 and a ground
`voltage level at a conductor 62. Power supply 30 preferably
`provides the VCC voltage level at +5 V for fixed circuit 27.
`Power supply 30 provides :1 +12 V signal at a conductor 61
`for interface 28. Also. power supply 30 provides a +5 V
`analog power signal. an analog ground power signal. and
`other power signals for capacitive touch sensor 15. Power
`supply 30 may be configured to provide a variety of Voltage
`levels such as +5 V. +12 V. —12 V. ground. or other necessary
`voltage levels.
`Isolation interface 28 includes an electrostatic discharge
`(ESD) protection circuit 70. a multiplexer control circuit 72.
`an analog switch circuit 74. a multiplexer circuit 76. a shunt
`regulator 85. and a preamplifier 78. Interface 28 is prefer-
`ably a floating circuit and is powered through a +12 V signal
`provided by conductor 61 from supply 30 through a current
`source 80. Capacitive touch sensor 15 may be configured for
`use with a variety of voltage levels such as ~12 V or +24 V
`in accordance with design criteria for sensor 15. Interface 28
`may utilize transformers. converters. or other devices for
`isolating array 20 from fixed circuit 27. Circuits within
`interface 28 are preferably powered between a positive
`power conductor 86 and a negative power conductor 89.
`Current source 80 provides power to shunt regulator 85
`which is referenced to an excitation signal (drive signal)
`from excitation driver 44 on a conductor 82. Therefore. the
`power signal between conductors 86 and 89 for interface 28
`is referenced to the excitation signal. The power rides on the
`excitation signal provided to interface 28 (e.g.. the power
`between conductors 86 and 89 is referenced to the excitation
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`signal). thereby isolating electromagnetic interference and
`allowing the excitation signals and sensor signals to be
`provided without ground interference from fixed circuit 27.
`Further. referencing isolation interface 28 to the excitation
`signal prevents small amounts of parasitic capacitance in
`multiplexer circuit 76 and ESD protection circuit 70 from
`affecting the integrity of the sensor signals and provides a
`high common mode rejection ratio.
`Sensor array 20 is coupled to control circuit 25 through
`isolation interface 28. Hence. array 20 is effectively floating
`or isolated (e.g.. array 20 is referenced to the excitation
`signal). Interface 23 is coupled to level shifting circuit 48 via
`conductors 92 and 94. multiplexer control circuit 72 via
`conductor 81. and excitation driver 44 via conductor 82.
`Powering sensor array 20 and interface 28 with a power
`signal upon which the excitation signal rides advantageously
`increases the accuracy of the touch localization by reducing
`common mode components on the sensor signals from
`preamplifier 78. Alternatively. isolation interface 28 may be
`powered by a DC to DC converter having outputs riding on
`the excitation signal.
`The operation of sensor 15 is discussed generally below
`with reference to FIG. 1. Processor 34. operating resident
`software. provides encoded bits or digital excitation control
`signals to register 54 so that control circuit 25 produces an
`excitation signal or waveform for eventual reception by
`sensor array 20. After the excitation signal is provided to
`array 20. processor 34 receives a digital representation of
`sensor signals Via A/D converter 52. The sensor signals are
`indicative of the capacitive loading on array 20. Processor
`34 determines the location of the touch by analyzing the
`sensor signals.
`Timing circuit 38. re-tirning circuit 40. waveform gen-
`erator 42. and excitation driver 44 cooperate to produce the
`excitation signal in response to excitation control signals
`provided by register 54. The excitation control signals
`program the duty cycle and frequency of the excitation
`signal and coordinate the reception of the sensor signals by
`multipleer circuit 76. The excitation signal is preferably an
`analog pulsing signal; an exemplary excitation signal is
`described in more detail with reference to FIG. 5 below.
`Alternatively. processor 34 directly produces the excitation
`signal through a digital-to—analog (D/A) converter. current
`mirrors. or other analog circuits and provides the excitation
`signal to driver 44. In this embodiment. processor 34 may be
`a microcomputer such as an 80C52. a digital signal
`processor. a general purpose microprocessor. or other simi-
`lar digital processor.
`Timing circuit 38 generates the digital representation of
`the excitation signal utilizing counters such as a Johnson
`decade counter and other logic circuitry (not shown). The
`digital representation is created from a pulse signal gener-
`ated from a 16 MHz signal provided by oscillator 32. The
`pulse signal is provided to re-timing circuit 40. Re-timing
`circuit 40 re-synchronizes the digital representation to elimi—
`nate propagation delays associated with register 54 and
`timing circuit 38 and reduces “jitter” and other noise asso-
`ciated with the pulse signal. The digital representation is
`provided to waveform generator 42. Alternatively. circuits
`38 and 40 may be eliminated and processor 34 may be
`configured to internally generate the digital representation of
`the excitation signal.
`Waveform generator 42 provides the excitation signal to
`excitation driver 44 in response to the digital representation.
`Waveform generator 42 is preferably an operational arnpli—
`fier (not shown in FIG. I) configured as an integrator with
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`analog switches which appropriately inject positive and
`negative current to the inputs of the amplifier so that the
`amplifier generates the excitation signal. In this preferred
`embodiment, the excitation signal is a burst of 16 trapezoidal
`waveforms. Each burst lasts 500 milliseconds; each wave—
`form is comprised of two complimentary ramps with a dwell
`period between the ramps. In this embodiment. the excita—
`tion signal has a 3 V peak-to—peak amplitude. For thicker
`dielectrics. substantially higher excitation voltage levels up
`to approximately 20 V peak-to-peak may be required.
`Excitation driver 44 provides the excitation signal at
`conductor 82 for reception by analog switch circuit 74.
`Excitation driver 44 is preferably an operational amplifier
`configured to have a gain of 28. Analog switch circuit 74 is
`controlled by switch control signals on conductor 75 pro-
`vided by multiplexer control circuit 72. The switch control
`signals open and close switches such as 4066 analog devices
`(not shown) in circuit 74 so that the excitation signal is
`appropriately provided to ESD protection circuit 70 and
`across link 100 to sensor array 20. Processor 34 generates
`the switch control signals in response to the resident soft—
`ware and provides the signals through level shifting circuit
`36 via conductor 86 to multiplexer control Circuit 72 so that
`the switches in analog switch circuit 74 are closed when the
`excitation signal is provided on conductor 82.
`The excitation signals are provided through ESD protec—
`tion circuit 70 to sensor array 20. Circuit 70 prevents large
`voltages and currents from passing between sensor array 20
`and interface 28 by clamping the signals on link 100. Also.
`circuit 70 provides proper biasing to array 20.
`When sensor array 20 is driven with the excitation signal.
`sensor array 20 provides sensor signals through link 100 and
`ESD protection circuit 70 to multiplexer circuit 76 for
`eventual analysis by processor 34. The sensor signals are
`indicative of the capacitive loading on array 20 and are
`generally 2—20 mV waveforms (depending upon the thick-
`ness of the dielectric) riding on the 3 V peak-to—peak
`excitation signal. Processor 34 controls multiplexer circuit
`76 by providing multiplexer control signals through level
`shifting circuit 36 to multiplex control circuit 72. The
`multipleer control signals cause multipleer circuit 76 to
`select or sample particular sensor signals from array 20.
`Preferably. multiplexer circuit 76 selects two sensor signals
`from array 20 and provides the sensor signals on conductors
`113 and 115.
`
`Processor 34 provides the multiplexer control signals to
`multipleer control circuit 72 in response to the resident
`software. Multiplexer circuit 76 is synchronized with the
`excitation signal so that signals resulting from bursts of 16
`trapezoidal waveforms of the excitation signal can be sensed
`on array 20. Multiplexer circuit 76 advantageously reduces
`the circuitry required to analyze the sensor signals by
`allowing processor 34 to choose particular or selected sensor
`signals. Alternatively. preamplifier 78 could be configured to
`receive a larger number of sensor signals which could be
`transmitted to processor 34 for analysis.
`Preamplifier 78 amplifies the sensor signals and provides
`the amplified sensor signals on conductors 92 and 94 to level
`shifting circuit 48. Level shifting circuits 36 and 48 translate
`the voltage levels between fixed circuit 27 and interface 28.
`Level shifting circuit 36 adjusts the voltage level of switch
`and multipleer control signals provided by processor 34 to
`interface 28. Similarly. level shifting circuit 48 adjusts the
`sensor signals provided by preamplifier 78 on conductors 92
`and 94 from the voltage level of interface 28 to the voltage
`level associated with fixed circuit 27.
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`Level shifting circuit 48 provides the sensor signals to
`synchronous rectifier 46 Via conductors 96 and 98. Synchro—
`nous rectifier 46 provides band pass filtering. rectifies the
`sensor signals. and stores the difference between the sensor
`signals as a sample signal. Synchronous rectifier 46 is
`controlled by re—tirning circuit 40 so that the sensor signals
`are rectified in accordance with the state of the excitation
`signal so that the AC sensor signals are appropriately stored
`as a dc sample signal. More particularly. synchronous rec-
`tifier 46 appropriately gates the negative and positive cycles
`of the excitation signal such that they both contribute to
`sample accumulation. Synchronous rectifier 46 provides
`band pass filtering centered on the frequency of the excita—
`tion signal (e.g.. 30 KHz).
`The sample signal is a charge accumulation associated
`with the diiference between sensor signals on conductors 96
`and 98; the car the charge. the larger the capacitive loading
`associated with sensor array 20. The sample signal is pro-
`vided on a conductor 121 and is representative of the
`diiference between the sensor signal on conductor 96 and the
`sensor signal on conductor 98. The use of a difference signal
`advantageously reduces parasitic capacitance effects and
`proximity effects associated with the bezel (not shown)