PATENT DOCUMENT

Publication Number: US-10725587-B2
Application Number: US-201916367024-A
Country: US
Kind Code: B2

Title: Front-end signal compensation

Abstract:
A touch surface device having improved sensitivity and dynamic range is disclosed. In one embodiment, the touch surface device includes a touch-sensitive panel having at least one sense node for providing an output signal indicative of a touch or no-touch condition on the panel; a compensation circuit, coupled to the at least one sense node, for generating a compensation signal that when summed with the output signal removes an undesired portion of the output signal so as to generated a compensated output signal; and an amplifier having an inverting input coupled to the output of the compensation circuit and a non-inverting input coupled to a known reference voltage.

Claims:
What is claimed is: 
     
       1. A front-end signal compensation apparatus, comprising:
 a compensation circuit configured for coupling to a sense node and receiving a sense signal indicative of a detected touch, the sense signal having a first amplitude during a no-touch condition and a second amplitude during a touch condition, the first amplitude being greater than the second amplitude; and 
 a signal generator coupled to the compensation circuit and configured for generating a compensation signal 180 degrees out of phase with respect to the sense signal, the compensation signal having a third amplitude between the first amplitude and the second amplitude; 
 wherein the compensation circuit is configured for summing the sense signal with the compensation signal to produce a compensated sense signal having the same phase as the sense signal during the no-touch condition and 180 degrees out of phase with respect to the sense signal during the touch condition. 
 
     
     
       2. The front-end signal compensation apparatus of  claim 1 , further comprising:
 an amplifier having an inverting input coupled to the compensation circuit and a non-inverting input coupled to a known reference voltage; and 
 a feedback capacitor coupled between an output of the amplifier and the inverting input. 
 
     
     
       3. The front-end signal compensation apparatus of  claim 2 , further comprising a mixer coupled to the amplifier and the signal generator for demodulating the output of the amplifier. 
     
     
       4. The front-end signal compensation apparatus of  claim 3 , further comprising an analog-to-digital converter (ADC) coupled to the mixer for generating a demodulated compensated sense signal. 
     
     
       5. The front-end signal compensation apparatus of  claim 1 , wherein the signal generator comprises:
 a digital-to-analog converter (DAC) for generating the compensation signal, the DAC coupled to the compensation circuit; and 
 a look-up table, coupled to the DAC, for storing and providing a digital code to the DAC, wherein the compensation signal is generated based on the digital code. 
 
     
     
       6. The front-end signal compensation apparatus of  claim 5 , wherein the DAC comprises a digital-to-analog voltage converter (VDAC) and a capacitor coupled to the VDAC. 
     
     
       7. The front-end signal compensation apparatus of  claim 5 , wherein the DAC comprises a digital-to-analog current converter (IDAC). 
     
     
       8. The front-end signal compensation apparatus of  claim 1 , wherein the third amplitude of the compensation signal is selected to be an average of the first and second amplitudes of the sense signal. 
     
     
       9. A touch sensing system incorporating the front-end signal compensation apparatus of  claim 1 , the touch sensing system comprising a touch sensor configured for generating the sense signal. 
     
     
       10. A method for front-end signal compensation, comprising:
 receiving a sense signal indicative of a detected touch, the sense signal having a first amplitude during a no-touch condition and a second amplitude during a touch condition, the first amplitude being greater than the second amplitude; 
 generating a compensation signal 180 degrees out of phase with respect to the sense signal, the compensation signal having a third amplitude between the first amplitude and the second amplitude; and 
 summing the sense signal with the compensation signal to produce a compensated sense signal having the same phase as the sense signal during the no-touch condition and 180 degrees out of phase with respect to the sense signal during the touch condition. 
 
     
     
       11. The method of  claim 10 , further comprising amplifying the compensated sense signal with a virtual-ground amplifier. 
     
     
       12. The method of  claim 11 , further comprising demodulating the amplified and compensated sense signal. 
     
     
       13. The method of  claim 12 , further comprising converting the demodulated, amplified and compensated sense signal from an analog signal to a digital signal. 
     
     
       14. The method of  claim 10 , further comprising generating the compensation signal by:
 accessing a look-up table to generate a digital code; and 
 providing the digital code to a digital-to-analog converter (DAC) to generate the compensation signal. 
 
     
     
       15. The method of  claim 10 , further comprising selecting the third amplitude of the compensation signal to be an average of the first and second amplitudes of the sense signal. 
     
     
       16. A front-end signal compensation apparatus, comprising:
 means for receiving a sense signal indicative of a detected touch, the sense signal having a first amplitude during a no-touch condition and a second amplitude during a touch condition, the first amplitude being greater than the second amplitude; 
 means for generating a compensation signal 180 degrees out of phase with respect to the sense signal, the compensation signal having a third amplitude between the first amplitude and the second amplitude; and 
 means for summing the sense signal with the compensation signal to produce a compensated sense signal having the same phase as the sense signal during the no-touch condition and 180 degrees out of phase with respect to the sense signal during the touch condition. 
 
     
     
       17. The front-end signal compensation apparatus of  claim 16 , further comprising means for amplifying the compensated sense signal. 
     
     
       18. The front-end signal compensation apparatus of  claim 17 , further comprising means for demodulating the amplified and compensated sense signal. 
     
     
       19. The front-end signal compensation apparatus of  claim 16 , the means for generating the compensation signal comprising:
 table look-up means for generating a digital code; and 
 means for converting the digital code to an analog signal to generate the compensation signal. 
 
     
     
       20. The front-end signal compensation apparatus of  claim 16 , further comprising means for selecting the third amplitude of the compensation signal to be an average of the first and second amplitudes of the sense signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/093,638, filed Apr. 7, 2016, and published on Aug. 4, 2016 as U.S. Publication No. 2016/0224185; which is a continuation of U.S. patent application Ser. No. 14/042,462, filed Sep. 30, 2013, and issued on Apr. 26, 2016 as U.S. Pat. No. 9,323,405; which is a continuation of U.S. patent application Ser. No. 13/284,732, filed Oct. 28, 2011, and issued on Oct. 8, 2013 as U.S. Pat. No. 8,553,004; which is a continuation of U.S. patent application Ser. No. 11/650,043, filed Jan. 3, 2007, and issued on Nov. 1, 2011 as U.S. Pat. No. 8,049,732; the entire contents of which are incorporated herein by reference in their entirety for all purposes. 
    
    
     FIELD OF THE INVENTION 
     This relates generally to electronic devices (e.g., a touch screen) capable of generating a dynamic output signal, and more particularly, to a method and system of compensating for undesired portions (e.g., a static portion) of the output signal. 
     BACKGROUND OF THE INVENTION 
     One example of an electronic device that generates dynamic output signals is a user input device for performing operations in a computer system. Such input devices generate output signals based on user operation of the device or user data or commands entered into the device. The operations generally correspond to moving a cursor and/or making selections on a display screen. By way of example, the input devices may include buttons or keys, mice, trackballs, touch pads, joy sticks, touch screens and the like. Touch pads and touch screens (collectively “touch surfaces”) are becoming increasingly popular because of their ease and versatility of operation as well as to their declining price. Touch surfaces allow a user to make selections and move a cursor by simply touching the surface, which may be a pad or the display screen, with a finger, stylus, or the like. In general, the touch surface recognizes the touch and position of the touch and the computer system interprets the touch and thereafter performs an action based on the touch. 
     Touch pads are well-known and ubiquitous today in laptop computers, for example, as a means for moving a cursor on a display screen. Such touch pads typically include a touch-sensitive opaque panel which senses when an object (e.g., finger) is touching portions of the panel surface. Touch screens are also well known in the art. Various types of touch screens are described in applicant&#39;s co-pending patent application Ser. No. 10/840,862, entitled “Multipoint Touchscreen,” filed May 6, 2004, which is hereby incorporated by reference in its entirety. As noted therein, touch screens typically include a touch-sensitive panel, a controller and a software driver. The touch-sensitive panel is generally a clear panel with a touch sensitive surface. The touch-sensitive panel is positioned in front of a display screen so that the touch sensitive surface covers the viewable area of the display screen. The touch-sensitive panel registers touch events and sends these signals to the controller. The controller processes these signals and sends the data to the computer system. The software driver translates the touch events into computer events. There are several types of touch screen technologies including resistive, capacitive, infrared, surface acoustic wave, electromagnetic, near field imaging, etc. Each of these devices has advantages and disadvantages that are taken into account when designing or configuring a touch screen. 
     In conventional touch surface devices, and other types of input devices, there is typically an operational amplifier that amplifies the output signal of the device. The output signal is a dynamic signal in that it changes between two or more states (e.g., a “touch” or “no touch” condition). In conventional devices, the amplifier may be followed by an output signal compensation circuit that provides a compensation signal to offset an undesired portion (e.g., static portion) of the output signal. The problem with this configuration is that the amplifier amplifies both the dynamic signal of interest as well as the undesired static or offset portion. 
     Additionally, by compensating the output signal after it has been amplified, conventional compensation methods provide poor utilization of the output dynamic range of the amplifier, which results in poor sensitivity in detecting dynamic changes in the output signal. 
     Furthermore, in devices wherein the output signal is a charge waveform (e.g., an output signal from a capacitive touch surface), a relatively large feedback capacitor is typically connected between the output of the amplifier and the inverting input of the amplifier in order to accommodate relatively large charge amplitudes at the inverting input of the amplifier. The charge amplitudes should be sufficiently large to provide a sufficiently high signal-to-noise (S/N) ratio. The large feedback capacitors, however, consume a significant amount of integrated circuit (IC) chip “real estate” and hence, add significant costs and size requirements to the IC chips. 
     SUMMARY OF THE INVENTION 
     The invention addresses the above and other needs by providing a new method and system for compensating for undesired portions (i.e., “offset portions”) of an output signal. In various embodiments, the invention is utilized in connection with a touch surface device, wherein offset compensation is provided to the output signals of the touch surface device before the output signal is provided to an input of an amplifier. Thus, the amplifier amplifies only a desired (e.g., dynamic) portion of the output signal. When the output signal is compensated in this fashion, changes in magnitude of the output signals due to a touch of the touch surface device, for example, reflect a much larger portion of the dynamic range of the amplifier, thereby providing more sensitivity and dynamic range to the touch surface device. 
     In one embodiment, the output signal of a touch surface device is summed with a compensation signal prior to being provided to an inverting input of an amplifier. The compensation signal has a desired amplitude, waveform, frequency and phase to provide a desired compensation to the output signal. In one embodiment, the compensation signal is generated by a compensation circuit that includes a look-up table, a digital to analog voltage converter (VDAC) and a compensation capacitor C COMP  for converting the output of the VDAC into a charge waveform that is used to compensate a charge waveform output of the touch surface device. The look-up table stores digital codes that are provided to the VDAC to generate the desired compensation signal. 
     In another embodiment, a charge compensation circuit includes a look-up table and a digital-to-analog current converter (IDAC). The look-up table stores digital codes that are provided to the IDAC to generate a desired current waveform that when viewed in the charge domain corresponds to a desired charge waveform to compensate a charge waveform output signal. 
     In a further embodiment, a compensation signal is generated by one or more capacitive nodes on a touch surface device that are insensitive to touch. A compensation drive signal, provided to one or more touch-insensitive nodes, is substantially 180 degrees out of phase with the drive signal provided to the touch-sensitive nodes of the touch surface device. The touch-insensitive nodes provide a compensation signal that is substantially 180 degrees out of phase with respect to an output signal generated by a touch sensitive node such that when summed together, a desired portion of the output signal is removed. Additionally, because the compensation signal is being generated by the touch surface device, variations in the output signal from a touch-sensitive node due to variations in processing or external conditions (e.g., temperature, dielectric thickness, etc.) are also exhibited by the compensation signal. Thus, the behavior and/or variations in the compensation signal “track” the behavior and/or variations in the output signals generated by the touch-sensitive portions of the touch surface device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a perspective view of a touch surface device capable utilizing an improved output signal compensation circuit and method, in accordance with one embodiment of the invention. 
         FIG. 2  is a block diagram of a computing device or system incorporating a touch surface device, in accordance with one embodiment of the invention. 
         FIGS. 3A and 3B  illustrate two possible arrangements of drive and sense electrodes in a touch screen, in accordance with various embodiments of the invention. 
         FIG. 4  illustrates a top view of transparent multipoint touch screen, in accordance with one embodiment of the present invention. 
         FIG. 5  is a partial front elevation view, in cross section of a display arrangement, in accordance with one embodiment of the present invention. 
         FIG. 6  is a simplified diagram of a mutual capacitance circuit, in accordance with one embodiment of the present invention. 
         FIG. 7  is a diagram of a charge amplifier, in accordance with one embodiment of the present invention. 
         FIG. 8  is a block diagram of a touch surface device and controller system, in accordance with one embodiment of the present invention. 
         FIGS. 9A and 9B  illustrate perspective side views of an exemplary capacitive sensing nodes (a.k.a., pixels) in “no touch” and “touch” states, respectively, in accordance with one embodiment of the present invention. 
         FIG. 10A  illustrates an exemplary drive signal waveform applied to a selected drive (e.g., row) electrode of a touch surface panel, in accordance with one embodiment of the present invention. 
         FIG. 10B  illustrates exemplary charge output waveforms (“touch” and “no touch”) generated by a sense (e.g., column) electrode of a touch surface panel, in accordance with one embodiment of the present invention. 
         FIG. 11  illustrates an exemplary analog sensing circuit or channel with front-end compensation, in accordance with one embodiment of the present invention. 
         FIG. 12  illustrates exemplary compensated signal waveforms representing a “no touch” and “max touch” state, respectively, in accordance with one embodiment of the present invention. 
         FIG. 13A  illustrates an exemplary compensation signal generator circuit, in accordance with one embodiment of the present invention. 
         FIG. 13B  illustrates another exemplary compensation signal generator circuit, in accordance with another embodiment of the present invention. 
         FIG. 14  illustrates a touch surface device and its drive circuitry, wherein portions of the touch surface device are utilized to generate a compensation signal, in accordance with one embodiment of the present invention. 
         FIG. 15  illustrates a top view of an exemplary touch surface panel wherein a top row of the touch surface panel is utilized to generate a compensation signal, in accordance with one embodiment of the invention. 
         FIGS. 16A and 16B  illustrate a touch-sensitive capacitive sensing node and a touch-insensitive node that is utilized to generate at least a portion of a compensation signal, in accordance one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the following description of preferred embodiments, reference is made to the accompanying drawings which form a part hereof, and in which it is shown by way of illustration specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention. Furthermore, although embodiments of the present invention are described herein in terms of devices and applications compatible with computer systems and devices manufactured by Apple Computer, Inc. of Cupertino, Calif., such embodiments are illustrative only and should not be considered limiting in any respect. 
       FIG. 1  is a perspective view of a touch screen display arrangement  30 , which includes a display  34  and a transparent touch screen  36  positioned in front of display  34 . Display  34  may be configured to display a graphical user interface (GUI) including perhaps a pointer or cursor as well as other information to the user. Transparent touch screen  36  is an input device that is sensitive to a user&#39;s touch, allowing a user to interact with the graphical user interface on display  34 . In general, touch screen  36  recognizes touch events on surface  38  of touch screen  36  and thereafter outputs this information to a host device. The host device may, for example, correspond to a computer such as a desktop, laptop, handheld or tablet computer. The host device interprets the touch event and thereafter performs an action based on the touch event. 
     In one embodiment, touch screen  36  is configured to recognize multiple touch events that occur simultaneously at different locations on touch sensitive surface  38 . That is, touch screen  36  allows for multiple contact points T 1 -T 4  to be tracked simultaneously. Touch screen  36  generates separate tracking signals S 1 -S 4  for each touch point T 1 -T 4  that occurs on the surface of touch screen  36  at the same time. In one embodiment, the number of recognizable touches may be about fifteen which allows for a user&#39;s ten fingers and two palms to be tracked along with three other contacts. The multiple touch events can be used separately or together to perform singular or multiple actions in the host device. Numerous examples of multiple touch events used to control a host device are disclosed in U.S. Pat. Nos. 6,323,846; 6,888,536; 6,677,932; 6,570,557, and co-pending U.S. patent application Ser. Nos. 11/015,434; 10/903,964; 11/048,264; 11/038,590; 11/228,758; 11/228,700; 11/228,737; 11/367,749, each of which is hereby incorporated by reference in its entirety. 
       FIG. 2  is a block diagram of a computer system  50 , employing a multi-touch touch screen. Computer system  50  may be, for example, a personal computer system such as a desktop, laptop, tablet, or handheld computer. The computer system could also be a public computer system such as an information kiosk, automated teller machine (ATM), point of sale machine (POS), industrial machine, gaming machine, arcade machine, vending machine, airline e-ticket terminal, restaurant reservation terminal, customer service station, library terminal, learning device, mobile telephone, audio/video player, etc. 
     Computer system  50  includes a processor  56  configured to execute instructions and to carry out operations associated with the computer system  50 . Computer code and data required by processor  56  are generally stored in storage block  58 , which is operatively coupled to processor  56 . Storage block  58  may include read-only memory (ROM)  60 , random access memory (RAM)  62 , hard disk drive  64 , and/or removable storage media such as CD-ROM, PC-card, floppy disks, and magnetic tapes. Any of these storage devices may also be accessed over a network. Computer system  50  also includes a display device  68  that is operatively coupled to the processor  56 . Display device  68  may be any of a variety of display types including liquid crystal displays (e.g., active matrix, passive matrix, etc.), cathode ray tubes (CRT), plasma displays, etc. Computer system  50  also includes touch screen  70 , which is operatively coupled to the processor  56  by I/O controller  66  and touch screen controller  76 . (The I/O controller  66  may be integrated with the processor  56 , or it may be a separate component.) In any case, touch screen  70  is a transparent panel that is positioned in front of the display device  68 , and may be integrated with the display device  68  or it may be a separate component. Touch screen  70  is configured to receive input from a user&#39;s touch and to send this information to the processor  56 . In most cases, touch screen  70  recognizes touches and the position and magnitude of touches on its surface. 
     The host processor  561  receives outputs from the touch screen controller  76  and performs actions based on the outputs. Such actions may include, but are not limited to, moving an object such as a cursor or pointer, scrolling or panning, adjusting control settings, opening a file or document, viewing a menu, making a selection, executing instructions, operating a peripheral device connected to the host device, answering a telephone call, placing a telephone call, terminating a telephone call, changing the volume or audio settings, storing information related to telephone communications such as addresses, frequently dialed numbers, received calls, missed calls, logging onto a computer or a computer network, permitting authorized individuals access to restricted areas of the computer or computer network, loading a user profile associated with a user&#39;s preferred arrangement of the computer desktop, permitting access to web content, launching a particular program, encrypting or decoding a message, and/or the like. The host processor  76  may also perform additional functions that may not be related to multi-touch (MT) panel processing, and may be coupled to program storage  58  and the display device  68  such as an LCD display for providing a user interface (UI) to a user of the device. 
     In one embodiment, the touch screen panel  70  can be implemented as a mutual capacitance device constructed as described below with reference to  FIGS. 3A and 3B . In this embodiment, the touch screen panel  70  is comprised of a two-layered electrode structure, with driving lines or electrodes on one layer and sensing lines or electrodes on the other. In either case, the layers are separated by a dielectric material (not shown). In the Cartesian arrangement of  FIG. 3A , one layer is comprised of N horizontal, preferably equally spaced row electrodes  81 , while the other layer is comprised of M vertical, preferably equally spaced column electrodes  82 . In a polar arrangement, illustrated in  FIG. 3B , the sensing lines may be concentric circles and the driving lines may be radially extending lines (or vice versa). As will be appreciated by those skilled in the art, other configurations based on a variety of coordinate systems are also possible. Additionally, it is understood that the invention is not necessarily limited to touch surface devices utilizing mutual capacitance sensing nodes. The invention may be implemented within other types of touch surface devices such as “self capacitance” devices, for example. 
     Each intersection  83  represents a pixel and has a characteristic mutual capacitance, C SIG . A grounded object (such as a finger) that approaches a pixel  83  from a finite distance shunts the electric field between the row and column intersection, causing a decrease in the mutual capacitance C SIG  at that location. In the case of a typical sensor panel, the typical signal capacitance C SIG  is about 1.0 picofarads (pF) and the change (ΔC SIG ) induced by a finger touching a pixel, is about 0.10 pF. These capacitance values are exemplary only and should not in any way limit the scope of the present invention. 
     The electrode material may vary depending on the application. In touch screen applications, the electrode material may be ITO (Indium Tin Oxide) on a glass substrate. In a touch tablet, which need not be transparent, copper on an FR4 substrate may be used. The number of sensing points  83  may also be widely varied. In touch screen applications, the number of sensing points  83  generally depends on the desired sensitivity as well as the desired transparency of the touch screen  70 . More nodes or sensing points generally increases sensitivity, but reduces transparency (and vice versa). 
     During operation, each row electrode (i.e., a drive electrode) is sequentially charged by driving it with a predetermined voltage waveform (discussed in greater detail below). The charge capacitively couples to the column electrodes (i.e., sense electrodes) at the intersections between the drive electrode and the sense electrodes. In alternative embodiments the column electrodes can be configured as the drive electrodes and the row electrodes can be configured as the sense electrodes. The capacitance of each intersection  83  is measured to determine the positions of multiple objects when they touch the touch surface. Sensing circuitry monitors the charge transferred and time required to detect changes in capacitance that occur at each node. The positions where changes occur and the magnitude of those changes are used to identify and quantify the multiple touch events. 
       FIG. 4  is a top view of a transparent multipoint touch screen  150 , in accordance with one embodiment of the present invention. As shown, the touch screen  150  includes a two layer grid of spatially separated lines or wires  152 . In most cases, the lines  152  on each layer are parallel one another. Furthermore, although in different planes, the lines  152  on the different layers are configured to intersect or cross in order to produce capacitive sensing nodes  154  (a.k.a., “pixels”), which each represent different coordinates in the plane of the touch screen  150 . The nodes  154  are configured to receive capacitive input from an object touching the touch screen  150  in the vicinity of the node  154 . When an object (e.g., a finger tip) is proximate the node  154 , the object steals charge thereby affecting the capacitance at the node  154 . It has been found that as a finger is pressed more firmly against the touch screen surface  150 , the surface area of the finger touching the touch screen  150  increases and a greater amount of charge is diverted away from the underlying sensing node(s)  154 . 
     The lines  152  on different layers serve two different functions. One set of lines  152 A drives a current therethrough while the second set of lines  152 B senses the capacitance coupling at each of the nodes  154 . In most cases, the top layer provides the driving lines  152 A while the bottom layer provides the sensing lines  152 B. The driving lines  152 A are connected to a voltage source (not shown) that separately drives the current through each of the driving lines  152 A. That is, the stimulus is only happening over one line while all the other lines are grounded. They may be driven similarly to a raster scan. Each sensing line  152 B is connected to a capacitive sensing circuit (not shown) that senses a charge and, hence, capacitance level for the sensing line  152 B. 
     When driven, the charge on the driving line  152 A capacitively couples to the intersecting sensing lines  152 B through the nodes  154  and the capacitive sensing circuits sense their corresponding sensing lines  152 B in parallel. Thereafter, the next driving line  152 A is driven, and the charge on the next driving line  152 A capacitively couples to the intersecting sensing lines  152 B through the nodes  154  and the capacitive sensing circuits sense all of the sensing lines  152 B in parallel. This happens sequentially until all the lines  152 A have been driven. Once all the lines  152 A have been driven, the sequence starts over (continuously repeats). As explained in further detail below, in one embodiment, the capacitive sensing circuits are fabricated on an application specific integrated circuit (ASIC), which converts analog capacitive signals to digital data and thereafter transmits the digital data over a serial bus to a host controller or microprocessor for processing. 
     The lines  152  are generally disposed on one or more optical transmissive members  156  formed from a clear material such as glass or plastic. By way of example, the lines  152  may be placed on opposing sides of the same member  156  or they may be placed on different members  156 . The lines  152  may be placed on the member  156  using any suitable patterning technique including for example, deposition, etching, printing and the like. Furthermore, the lines  152  can be made from any suitable transparent conductive material. By way of example, the lines may be formed from indium tin oxide (ITO). The driving lines  152 A may be coupled to the voltage source through a flex circuit  158 A, and the sensing lines  152 B may be coupled to the sensing circuits via a flex circuit  158 B. The sensor ICs may be attached to a printed circuit board (PCB). 
     The distribution of the lines  152  may be widely varied. For example, the lines  152  may be positioned almost anywhere in the plane of the touch screen  150 . The lines  152  may be positioned randomly or in a particular pattern about the touch screen  150 . With regards to the later, the position of the lines  152  may depend on the coordinate system used. For example, the lines  152  may be placed in rows and columns for Cartesian coordinates or concentrically and radially for polar coordinates. When using rows and columns, the rows and columns may be placed at various angles relative to one another. For example, they may be vertical, horizontal or diagonal. 
       FIG. 5  is a partial front elevation view, in cross section of an exemplary display arrangement  170 . The display arrangement  170  includes an LCD display  172  and a touch screen  174  positioned over the LCD display  172 . The touch screen may for example correspond to the touch screen shown in  FIG. 4 . The LCD display  172  may correspond to any conventional LCD display known in the art. Although not shown, the LCD display  172  typically includes various layers including a fluorescent panel, polarizing filters, a layer of liquid crystal cells, a color filter and the like. 
     The touch screen  174  includes a transparent sensing layer  176  that is positioned over a first glass member  178 . The sensing layer  176  includes a plurality of sensor lines  177  positioned in columns (which extend in and out of the page). The first glass member  178  may be a portion of the LCD display  172  or it may be a portion of the touch screen  174 . For example, it may be the front glass of the LCD display  172  or it may be the bottom glass of the touch screen  174 . The sensor layer  176  is typically disposed on the glass member  178  using suitable transparent conductive materials and patterning techniques. In some cases, it may be desirable to coat the sensor layer  176  with material of similar refractive index to improve the visual appearance, i.e., make it more uniform. 
     The touch screen  174  also includes a transparent driving layer  180  that is positioned over a second glass member  182 . The second glass member  182  is positioned over the first glass member  178 . The sensing layer  176  is therefore sandwiched between the first and second glass members  178  and  182 . The second glass member  182  provides an insulating layer between the driving and sensing layers  176  and  180 . The driving layer  180  includes a plurality of driving lines  181  positioned in rows (extend to the right and left of the page). The driving lines  181  are configured to intersect or cross the sensing lines  177  positioned in columns in order to form a plurality of capacitive coupling nodes  182 . Like the sensing layer  176 , the driving layer  180  is disposed on the glass member  182  using suitable materials and patterning techniques. Furthermore, in some cases, it may be necessary to coat the driving layer  180  with material of similar refractive index to improve the visual appearance. Although the sensing layer is typically patterned on the first glass member, it should be noted that in some cases it may be alternatively or additionally patterned on the second glass member. 
     The touch screen  174  also includes a protective cover sheet  190  disposed over the driving layer  180 . The driving layer  180  is therefore sandwiched between the second glass member  182  and the protective cover sheet  190 . The protective cover sheet  190  serves to protect the under layers and provide a surface for allowing an object to slide thereon. The protective cover sheet  190  also provides an insulating layer between the object and the driving layer  180 . The protective cover sheet is suitably thin to allow for sufficient coupling. The protective cover sheet  190  may be formed from any suitable clear material such as glass and plastic. In addition, the protective cover sheet  190  may be treated with coatings to reduce friction or sticking when touching and reduce glare when viewing the underlying LCD display  172 . By way of example, a low friction/anti reflective coating may be applied over the cover sheet  190 . Although the line layer is typically patterned on a glass member, it should be noted that in some cases it may be alternatively or additionally patterned on the protective cover sheet. 
     The touch screen  174  also includes various bonding layers  192 . The bonding layers  192  bond the glass members  178  and  182  as well as the protective cover sheet  190  together to form the laminated structure and to provide rigidity and stiffness to the laminated structure. In essence, the bonding layers  192  help to produce a monolithic sheet that is stronger than each of the individual layers taken alone. In most cases, the first and second glass members  178  and  182  as well as the second glass member and the protective sheet  182  and  190  are laminated together using a bonding agent such as glue. The compliant nature of the glue may be used to absorb geometric variations so as to form a singular composite structure with an overall geometry that is desirable. In some cases, the bonding agent includes an index matching material to improve the visual appearance of the touch screen  170 . 
       FIG. 6  is a simplified diagram of a mutual capacitance circuit  220 , in accordance with one embodiment of the present invention. The mutual capacitance circuit  220  includes a driving line  222  and a sensing line  224  that are spatially separated by a capacitive coupling node  226 . The driving line  222  is electrically coupled to a voltage source  228 , and the sensing line  224  is electrically coupled to a capacitive sensing circuit  230 . The driving line  222  is configured to carry a current to the capacitive coupling node  226 , and the sensing line  224  is configured to carry a current to the capacitive sensing circuit  230 . When no object is present, the capacitive coupling at the node  226  stays fairly constant. When an object  232  such as a finger is placed proximate the node  226 , the capacitive coupling through the node  226  changes. The object  232  effectively shunts some of the electromagnetic field away so that the charge formed across the node  226  decreases. The change in capacitive coupling changes the current that is carried by the sensing lines  224 . The capacitive sensing circuit  230  notes the current change and the position of the node  226  where the current change occurred and reports this information in a raw or in some processed form to a host controller or microprocessor. Such sensing occurs for each node at a rapid scan rate so that from the perspective of a user it appears that all nodes are sensed simultaneously. 
     In one embodiment, the capacitive sensing circuit  230  includes an input filter  236  for eliminating parasitic or stray capacitance  237 , which may for example be created by the large surface area of the row and column lines relative to the other lines and the system enclosure at ground potential. Generally speaking, the filter rejects stray capacitance effects so that a clean representation of the charge transferred across the node  226  is outputted. That is, the filter  236  produces an output that is not dependent on the parasitic or stray capacitance, but rather on the capacitance at the node  226 . As a result, a more accurate output is produced. 
       FIG. 7  is a diagram of a charge amplifier  240  that may be used as the filter  236 , in accordance with one embodiment of the present invention. As shown, the amplifier includes a non inverting input that is held at a constant voltage (e.g., a reference voltage or ground), and an inverting input that is coupled to the node  226 . The output of the amplifier  240  is coupled back to the inverting input through a feedback capacitor (C FB )  242 . As is known in the art, in this configuration, the amplifier  240  and C FB    242  eliminate stray capacitance that may otherwise effect the measurement of capacitance or change in capacitance (ΔC SIG ) at the capacitive sensing node  226 . Because of the characteristics of the amplifier  240 , any charge that appears across C STRAY  will be equal to the charge at the output of the amplifier and, therefore, no matter how much stray capacitance C STRAY  is added to the inverting input, the net charge across C SIG  will always be zero. In one embodiment, the inverting amplifier  240  in combination with the C FB    242  perform the following tasks: (1) charge to voltage conversion, (2) charge amplification, (3) rejection of stray capacitance present at the column electrode, (4) anti aliasing, and (5) gain equalization at different frequencies. Charge to voltage conversion is performed by the feedback capacitor C FB  in the feedback path of the amplifier  240 . 
     In one embodiment, the functions of driving each row electrode and sensing the charge transfer on each corresponding column electrode are performed by a multipoint touch screen controller system  300 , as shown in  FIG. 8 . The controller system  100  includes a touch surface device  301  (e.g., a touch screen or touch pad), an application specific integrated circuit (ASIC) chip  305  for receiving output (e.g., touch data) from the touch surface  301 , a microprocessor  307  for receiving and processing digital data from the ASIC  305 , a level shifter or voltage multiplier  310  for generating drive signals of a desired amplitude, and a decoder  311  for decoding timing signals from the microprocessor and applying the drive signal to an appropriate row electrode. 
     In one embodiment, the ASIC  305  receives analog signals (e.g., voltage waveforms) from each column electrode  82  ( FIG. 3 a   ) of the touch surface device  301  indicating a touch or no-touch condition at a respective capacitive sensing node  83  corresponding to an intersection of the column electrode  82  and a selected, driven row electrode  81  of the touch surface device  301 . The ASIC  305  converts the analog signals received from the node  83  of the touch surface  301  into digital signals which are then received and processed by the microprocessor  307  in order to sense touch and/or multi-touch states. In one embodiment, the ASIC  305  contains a plurality of inverting amplifier  240  and feedback capacitor  242  circuits, similar to that shown in  FIG. 7 , each coupled to respective column electrodes of the touch surface device  301 . 
     The ASIC  305  further generates all the drive waveforms necessary to scan the sensor panel and provides those waveforms to the level shifter  310 , which amplifies the drive waveforms. In one embodiment, the microprocessor  307  sends a clock signal  321  to set the timing of the ASIC  305 , which in turn generates the appropriate timing waveforms  322  to create the row stimuli to the touch surface device  301 . Decoder  311  decodes the timing signals to drive each row of the touch surface  301  in sequence. Level shifter  310  converts the timing signals  322  from the signaling level (e.g., 3.3 V p-p ) to the level used to drive the touch surface device  301  (e.g., 18V p-p ). 
     In one embodiment, it is desirable to drive the panel at multiple different frequencies for noise rejection purposes. Noise that exists at a particular drive frequency may not, and likely will not exist at the other frequencies. In one embodiment, each sensor panel row is stimulated with three bursts of twelve square wave cycles (50% duty-cycle, 18V amplitude), while the remaining rows are kept at ground. For better noise rejection, the frequency of each burst is different. Exemplary burst frequencies are 140 kHz, 200 kHz, and 260 Khz. A more detailed discussion of this “frequency hopping” method is provided in a commonly-owned and concurrently pending patent application entitled “Scan Sequence Generator” (U.S. application Ser. No. 11/650,046), the entirety of which is incorporated by reference herein. 
     During each burst of pulses, ASIC  305  takes a measurement of the column electrodes. This process is repeated for all remaining rows in the sensor panel. After all rows have been scanned in a single scan cycle, the measurement results are used to provide one or more images of the touch/no-touch state of the touch surface  301 , each image taken at a different stimulus frequency. The images are stored in a memory (not shown) accessible by the microprocessor  307  and processed to determine a no-touch, touch or multi-touch condition. 
       FIG. 9A  illustrates a side view of an exemplary sensing node (a.k.a., pixel)  350  in a steady-state (no-touch) condition. The node  350  is located at an intersection of a row electrode  352  and a column electrode  354 , separated by a dielectric  356 . An electric field illustrated by electric field lines  358  between the column  354  and row  352  traces or electrodes create a mutual capacitance, C SIG , between the row and column electrodes when a driving signal or stimulus is applied to the row electrode or trace  352 . 
       FIG. 9B  is a side view of the exemplary node  350  in a dynamic (touch) condition. A user&#39;s finger  360 , which has been placed on or near the node  350 , blocks some of the electric field lines  358  between the row  352  and column  354  electrodes (those fringing fields that exit the dielectric and pass through the air above the row electrode). These electric field lines are shunted to ground through the capacitance inherent in the finger, and as a result, the steady state signal capacitance C SIG  is reduced by ΔC SIG . Therefore, the signal capacitance at the node  350  becomes C SIG −ΔC SIG , where C SIG  represents the static (no touch) component and ΔC SIG  represents the dynamic (touch) component. Note that C SIG −ΔC SIG  may always be nonzero due to the inability of a finger or other object to block all electric fields, especially those electric fields that remain entirely within the dielectric. In addition, it should be understood that as a finger is pushed harder or more completely onto the touch surface, the finger will tend to flatten and increase in surface area, thereby blocking more of the electric fields. Thus, ΔC SIG  may be variable and representative of how completely the finger is pushing down on the panel (i.e., a range from “no-touch” to “full-touch”). 
       FIG. 10A  illustrates an exemplary driving signal waveform  402  that may be provided to a row electrode  81  ( FIG. 3A ). In this example, the driving signal is a square wave signal having a peak-to-peak amplitude of approximately 20 V p-p . Since charge (Q) equals voltage (V) multiplied by capacitance (C), if the mutual capacitance (C SIG ) at a sensing node  83  is 1.0 pF, for example, the output at the corresponding column electrode  82  will be a square wave having an amplitude of 20 pico-coulombs peak-to-peak (pC p-p ) when viewed in the charge domain, as shown by solid line  404  in  FIG. 10B . When a finger or other object comes in close proximity to the node  83 , its mutual capacitance value will decrease to C SIG −ΔC SIG , as discussed above. If, for example, the decreased capacitance C SIG −ΔC SIG =0.9 pF, then the output at the column electrode  82  will be a square wave having a peak-to-peak amplitude of 18 pC p-p , as indicated by the dashed waveform  406  in  FIG. 10B  (these figures are not necessarily drawn to scale). Thus, the difference in charge output from a column electrode between a “no-touch” condition and a “touch” condition would be 20−18=2 pC p-p  in this example. 
       FIG. 11  illustrates a capacitive sensing circuit  500  with front-end charge compensation, in accordance with one embodiment of the invention. Depending on whether a touch condition is being sensed at a respective capacitive sensing node, as described above, a charge waveform  404  (no touch) or  406  (touch) will be received at a first input of a signal summing circuit or multiplier  502 . The sensing circuit  500  further includes a digital-analog-converter (DAC)  504  which provides a compensation waveform signal  506  to a second input of the multiplier  502 . The compensation waveform signal  506  is generated to have substantially the same frequency as the charge waveform  404  or  406  but substantially 180 degrees out of phase with the charge waveform  404  or  406  outputted from the touch surface device  301  ( FIG. 8 ). The amplitude of the compensation signal  506  may be selected to achieve any desired level of offset and/or amplitude range for the resulting compensated signal  508  outputted by the multiplier  502 . In one embodiment, the peak-to-peak amplitude of the compensation signal  506  is selected to be the average of the amplitudes of waveforms  404  and  406 . For example, if the amplitude of waveform  404  is 20 pC p-p  and the amplitude of waveform  406  is 18 pC p-p  (e.g., at full touch), the amplitude of the compensation waveform  506  would be selected to be 19 pC p-p . Various advantages of selecting the amplitude of the compensation waveform  506  to be the average of the amplitudes of waveforms  404  and  406  are discussed in further detail below. 
     The capacitive sensing circuit  500  further includes a look-up table  510  that provides a digital signal to the DAC  504 , which the DAC  504  converts into the desired compensation waveform  506 , having a desired amplitude, shape, frequency and phase. Various embodiments of the DAC  504  are described in further detail below. In one embodiment, the look-up table  510  is pre-programmed to provide digital codes to the DAC  504  to generate predetermined compensation waveforms  506  corresponding to each drive signal frequency. A control signal  511  generated by row or channel scan logic circuitry (not shown) within the ASIC  305  controls what outputs will be provided to the DAC  504  and mixer  512  (described below). In various embodiments, the look-up table  510  may be implemented as one or more look-up tables residing in a memory of the ASIC  305 . Thus, in the embodiment illustrated in  FIG. 11 , a front-end compensation circuit is provided that includes the summing circuit  502 , the DAC  504  and the look-up table  510 . 
     The compensated output signal  508  from the summing circuit  502  is provided to an inverting input of the operational amplifier  240 . Since the compensated signal  508  is a charge waveform, the feedback capacitor  242  converts the charge waveform into a voltage waveform according to the equation Q=C FB V out . or V out .=Q/C FB , where Q is the amplitude of the compensated waveform  508  and V out . is the amplitude of the resulting voltage waveform  512  at the output of the amplifier  240 . It will be appreciated that since the peak-to-peak amplitude of the compensated waveform  508  (e.g., 0-2 pC p-p ) is significantly smaller than the amplitude of the uncompensated waveforms  404  or  406 , the value of C FB  may be significantly reduced (e.g., by a factor of 10-20 times) while maintaining desired voltage ranges (e.g., CMOS levels) for V out    512  at the output of the amplifier  240 . For example, to achieve a dynamic range of 1 volt, peak-to-peak (V p-p ) at the output of the amplifier  240 , if the signal at the inverting input of the amplifier is 20 pC p-p  (uncompensated), then C FB  must be equal to 20 pF. In contrast, if the maximum amplitude of the signal at the inverting input of the amplifier  240  is 2 pC p-p  (compensated), then C FB  must only equal 2 pF to provide a dynamic range of 1 V p-p . This reduction in size of C FB  is a significant advantage in terms of chip cost and “real estate” for the ASIC  305  ( FIG. 8 ), which, in one embodiment, can contain multiple capacitive sensing circuits  500  at its input stage. Capacitors require a relatively large die area in integrated circuits, which add to their costs and limit the number of devices (e.g., transistors) that can be integrated onto the IC chip. Therefore, it is advantageous to decrease the size of capacitors in IC chips when possible. 
     As shown in  FIG. 11 , the output waveform  511  of the amplifier  240  is provided to a first input of a mixer  514 . Since the waveform  511  is a square wave, which may create undesirable harmonics, a demodulation waveform  516 , which may be a sine wave digitally generated from the look-up table  510 , is synchronized to the output signal  511  and provided to a second input of the mixer  514 . In one embodiment, the mixer  514  demodulates the output  511  of the charge amplifier by subtracting the demodulation waveform  516  from the output signal  511  to provide better noise rejection. The mixer  514  rejects all frequencies outside its passband, which may be about +/−30 kHz around the frequency of the demodulation waveform  516 . This noise rejection may be beneficial in noisy environment with many sources of noise, such as 802.11, Bluetooth, etc. In some embodiments, the mixer  514  may be implemented as a synchronous rectifier that outputs a rectified Gaussian sine wave. The output of the mixer is provided to an analog-to-digital converter (ADC)  518 , which converts the analog signals into corresponding digital signals for storage and processing by the microprocessor  307  ( FIG. 8 ). 
     In one embodiment, the ADC  518  may be a sigma-delta converter, which may sum a number of consecutive digital values and average them to generate a result. However, other types of ADCs (such as a voltage to frequency converter with a subsequent counter stage) could be used. The ADC typically performs two functions: (1) it converts the offset compensated waveform from the mixer  514  to a digital value; and (2) it performs low pass filtering functions, e.g., it averages a rectified signal coming out of the mixer arrangement. The offset compensated, demodulated signal looks like a rectified Gaussian shaped sine wave, whose amplitude is a function of C FB  and C SIG . The ADC result returned to the microprocessor  307  is typically the average of that signal. 
     It is appreciated that the front-end charge compensation provided by the summing circuit  502  also significantly improves utilization of the dynamic range of the amplifier  240 . Referring again to  FIG. 10B , the uncompensated charge waveforms generated by a capacitive sensing node  83  ( FIG. 3 ) may be in range of 18 pC p-p  (max touch) to 20 pC p-p  (no touch). If these signals were provided directly to the inverting input of the amplifier  240 , and if the feedback capacitor (C m ) is equal to 10 pF, for example, the output of the amplifier will be a voltage waveform in the range of 1.8 V p-p  (max touch) to 2.0 V p-p  (no touch). Thus, the dynamic range utilized to sense the difference between a no touch condition and max touch condition would be only 0.2 V p-p , which represents a poor utilization of the dynamic range of the amplifier  240 . 
       FIG. 12  illustrates a compensated charge square waveform  600  when the output charge square waveform  404  ( FIG. 10B ) having an amplitude of 20 pC p-p , which represents a “no touch” state, is compensated with a compensation waveform  506  ( FIG. 11 ) having an amplitude of 19 pC p-p  at the same frequency but 180 degrees out of phase with the output waveform  404 . The resulting compensated waveform  600  is a charge waveform having an amplitude of 1 pC p-p  with a phase that is the same as the original output waveform  404 .  FIG. 12  further illustrates a compensated charge square waveform  602  when the output charge square waveform  406  having an amplitude of 18 pC p-p , which represents a “max touch” state, is compensated with the compensation waveform  506 . The resulting compensated waveform  602  is a charge waveform having an amplitude of 1 pC p-p  with a phase that is the same as the compensation waveform  506  and opposite the phase of the waveform  600 . 
     Thus, with front-end charge compensation, the charge waveform provided to the inverting input of the amplifier  240  swings from +0.5 pC to −0.5 pC and its phase shifts 180 degrees as the output levels transition from a “no touch” state to a “max touch” state. If the feedback capacitor (C FB )  242  is equal to 1 pF, for example, the output of the amplifier  240  will mirror the inverting input and will swing from +0.5 V to −0.5 V from a “no touch” state to a “max touch” state. Thus, in this example, the utilizing of the dynamic range of the amplifier  240  is increased from 0.2 V p-p  to 1.0 V p-p , which is a significant improvement. Additionally, the phase of the amplifier output will shift by 180 degrees at approximately a midpoint (e.g., a “medium touch” state) during the transition from a “no touch” state to a “max touch” state. This phase shift can be utilized to provide additional information concerning the level of pressure being exerted by a touch or a type of touch. As mentioned above, as a finger is pressed more firmly onto a touch surface, it tends to flatten and increase in surface area, thereby stealing more charge from the sensing node and reducing C SIG . Thus, the compensated waveform  600  will decrease in peak-to-peak amplitude from a “no touch” state to a “medium touch” state, at which point the compensated waveform  600  is ideally a flat line having an amplitude of 0 V p-p . As a finger is pressed harder onto the touch surface, the compensated waveform will transition from a “medium touch” state to a “max touch” state and shift 180 degrees in phase. Additionally, its amplitude will gradually increase as the finger is pressed down harder until the compensated waveform reaches the “max touch” state waveform  602 , as shown in  FIG. 12 . 
       FIG. 13A  illustrates an exemplary compensation signal generator  504  ( FIG. 11 ) that includes a digital-to-analog voltage converter (VDAC)  610  and compensation capacitor (C COMP )  612 , in accordance with one embodiment of the invention. The VDAC  610  receives digital signals from a look-up table (e.g., look-up table  510  in  FIG. 11 ) and generates a corresponding analog voltage signal (e.g., a square wave) having a desired amplitude, frequency and phase. This analog voltage signal is then provided to C COMP    612 , which converts the voltage waveform into a charge waveform  614  for compensating a charge waveform (e.g.,  404  or  406 ) outputted by a touch surface device, for example. This exemplary compensation circuit provides the benefit of increasing the dynamic range of the analog sensing circuit, which includes the amplifier  240  and feedback capacitor (C FB )  242  discussed above. However, one potential drawback to this compensation circuit is the necessity of the compensation capacitor (C COMP )  612 . This capacitor will typically be approximately the same size or on the same order of magnitude in size as C F . Therefore, the cost and chip “real estate” savings achieved by the reduction in size of C FB    242  is offset by the need for C COMP    612 . 
       FIG. 13B  illustrates another exemplary compensation signal generator  504  comprising a digital-to-analog current converter (IDAC)  616 , in accordance with another embodiment of the invention. In this embodiment, IDAC  616  generates a periodic current waveform  618  having a desired amplitude, frequency and phase based on digital codes received from the look-up table  510  ( FIG. 11 ). Since current represents a change in charge over time (or, in other words, charge is the integral of current over time), the current waveform  618  resembles a square wave when viewed in the charge domain. Thus, the current waveform  618  can be provided directly to the input of the summing circuit  502  ( FIG. 11 ) to compensate the output charge waveform  404  or  406  from the touch surface device. However, because the current waveform  618  is characterized by a plurality of periodically alternating current spikes, it presents potentially difficult timing issues when trying to achieve an accurate 180 degree phase shift between the current waveform  618  and the output charge waveform  404  or  406  from the touch surface device. Thus, the IDAC  616  provides both the benefits of increasing the dynamic range of the analog sensing circuit  500  ( FIG. 11 ) and decreasing IC chip cost and “real estate” requirements due to C (since C COMP  is not required). However, the IDAC  616  is less tolerant of phase mismatches and can add to the complexity of the timing logic requirements of the sensing circuit  500 . In one embodiment, in order to minimize phase mismatches and the effects of any phase mismatches between a compensation signal and an output signal, the invention utilizes the methods and circuits described in a co-pending and commonly-owned patent application entitled “Minimizing Mismatch During Phase Compensation” (U.S. application Ser. No. 11/650,038), the entirety of which is incorporated by reference herein. 
       FIG. 14  illustrates a touch surface system  700  that includes a high-voltage level shifter and decoder unit  702  and touch surface panel  704 , in accordance with one embodiment of the invention. In this embodiment, the touch surface panel  704  utilizes two touch-insensitive portions (e.g., top and bottom rows) of the touch surface panel  704  to provide substantially fixed mutual capacitance values C COMP1  and C COMP2 , respectively. When a 180-degree phase-shifted drive signal is applied to the top and bottom rows, C COMP1  and C COMP2  generate two compensation signals that, when summed, compensate the output signals provided by the capacitive nodes of a selected touch-sensitive row, as described above. The level shifter/decoder unit  702  performs functions similar to the level shifter  310  and decoder  311  described above with respect to  FIG. 8 . In this embodiment, however, the functionality of these devices is integrated into a single IC chip  702 . The level shifter/decoder unit  702  receives an input waveform  706  from ASIC  305  ( FIG. 8 ), which may be a series of twelve square wave pulses having a peak-to-peak amplitude of 2 V p-p , for example. The level shifter/decoder unit  702  then amplifies this signal to 20 V p-p , for example, and provides this drive signal to a selected row (1−n) based on a timing signal or MUX control signal  708  received from a microprocessor  307  ( FIG. 8 ) or the ASIC  305 . 
     The level shifter/decoder unit  702  further includes a plurality of selection switches  710 , which close when a corresponding row has been selected to be driven by the amplified drive signal. In one embodiment, the level shifter/decoder unit  702  has an output driver  712  corresponding to each row of the panel  704 . In alternative embodiments multiple rows may be connected to the output of one or more drivers  712  via a multiplexing/demultiplexing circuit arrangement. The level shifter/decoder unit  702  further includes an inverting gate  714  which inverts the incoming drive signal  706  to produce a 180-degree phase-shifted compensation signal that is provided to a top compensation row (C COMP1 ) and a bottom compensation row (C COMP2 ) of the touch surface panel  704 . 
     The touch surface panel  704  includes a plurality of capacitive sensing nodes, C SIG(n, m) , arranged in an (n×m) matrix, where n represents the number of touch sensitive row electrodes and m represents the number of column electrodes, which through mutual capacitance, provide output signals indicative of touch or no-touch conditions on the panel  704 . The panel  704  further includes a top row or strip that is touch-insensitive and provides a substantially fixed capacitance of C COMP1 . A bottom strip of the panel is also touch-insensitive and provides a substantially fixed capacitance of C COMP2 . As discussed above, the drive signal applied to the top and bottom touch-insensitive rows is 180 degrees out of phase with the drive signal applied to a selected touch-sensitive row. Each column electrode is always connected to the touch-insensitive rows and selectively connected to a touch-sensitive row (1−n) one at a time. Thus, the compensated capacitance seen at the output of each column electrode is effectively C SIG −(C COMP1 +C COMP2 ). 
     In one embodiment, the top and bottom strips of the panel  704  are designed so that the values of C COMP1  and C COMP2  satisfy the following equation: C COMP1  C COMP2 =(2C SIG −ΔC SIG )/2. 
     In the above equation, ΔC SIG  represents the change in mutual capacitance due to a max touch condition, as discussed above. With this design, the effective compensation signal provided by C COMP1  and C COMP2  has an amplitude that is equal to the average of the amplitude of the capacitive sensing node outputs when the node is experiencing a “no touch” state (C SIG ) and a “max touch” state (C SIG −ΔC SIG ). Some of the advantages of designing the amplitude of the compensation signal to be equal to the average of the output values corresponding to a “no touch” state and a “max touch” state are discussed above with respect to  FIG. 12 . 
       FIG. 15  illustrates a perspective view of a top portion of the touch surface panel  704 , in accordance with one embodiment of the invention. The panel  704  includes a plurality of row electrodes  706  separated by a dielectric layer (not shown) from a plurality of column electrodes  708 . In this embodiment, the column electrodes  708  are formed on top of the row electrodes  706  and are substantially orthogonal to the row electrodes. However, other configurations and arrangements would be readily apparent to those of skill in the art. As discussed above, a capacitive sensing node or “pixel” is formed at the intersection of each row electrode  706  and each column electrode  708 . A mutual capacitance is formed at each node when a drive signal is applied to a corresponding row electrode (i.e., drive electrode). In this embodiment, the row electrodes  706  are configured as the drive electrodes and the column electrodes  708  are configured as sense electrodes. In alternative embodiments, the row electrodes  706  may be configured as the sense electrodes and the column electrodes  708  may be configured as the drive electrodes which are driven by a drive input signal applied to the panel  704 . 
     The top row  710 , however, is touch-insensitive due to the configuration and arrangement of the top portions  712  of the column electrodes  708  above the top row  710 . As shown in  FIG. 15 , the top portions  712  of the column electrodes  708  are expanded to substantially cover the entirety of the top row  710 , thereby shielding the mutual capacitance (C COMP1 ) formed between the top row  710  and respective top portions  712  of the column electrodes  708  from any shunting effects that would otherwise be caused by a finger or other object touching the top portion of the panel  704 . Although the top portions  712  are expanded, they do not make electrical contact with adjacent top portions  712 . A bottom touch-insensitive row (not shown) is formed in a similar fashion. 
     It is understood in alternative embodiments there may only be one touch-insensitive row, or any number of desired touch-insensitive rows or columns. For example, the insensitive portions of the panel  704  may be configured along one or both side edges of the panel  704  instead of the top and bottom edges. In such a configuration, the row electrodes  706  can be formed on top of the column electrodes  708  with the ends of the row electrodes  706  expanded in a similar manner as the expanded portions  712  of the column electrodes  708 . 
       FIGS. 16A and 16B  illustrate the effects of a finger touching a touch-sensitive and a touch-insensitive portion of the panel  704 , respectively. Referring to  FIG. 16A , when a finger  800  or other object touches or comes in sufficient proximity to (collectively referred to as a “touch” herein) a sensing node formed at an intersection of a column electrode  708  and a row electrode  706 , the finger or object blocks or “steals” some of the electric field lines  802  between the column  708  and row  706 . These electric field lines are shunted to ground through the capacitance inherent in the finger, and as a result, the steady state signal capacitance C SIG  is reduced by ΔC SIG . Therefore, the signal capacitance at the node becomes C SIG −ΔC SIG , where C SIG  represents the static (no touch) component and ΔC SIG  represents the dynamic (touch) component. 
       FIG. 16B  illustrates the shielding effects of the expanded portions  712  of the column electrodes  708  when a finger touches the corresponding top or bottom portion of the panel  704 . Since the expanded portions  712  of the column electrodes substantially cover the entirety of the underlying row electrode  710 , the finger  800  is shielded from the underlying electrodes and does not affect the electric field lines  802  between the electrodes. Thus, there is no shunting effect or sensitivity to a touch event at this portion of the panel  704 . As discussed above, the touch-insensitive top and bottom rows or portions of the panel  704  may be designed to provide desired values of C COMP1  and C COMP2 , respectively. Those of skill in the art would readily know how to achieve such desired mutual capacitance values in order to generate a desired compensation signal as described herein. 
     One advantage of generating the compensation signal through the panel  704 , as described above, is that the compensation signal will substantially “track” any variations in the mutual capacitance (C SIG ) values present at the touch-sensitive portions of the panel  704  that may be due to, for example, variations in operating parameters (e.g., temperature) and/or processing parameters (e.g., dielectric thickness). Thus, the compensation signal will mimic any variations in the output signals from the touch-sensitive portions of the panel  704 . One disadvantage, however, is that the effective surface area of the panel  704  for receiving touch inputs is slightly reduced. 
     While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not of limitation. For example, although the disclosure is primarily directed at devices that utilize capacitive sensing, some or all of the features described herein may be applied to other sensing methodologies. Additionally, although embodiments of this invention are primarily described herein for use with touch sensor panels, proximity sensor panels, which sense “hover” events or conditions, may also be used to generate modulated output signals for detection by the analog channels. Proximity sensor panels are described in Applicants&#39; co-pending U.S. application Ser. No. 11/649,998 entitled “Proximity and Multi-Touch Sensor Detection and Demodulation,” the contents of which are incorporated herein by reference in its entirety. As used herein, “touch” events or conditions should be construed to encompass “hover” events and conditions and “touch surface panels” should be construed to encompass “proximity sensor panel.” Likewise, the various diagrams may depict an example architectural or other configuration for the invention, which is done to aid in understanding the features and functionality that can be included in the invention. The invention is not restricted to the illustrated example architectures or configurations, but can be implemented using a variety of alternative architectures and configurations. Additionally, although the invention is described above in terms of various exemplary embodiments and implementations, it should be understood that the various features and functionality described in one or more of the individual embodiments are not limited in their applicability to the particular embodiment with which they are described, but instead can be applied, alone or in some combination, to one or more of the other embodiments of the invention, whether or not such embodiments are described and whether or not such features are presented as being a part of a described embodiment. Thus the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments. 
     Terms and phrases used in this document, and variations thereof, unless otherwise expressly stated, should be construed as open ended as opposed to limiting. As examples of the foregoing: the term “including” should be read as mean “including, without limitation” or the like; the term “example” is used to provide exemplary instances of the item in discussion, not an exhaustive or limiting list thereof; and adjectives such as “conventional,” “traditional,” “normal,” “standard,” “known” and terms of similar meaning should not be construed as limiting the item described to a given time period or to an item available as of a given time, but instead should be read to encompass conventional, traditional, normal, or standard technologies that may be available or known now or at any time in the future. Likewise, a group of items linked with the conjunction “and” should not be read as requiring that each and every one of those items be present in the grouping, but rather should be read as “and/or” unless expressly stated otherwise. Similarly, a group of items linked with the conjunction “or” should not be read as requiring mutual exclusivity among that group, but rather should also be read as “and/or” unless expressly stated otherwise. Furthermore, although items, elements or components of the invention may be described or claimed in the singular, the plural is contemplated to be within the scope thereof unless limitation to the singular is explicitly stated. The presence of broadening words and phrases such as “one or more,” “at least,” “but not limited to” or other like phrases in some instances shall not be read to mean that the narrower case is intended or required in instances where such broadening phrases may be absent.

Metadata:
Filing Date: 20190327
Publication Date: 20200728
Grant Date: 20200728
Priority Date: 20070103
Inventors: HOTELLING, STEVE PORTER
LAND, BRIAN RICHARDS
Assignee: APPLE INC
CPC Classifications: [{"code": "G06F3/0445", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/04182", "inventive": true, "first": true, "tree": "[]"}, {"code": "G06F3/0446", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/044", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F2203/04107", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F3/0418", "inventive": true, "first": true, "tree": "[]"}, {"code": "G06F2203/04104", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F3/044", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F2203/04104", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F2203/04107", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F3/0418", "inventive": true, "first": true, "tree": "[]"}]
Family ID: 39247162