PATENT DOCUMENT

Publication Number: US-10120520-B2
Application Number: US-201715663271-A
Country: US
Kind Code: B2

Title: Touch sensor panel with multi-power domain chip configuration

Abstract:
A touch sensing system is disclosed. The touch sensing system includes a guard signal generation chip operating in a first power domain referenced to a first voltage, the guard signal generation chip configured to generate a guard signal. A touch sensing chip operates in a second power domain, different from the first power domain, referenced to the guard signal, the touch sensing chip configured to sense touch at one or more touch electrodes included in a touch sensor panel operating in the second power domain referenced to the guard signal, and the touch sensing chip a different chip than the guard signal generation chip. A voltage regulator is configured to selectively regulate a voltage of the guard signal at the touch sensing chip.

Claims:
The invention claimed is: 
     
       1. A touch sensing system comprising:
 a guard signal generation chip operating in a first power domain referenced to a first voltage, the guard signal generation chip configured to generate a guard signal; 
 a touch sensing chip operating in a second power domain, different from the first power domain, referenced to the guard signal, the touch sensing chip configured to sense touch at one or more touch electrodes included in a touch sensor panel operating in the second power domain referenced to the guard signal, and the touch sensing chip a different chip than the guard signal generation chip; and 
 a voltage regulator configured to selectively regulate a voltage of the guard signal at the touch sensing chip. 
 
     
     
       2. The touch sensing system of  claim 1 , wherein the first power domain is referenced to a chassis ground of an electronic device in which the touch sensing system is included, or to earth ground. 
     
     
       3. The touch sensing system of  claim 1 , wherein the guard signal comprises an AC voltage. 
     
     
       4. The touch sensing system of  claim 1 , wherein the touch sensing chip is disposed on a guard plane electrically connected to the guard signal generation chip and configured to be driven with the guard signal. 
     
     
       5. The touch sensing system of  claim 1 , wherein the touch sensing chip includes touch sensing circuitry configured to sense touch at the one or more touch electrodes using the guard signal. 
     
     
       6. The touch sensing system of  claim 1 , further comprising:
 a flex circuit including:
 one or more traces configured to electrically couple the touch sensing chip to the touch sensor panel; and 
 one or more shields configured to isolate the one or more traces from the first power domain, 
 
 wherein:
 the touch sensing chip is disposed on a guard plane configured to isolate the touch sensing chip from the first power domain, 
 the touch sensor panel includes one or more shields configured to isolate the one or more touch electrodes from the first power domain, and 
 the guard plane, the one or more shields included in the flex circuit, and the one or more shields included in the touch sensor panel are electrically connected to the guard signal generation chip and are configured to be driven by the guard signal. 
 
 
     
     
       7. The touch sensing system of  claim 1 , wherein the voltage regulator is configured to:
 while the guard signal is in a first state, regulate the voltage of the guard signal at the touch sensing chip to a respective voltage, and 
 while the guard signal is in a second state, different from the first state, forgo regulating the voltage of the guard signal at the touch sensing chip to the respective voltage. 
 
     
     
       8. The touch sensing system of  claim 7 , wherein the guard signal is in the first state when the guard signal is in a low voltage state, and the guard signal is in the second state when the guard signal is in a high voltage state. 
     
     
       9. The touch sensing system of  claim 8 , wherein the respective voltage is a low voltage. 
     
     
       10. The touch sensing system of  claim 7 , wherein the guard signal is in the first state when the guard signal is in a high voltage state, and the guard signal is in the second state when the guard signal is in a low voltage state. 
     
     
       11. The touch sensing system of  claim 10 , wherein the respective voltage is a high voltage. 
     
     
       12. The touch sensing system of  claim 1 , wherein the voltage regulator comprises an amplifier including:
 an input electrically coupled to the guard signal at the touch sensing chip; and 
 an output electrically coupled to the guard signal generation chip. 
 
     
     
       13. The touch sensing system of  claim 12 , wherein the voltage regulator is configured to adjust the output of the amplifier based on a deviation of the guard signal at the touch sensing chip from a target voltage, the adjustment of the output of the amplifier causing the guard signal generation chip to adjust the guard signal. 
     
     
       14. The touch sensing system of  claim 1 , further comprising:
 a host chip operating in the first power domain, the host chip communicatively coupled to the touch sensing chip via a communication link, and the host chip configured to communicate data to the touch sensing chip via the communication link. 
 
     
     
       15. The touch sensing system of  claim 14 , further comprising one or more level shifters included in the communication link, the one or more level shifters configured to adjust a level of the data from the first power domain to the second power domain. 
     
     
       16. The touch sensing system of  claim 15 , wherein the one or more level shifters are configured to be selectively bypassed during the communication of the data from the host chip to the touch sensing chip. 
     
     
       17. The touch sensing system of  claim 16 , wherein during the communication of the data from the host chip to the touch sensing chip, the one or more level shifters are configured to be:
 bypassed when the guard signal generation chip is not generating the guard signal, and 
 not bypassed when the guard signal generation chip is generating the guard signal. 
 
     
     
       18. The touch sensing system of  claim 16 , wherein during the communication of the data from the host chip to the touch sensing chip, the one or more level shifters are configured to be:
 bypassed when the guard signal is in a low state, and 
 not bypassed when the guard signal is in a high state. 
 
     
     
       19. The touch sensing system of  claim 1 , wherein the guard signal generation chip comprises a direct digital synthesizer, a digital-to-analog converter and a buffer, an output of the direct digital synthesizer coupled to an input of the digital-to-analog converter, an output of the digital-to-analog converter coupled to an input of the buffer, and an output of the buffer outputting the guard signal generated by the guard signal generation chip. 
     
     
       20. The touch sensing system of  claim 1 , wherein the touch sensing chip is configured to receive a low voltage and a high voltage, the low voltage corresponding to the guard signal, and the high voltage based on the low voltage and generated using a capacitor and a switch, the switch configured to limit an amount of current flowing into the capacitor to less than a threshold amount. 
     
     
       21. The touch sensing system of  claim 1 , wherein:
 the guard signal generation chip includes circuitry configured to generate the guard signal, 
 the circuitry is configured to operate as a linear buffer when the guard signal is in a low state, and 
 the circuitry is configured to operate as a push-pull buffer when the guard signal is in a high state. 
 
     
     
       22. The touch sensing system of  claim 1 , wherein the touch sensing chip is configured to selectively reset a sense amplifier that is configured to sense the touch at the one or more touch electrodes based on one or more of a number of touch electrodes the sense amplifier is sensing simultaneously and a spectral scan of the touch sensor panel. 
     
     
       23. An electronic device comprising:
 a touch sensor panel including one or more touch electrodes; 
 a guard signal generation chip operating in a first power domain referenced to a first voltage, the guard signal generation chip configured to generate a guard signal; 
 a touch sensing chip operating in a second power domain, different from the first power domain, referenced to the guard signal, the touch sensing chip configured to sense touch at the one or more touch electrodes included in the touch sensor panel operating in the second power domain referenced to the guard signal, and the touch sensing chip a different chip than the guard signal generation chip; and 
 a voltage regulator configured to selectively regulate a voltage of the guard signal at the touch sensing chip. 
 
     
     
       24. A method for operating a touch sensing system, the method comprising:
 operating a guard signal generation chip in a first power domain referenced to a first voltage, the guard signal generation chip configured to generate a guard signal; 
 operating a touch sensing chip in a second power domain, different from the first power domain, referenced to the guard signal, the touch sensing chip configured to sense touch at one or more touch electrodes included in a touch sensor panel operating in the second power domain referenced to the guard signal, and the touch sensing chip a different chip than the guard signal generation chip; and 
 selectively regulating a voltage of the guard signal at the touch sensing chip. 
 
     
     
       25. The touch sensing system of  claim 24 , wherein the switching circuitry is configured to:
 couple the voltage input of the touch sensing chip to the guard signal when the state of the guard signal is a first state, and 
 couple the voltage input of the touch sensing chip to the first voltage when the state of the guard signal is a second state, different from the first state. 
 
     
     
       26. A touch sensing system comprising:
 a guard signal generation chip operating in a first power domain referenced to a first voltage, the guard signal generation chip configured to generate a guard signal; 
 a touch sensing chip operating in a second power domain, different from the first power domain, referenced to the guard signal, the touch sensing chip configured to sense touch at one or more touch electrodes included in a touch sensor panel operating in the second power domain referenced to the guard signal, and the touch sensing chip a different chip than the guard signal generation chip; and 
 switching circuitry configured to selectively couple a voltage input of the touch sensing chip to the guard signal generation chip based on a state of the guard signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Non-Provisional application which claims the benefit of U.S. Provisional Patent Application No. 62/458,925, filed Feb. 14, 2017, and U.S. Provisional Patent Application No. 62/399,230, filed Sep. 23, 2016, and U.S. Provisional Patent Application No. 62/368,798, filed Jul. 29, 2016, the contents of which are incorporated herein by reference in their entirety for all purposes. 
    
    
     FIELD OF THE DISCLOSURE 
     This relates generally to touch sensor panels, and more particularly to touch sensor panels in which a touch sensing chip operates in a different power domain than other chips in the touch sensing system. 
     BACKGROUND OF THE DISCLOSURE 
     Many types of input devices are presently available for performing operations in a computing system, such as buttons or keys, mice, trackballs, joysticks, touch sensor panels, touch screens and the like. Touch screens, in particular, are becoming increasingly popular because of their ease and versatility of operation as well as their declining price. Touch screens can include a touch sensor panel, which can be a clear panel with a touch-sensitive surface, and a display device such as a liquid crystal display (LCD) that can be positioned partially or fully behind the panel so that the touch-sensitive surface can cover at least a portion of the viewable area of the display device. Touch screens can allow a user to perform various functions by touching the touch sensor panel using a finger, stylus or other object at a location often dictated by a user interface (UI) being displayed by the display device. In general, touch screens can recognize a touch and the position of the touch on the touch sensor panel, and the computing system can then interpret the touch in accordance with the display appearing at the time of the touch, and thereafter can perform one or more actions based on the touch. In the case of some touch sensing systems, a physical touch on the display is not needed to detect a touch. For example, in some capacitive-type touch sensing systems, fringing electrical fields used to detect touch can extend beyond the surface of the display, and objects approaching near the surface may be detected near the surface without actually touching the surface. 
     Capacitive touch sensor panels can be formed by a matrix of substantially transparent or non-transparent conductive plates (e.g., touch electrodes) made of materials such as Indium Tin Oxide (ITO). It is due in part to their substantial transparency that some capacitive touch sensor panels can be overlaid on a display to form a touch screen, as described above. Some touch screens can be formed by at least partially integrating touch sensing circuitry into a display pixel stackup (i.e., the stacked material layers forming the display pixels). 
     In some cases, parasitic or stray capacitances can exist between the touch electrodes used for sensing touch on the touch sensor panels, and other components of the devices in which the touch sensor panels are included, which can be referenced to a chassis or earth ground. These parasitic or stray capacitances can introduce errors and/or offsets into the touch outputs of the touch sensor panels, and can therefore degrade touch sensing dynamic range. Therefore, it can be beneficial to reduce or eliminate such parasitic or stray capacitances. 
     SUMMARY OF THE DISCLOSURE 
     As previously mentioned, parasitic or stray capacitances can exist between touch electrodes on touch sensor panels and other components of the devices in which the touch sensor panels are included, which can introduce errors and/or offsets into the touch outputs of the touch sensor panels, therefore degrading touch sensing dynamic range. The examples of the disclosure provide various touch sensing system configurations in which a touch sensing chip can be operated in a different power domain than other chips in the touch sensing system (i.e., the touch sensing chip can be referenced to a guard ground that can be different from a chassis or earth ground to which the other chips in the system can be referenced). Doing so can reduce or eliminate stray or parasitic capacitances that can exist between the touch electrodes and chassis or earth ground. This, in turn, can improve the touch sensing performance of the system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-1D  illustrate an example mobile telephone, an example media player, an example personal computer and an example tablet computer that can each include an exemplary touch screen according to examples of the disclosure. 
         FIG. 2  is a block diagram of an example computing system that illustrates one implementation of an example self-capacitance touch screen according to examples of the disclosure. 
         FIG. 3A  illustrates an exemplary touch sensor circuit corresponding to a self-capacitance touch node electrode and sensing circuit according to examples of the disclosure. 
         FIG. 3B  illustrates an exemplary touch sensor circuit corresponding to a mutual-capacitance drive and sense line and sensing circuit according to examples of the disclosure. 
         FIG. 4A  illustrates a touch screen with touch electrodes arranged in rows and columns according to examples of the disclosure. 
         FIG. 4B  illustrates a touch screen with touch node electrodes arranged in a pixelated touch node electrode configuration according to examples of the disclosure. 
         FIGS. 5A-5B  illustrate an exemplary touch sensor panel configuration in which the touch sensing circuitry of the touch sensor panel is included in an electronic chip (e.g., an integrated circuit, etc.) that is referenced to earth or chassis ground according to examples of the disclosure. 
         FIGS. 6A-6E  illustrate an exemplary touch sensor panel configuration in which the touch sensing circuitry of the touch sensor panel is included on an electronic chip (e.g., an integrated circuit, etc.) that is referenced to a guard ground rather than a chassis or earth ground according to examples of the disclosure. 
         FIG. 7A  illustrates an exemplary guard stimulus voltage delivery configuration for delivering a guard stimulus voltage to a touch sensing chip according to examples of the disclosure. 
         FIG. 7B  illustrates an exemplary guard signal at node A in  FIG. 7A  according to examples of the disclosure. 
         FIG. 8A  illustrates an exemplary guard stimulus voltage delivery configuration for delivering a guard stimulus voltage to a touch sensing chip using a voltage regulator according to examples of the disclosure. 
         FIG. 8B  illustrates exemplary details of a guard source and a voltage regulator according to examples of the disclosure. 
         FIG. 8C  illustrates an exemplary guard signal at node A of  FIGS. 8A-8B  resulting from low- and/or high-side voltage regulation according to examples of the disclosure. 
         FIG. 8D  illustrates another exemplary guard stimulus voltage delivery configuration for delivering a guard stimulus voltage to a touch sensing chip in which a capacitor is selectively coupled to node A of  FIG. 7A  according to examples of the disclosure. 
         FIG. 8E  illustrates an exemplary timing diagram that relates the guard signal, the RESET signal, and the output of a sensing circuit during the DC restore mode according to examples of the disclosure. 
         FIG. 8F  illustrates an exemplary timing diagram that relates the guard signal, the RESET signal, and the output of a sensing circuit during the scan step RESET mode according to examples of the disclosure. 
         FIG. 9  illustrates an exemplary level shifter configuration according to examples of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description of examples, reference is made to the accompanying drawings which form a part hereof, and in which it is shown by way of illustration specific examples that can be practiced. It is to be understood that other examples can be used and structural changes can be made without departing from the scope of the disclosed examples. 
     Some capacitive touch sensor panels can be formed by a matrix of substantially transparent or non-transparent conductive plates (e.g., touch electrodes) made of materials such as Indium Tin Oxide (ITO), and some touch screens can be formed by at least partially integrating touch sensing circuitry into a display pixel stackup (i.e., the stacked material layers forming the display pixels). In some cases, parasitic or stray capacitances can exist between the touch electrodes used for sensing touch on the touch sensor panels, and other components of the devices in which the touch sensor panels are included, which can be referenced to a chassis or earth ground. These parasitic or stray capacitances can introduce errors and/or offsets into the touch outputs of the touch sensor panels, therefore degrading touch sensing dynamic range. Therefore, it can be beneficial to reduce or eliminate such parasitic or stray capacitances. The examples of the disclosure provide various touch sensing system configurations in which a touch sensing chip can be operated in a different power domain than other chips in the touch sensing system (i.e., the touch sensing chip can be referenced to a guard ground that can be different from a chassis or earth ground to which the other chips in the system can be referenced). Doing so can reduce or eliminate stray or parasitic capacitances that can exist between the touch electrodes and chassis or earth ground. This, in turn, can improve the touch sensing performance of the system. 
       FIGS. 1A-1D  illustrate example systems in which a touch screen according to examples of the disclosure may be implemented.  FIG. 1A  illustrates an example mobile telephone  136  that includes a touch screen  124 .  FIG. 1B  illustrates an example digital media player  140  that includes a touch screen  126 .  FIG. 1C  illustrates an example personal computer  144  that includes a touch screen  128 .  FIG. 1D  illustrates an example tablet computer  148  that includes a touch screen  130 . It is understood that the above touch screens can be implemented in other devices as well, including in wearable devices. 
     In some examples, touch screens  124 ,  126 ,  128  and  130  can be based on self-capacitance. A self-capacitance based touch system can include a matrix of small, individual plates of conductive material that can be referred to as touch node electrodes (as described below with reference to touch screen  220  in  FIG. 2 ). For example, a touch screen can include a plurality of individual touch node electrodes, each touch node electrode identifying or representing a unique location on the touch screen at which touch or proximity (i.e., a touch or proximity event) is to be sensed, and each touch node electrode being electrically isolated from the other touch node electrodes in the touch screen/panel. Such a touch screen can be referred to as a pixelated self-capacitance touch screen, though it is understood that in some examples, the touch node electrodes on the touch screen can be used to perform scans other than self-capacitance scans on the touch screen (e.g., mutual capacitance scans). During operation, a touch node electrode can be stimulated with an AC waveform, and the self-capacitance to ground of the touch node electrode can be measured. As an object approaches the touch node electrode, the self-capacitance to ground of the touch node electrode can change. This change in the self-capacitance of the touch node electrode can be detected and measured by the touch sensing system to determine the positions of multiple objects when they touch, or come in proximity to, the touch screen. In some examples, the electrodes of a self-capacitance based touch system can be formed from rows and columns of conductive material, and changes in the self-capacitance to ground of the rows and columns can be detected, similar to above. In some examples, a touch screen can be multi-touch, single touch, projection scan, full-imaging multi-touch, capacitive touch, etc. 
     In some examples, touch screens  124 ,  126 ,  128  and  130  can be based on mutual capacitance. A mutual capacitance based touch system can include drive and sense lines that may cross over each other on different layers, or may be adjacent to each other on the same layer. The crossing or adjacent locations can be referred to as touch nodes. During operation, the drive line can be stimulated with an AC waveform and the mutual capacitance of the touch node can be measured. As an object approaches the touch node, the mutual capacitance of the touch node can change. This change in the mutual capacitance of the touch node can be detected and measured by the touch sensing system to determine the positions of multiple objects when they touch, or come in proximity to, the touch screen. 
       FIG. 2  is a block diagram of an example computing system  200  that illustrates one implementation of an example self-capacitance touch screen  220  according to examples of the disclosure. It is understood that computing system  200  can instead include a mutual capacitance touch screen, as described above, though the examples of the disclosure will be described assuming a self-capacitance touch screen is provided. Computing system  200  can be included in, for example, mobile telephone  136 , digital media player  140 , personal computer  144 , tablet computer  148 , or any mobile or non-mobile computing device that includes a touch screen, including a wearable device. Computing system  200  can include a touch sensing system including one or more touch processors  202 , peripherals  204 , a touch controller  206 , and touch sensing circuitry (described in more detail below). Peripherals  204  can include, but are not limited to, random access memory (RAM) or other types of memory or storage, watchdog timers and the like. Touch controller  206  can include, but is not limited to, one or more sense channels  208  and channel scan logic  210 . Channel scan logic  210  can access RAM  212 , autonomously read data from sense channels  208  and provide control for the sense channels. In some examples, RAM  212  can contain various configuration information for specific touch screen  220  scans performed by channel scan logic  210  (e.g., scan specific configuration information for sense channels  208 ), can receive and/or store touch data from sense channels  208 , and can be managed by channel scan logic  210 . In addition, channel scan logic  210  can control sense channels  208  to generate stimulation signals at various frequencies and phases that can be selectively applied to the touch nodes of touch screen  220 , as described in more detail below. In some examples, touch controller  206 , touch processor  202  and peripherals  204  can be integrated into a single application specific integrated circuit (ASIC), and in some examples can be integrated with touch screen  220  itself. 
     Touch screen  220  can include touch sensing circuitry that can include a capacitive sensing medium having a plurality of electrically isolated touch node electrodes  222  (e.g., a pixelated self-capacitance touch screen). Touch node electrodes  222  can be coupled to sense channels  208  in touch controller  206 , can be driven by stimulation signals from the sense channels through drive/sense interface  225 , and can be sensed by the sense channels through the drive/sense interface as well, as described above. In some examples, drive/sense interface  225  can be implemented in the touch controller  206 , or can be implemented in a chip separate from touch controller  206 . Additional exemplary details of how drive/sense interface  225  can be implemented can be found in U.S. patent application Ser. No. 15/009,774, filed Jan. 28, 2016, entitled “Flexible Self Capacitance and Mutual Capacitance Touch Sensing System Architecture,” the entire contents of which is hereby incorporated by reference for all purposes. Labeling the conductive plates used to detect touch (i.e., touch node electrodes  222 ) as “touch node” electrodes can be particularly useful when touch screen  220  is viewed as capturing an “image” of touch (e.g., a “touch image”). In other words, after touch controller  206  has determined an amount of touch detected at each touch node electrode  222  in touch screen  220 , the pattern of touch node electrodes in the touch screen at which a touch occurred can be thought of as a touch image (e.g., a pattern of fingers touching the touch screen). 
     Computing system  200  can also include a host processor  228  for receiving outputs from touch processor  202  and performing actions based on the outputs. For example, host processor  228  can be connected to program storage  232  and a display controller, such as an LCD driver  234 . The LCD driver  234  can provide voltages on select (e.g., gate) lines to each pixel transistor and can provide data signals along data lines to these same transistors to control the pixel display image as described in more detail below. Host processor  228  can use LCD driver  234  to generate a display image on touch screen  220 , such as a display image of a user interface (UI), and can use touch processor  202  and touch controller  206  to detect a touch on or near touch screen  220 . The touch input can be used by computer programs stored in program storage  232  to perform actions that can 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. Host processor  228  can also perform additional functions that may not be related to touch processing. It is understood that in some examples, touch screen  220  need not be integrated in a display module or stackup (e.g., need not be in-cell), but can instead be separate from the display module or stackup (e.g., a discrete touch sensor panel that is not part of a display, and is merely overlaid on the display or is separate from the display). 
     Note that one or more of the functions described herein, including the configuration of switches, can be performed by firmware stored in memory (e.g., one of the peripherals  204  in  FIG. 2 ) and executed by touch processor  202 , or stored in program storage  232  and executed by host processor  228 . The firmware can also be stored and/or transported within any non-transitory computer-readable storage medium for use by or in connection with an instruction execution system, apparatus, or device, such as a computer-based system, processor-containing system, or other system that can fetch the instructions from the instruction execution system, apparatus, or device and execute the instructions. In the context of this document, a “non-transitory computer-readable storage medium” can be any medium (excluding signals) that can contain or store the program for use by or in connection with the instruction execution system, apparatus, or device. The computer-readable storage medium can include, but is not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus or device, a portable computer diskette (magnetic), a random access memory (RAM) (magnetic), a read-only memory (ROM) (magnetic), an erasable programmable read-only memory (EPROM) (magnetic), a portable optical disc such a CD, CD-R, CD-RW, DVD, DVD-R, or DVD-RW, or flash memory such as compact flash cards, secured digital cards, USB memory devices, memory sticks, and the like. 
     The firmware can also be propagated within any transport medium for use by or in connection with an instruction execution system, apparatus, or device, such as a computer-based system, processor-containing system, or other system that can fetch the instructions from the instruction execution system, apparatus, or device and execute the instructions. In the context of this document, a “transport medium” can be any medium that can communicate, propagate or transport the program for use by or in connection with the instruction execution system, apparatus, or device. The transport medium can include, but is not limited to, an electronic, magnetic, optical, electromagnetic or infrared wired or wireless propagation medium. 
       FIG. 3A  illustrates an exemplary touch sensor circuit  300  corresponding to a self-capacitance touch node electrode  302  and sensing circuit  314  according to examples of the disclosure. Touch node electrode  302  can correspond to touch node electrode  222 . Touch node electrode  302  can have an inherent self-capacitance Cstray  307  to ground associated with it, and also an additional self-capacitance C  304  to ground that is formed when an object, such as finger  305 , is in proximity to or touching the electrode. The total self-capacitance to ground of touch node electrode  302  can be illustrated as capacitance C  304 +Cstray  307 . Finger  305  can have capacitance Cbody  309  to ground. Note that Cbody  309  can typically be much larger than C  304  such that the total series capacitance of C  304  and Cstray  307  can be approximately C  304 . Touch node electrode  302  can be coupled to sensing circuit  314 . Sensing circuit  314  can include an operational amplifier  308 , feedback resistor  312  and feedback capacitor  310 , although other configurations can be employed. For example, feedback resistor  312  can be replaced by a switched capacitor resistor in order to minimize a parasitic capacitance effect that can be caused by a variable feedback resistor. In some examples, switch  315  can be coupled across feedback resistor  312  (e.g., in parallel with feedback resistor  312  and/or feedback capacitor  310 ), and switch  315  can be controlled by signal RESET (e.g., the RESET signal can control whether switch  315  is open or closed). By closing and opening switch  315 , the touch sensing system of the disclosure can dynamically change the feedback impedance of sensing circuit  314 , which can change its operational characteristics. Details about the operation of switch  315  and the RESET signal will be provided later. 
     Touch node electrode  302  can be coupled to the inverting input (−) of operational amplifier  308 . An AC voltage source  306  (Vac) can be coupled to the non-inverting input (+) of operational amplifier  308 . As such, touch sensor circuit  300  can be configured to sense changes in the total self-capacitance  304  of the touch node electrode  302  induced by a finger or object either touching or in proximity to the touch sensor panel. The output of sensing circuit  314  Vo can be filtered and heterodyned or homodyned by being fed into multiplier  328 , where Vo can be multiplied with local oscillator  330  to produce Vdetect. Vdetect can be inputted into filter  332 . One skilled in the art will recognize that the placement of filter  332  can be varied; thus, the filter can be placed after multiplier  328 , as illustrated, or two filters can be employed: one before the multiplier and one after the multiplier. In some examples, there can be no filter at all. The direct current (DC) portion of Vdetect can be used to determine if a touch or proximity event has occurred; for example, the DC portion of Vdetect can be used by a processor to determine the presence of a proximity or touch event, or the output can be inputted into a discrete logic network to determine the presence of a proximity or touch event. Note that while  FIG. 3A  indicates the demodulation at multiplier  328  occurs in the analog domain, output Vo may be digitized by an analog-to-digital converter (ADC), and blocks  328 ,  332  and  330  may be implemented in a digital fashion (e.g.,  328  can be a digital demodulator,  332  can be a digital filter, and  330  can be a digital NCO (Numerical Controlled Oscillator). 
       FIG. 3B  illustrates an exemplary touch sensor circuit  350  corresponding to a mutual-capacitance drive  322  and sense  326  line and sensing circuit  314  according to examples of the disclosure. Drive line  322  can be stimulated by stimulation signal  306  (e.g., an AC voltage signal). Stimulation signal  306  can be capacitively coupled to sense line  326  through mutual capacitance  324  between drive line  322  and the sense line. When a finger or object  305  approaches the touch node created by the intersection of drive line  322  and sense line  326 , mutual capacitance  324  can be altered as indicated by capacitances C FD    311  and C FS    313 , which can be formed between drive line  322 , finger  305  and sense line  326 . This change in mutual capacitance  324  can be detected to indicate a touch or proximity event at the touch node, as described previously and below. The sense signal coupled onto sense line  326  can be received by sensing circuit  314 . Sensing circuit  314  can include operational amplifier  308  and at least one of a feedback resistor  312  and a feedback capacitor  310 . In some examples, switch  315  can be coupled across feedback resistor  312  (e.g., in parallel with feedback resistor  312  and/or feedback capacitor  310 ), and switch  315  can be controlled by signal RESET (e.g., the RESET signal can control whether switch  315  is open or closed). By closing and opening switch  315 , the touch sensing system of the disclosure can dynamically change the feedback impedance of sensing circuit  314 , which can change its operational characteristics. Details about the operation of switch  315  and the RESET signal will be provided later. 
       FIG. 3B  illustrates a general case in which both resistive and capacitive feedback elements are utilized. The sense signal (referred to as Vin) can be inputted into the inverting input of operational amplifier  308 , and the non-inverting input of the operational amplifier can be coupled to a reference voltage Vref. Operational amplifier  308  can drive its output to voltage Vo to keep Vin substantially equal to Vref, and can therefore maintain Vin constant or virtually grounded. Therefore, the gain of sensing circuit  314  can be mostly a function of the ratio of mutual capacitance  324  and the feedback impedance, comprised of resistor  312  and/or capacitor  310 . The output of sensing circuit  314  Vo can be filtered and heterodyned or homodyned by being fed into multiplier  328 , where Vo can be multiplied with local oscillator  330  to produce Vdetect. Vdetect can be inputted into filter  332 . One skilled in the art will recognize that the placement of filter  332  can be varied; thus, the filter can be placed after multiplier  328 , as illustrated, or two filters can be employed: one before the multiplier and one after the multiplier. In some examples, there can be no filter at all. The direct current (DC) portion of Vdetect can be used to determine if a touch or proximity event has occurred. Note that while  FIG. 3B  indicates the demodulation at multiplier  328  occurs in the analog domain, output Vo may be digitized by an ADC, and blocks  328 ,  332  and  330  may be implemented in a digital fashion (e.g.,  328  can be a digital demodulator,  332  can be a digital filter, and  330  can be a digital NCO (Numerical Controlled Oscillator). 
     Referring back to  FIG. 2 , in some examples, touch screen  220  can be an integrated touch screen in which touch sensing circuit elements of the touch sensing system can be integrated into the display pixel stackups of a display. The circuit elements in touch screen  220  can include, for example, elements that can exist in LCD or other displays, such as one or more pixel transistors (e.g., thin film transistors (TFTs)), gate lines, data lines, pixel electrodes and common electrodes. In a given display pixel, a voltage between a pixel electrode and a common electrode can control a luminance of the display pixel. The voltage on the pixel electrode can be supplied by a data line through a pixel transistor, which can be controlled by a gate line. It is noted that circuit elements are not limited to whole circuit components, such as a whole capacitor, a whole transistor, etc., but can include portions of circuitry, such as only one of the two plates of a parallel plate capacitor. 
       FIG. 4A  illustrates touch screen  400  with touch electrodes  404  and  406  arranged in rows and columns according to examples of the disclosure. Specifically, touch screen  400  can include a plurality of touch electrodes  404  disposed as rows, and a plurality of touch electrodes  406  disposed as columns. Touch electrodes  404  and touch electrodes  406  can be on the same or different material layers on touch screen  400 , and can intersect with each other while remaining electrically isolated from each other, as illustrated in  FIG. 4A . In some examples, touch screen  400  can sense the self-capacitance of touch electrodes  404  and  406  to detect touch and/or proximity activity on touch screen  400 , and in some examples, touch screen  400  can sense the mutual capacitance between touch electrodes  404  and  406  to detect touch and/or proximity activity on touch screen  400 . 
       FIG. 4B  illustrates touch screen  402  with touch node electrodes  408  arranged in a pixelated touch node electrode configuration according to examples of the disclosure. Specifically, touch screen  402  can include a plurality of individual touch node electrodes  408 , each touch node electrode identifying or representing a unique location on the touch screen at which touch or proximity (i.e., a touch or proximity event) is to be sensed, and each touch node electrode being electrically isolated from the other touch node electrodes in the touch screen/panel, as previously described. Touch node electrodes  408  can be on the same or different material layers on touch screen  400 . In some examples, touch screen  400  can sense the self-capacitance of touch node electrodes  408  to detect touch and/or proximity activity on touch screen  400 , and in some examples, touch screen  400  can sense the mutual capacitance between touch node electrodes  408  to detect touch and/or proximity activity on touch screen  400 . 
     In some examples, the touch sensing circuitry of a touch screen or touch sensor panel (e.g., touch sensing circuitry as described with reference to  FIGS. 2 and 3A-3B ) can be fabricated in an electronic chip (e.g., an integrated circuit, etc.), and the electronic chip and/or the circuitry included in the electronic chip can operate with respect to a reference voltage provided by the chassis of the electronic device (“chassis ground”) in which the touch screen or touch sensor panel is included (e.g., devices  136 ,  140 ,  144  and  148  in  FIGS. 1A-1D ). In some examples this chassis ground can be a grounding pathway from the chassis through a user operating the electronic device to earth ground. In some examples, this chassis ground can be the same as earth ground. However, in some examples, operating the electronic chip and/or the circuitry included in the electronic chip with respect to chassis or earth ground can result in undesirable touch sensing performance, as will be described in more detail below. 
       FIGS. 5A-5B  illustrate an exemplary touch sensor panel configuration  500  in which the touch sensing circuitry of the touch sensor panel is included in an electronic chip (e.g., an integrated circuit, etc.) that is referenced to earth or chassis ground according to examples of the disclosure. Specifically, in configuration  500  of  FIG. 5A , a touch sensor panel is included in a device (e.g., devices  136 ,  140 ,  144  and  148  in  FIGS. 1A-1D ) having device chassis  502 . Chassis  502  can be grounded to earth ground  506  (e.g., via a user holding the device or otherwise in contact with the device), or can be grounded to a separate device ground (not illustrated). Chassis  502  can include electronic chip  504 , which can include touch sensing circuitry for sensing touch on the touch sensor panel included in the device of  FIG. 5A . For example, chip  504  can include touch controller  206 , drive/sense interface  225  and/or touch processor  202  of  FIG. 2  and/or the touch sensing circuits of  FIGS. 3A-3B . Chip  504  and/or the touch sensing circuitry in chip  504  can be referenced to chassis  502  (e.g., referenced to earth ground  506 ). Chip  504  can be coupled, via one or more traces, to touch node electrode  508 , which can be a touch node electrode included in the touch sensor panel of the device of  FIG. 5A . Chip  504  can also be coupled to other touch node electrodes included in the touch sensor panel, though a single touch node electrode  508  is illustrated for ease of description. Chip  504  can measure the self-capacitance of touch node electrode  508  to detect proximity activity at touch node electrode  508 , as discussed with reference to  FIG. 3A . 
       FIG. 5B  illustrates various capacitances associated with proximity detection using touch sensor panel configuration  500  of  FIG. 5A  according to examples of the disclosure. Specifically, finger (or object)  510  can be in proximity to touch node electrode  508 . Finger  510  can be grounded to earth ground  506  through capacitance  512  (e.g., C body ), which can represent a capacitance from finger  510  through a user&#39;s body to earth ground  506 . Capacitance  514  (e.g., C touch ) can represent a capacitance between finger  510  and touch node electrode  508 , and can be the capacitance of interest in determining how close finger  510  is to touch node electrode  508 . Capacitance  514  can be measured by sense circuitry  522  (e.g., as described with reference to  FIG. 3A ) included in chip  504  to determine an amount of touch at touch node electrode  508 . However, because touch node electrode  508  can be in proximity to chassis  502  (e.g., due to being included in the device of which chassis  502  is a part), which can be grounded to earth ground  506 , parasitic or stray capacitances can exist between touch node electrode  508  and chassis  502  (represented by capacitance  516  (e.g., C p )) and/or between traces that connect touch node electrode  508  to sense circuitry  522  and chassis  502  (represented by capacitance  518  (e.g., C s )). These stray capacitances  516  and  518  can also be measured by sense circuitry  522 , and can create an offset (e.g., from zero output signal) in the output signal of sense circuitry  522 , which can reduce the touch signal-to-noise ratio (or the touch dynamic range) of sense circuitry  522 . This, in turn, can reduce the range of touch-related capacitances (e.g., C touch    514 ) that sense circuitry  522  can detect, thus potentially limiting the touch sensing performance of the touch sensor panel in which touch node electrode  508  is included. 
     In order to reduce or eliminate parasitic or stray capacitances that may be measured by sense circuitry in a touch sensing chip of a touch sensor panel, a guard plane can be established between the touch-related components of the touch sensor panel (e.g., touch node electrode  508 , touch sensing chip  504 , etc.) and one or more portions of chassis  502  (e.g., the surface of chassis  502  corresponding to a back plate of the device). The guard plane, including the touch sensing chip (e.g., integrated circuit, etc.), can be referenced to a guard potential that can mirror or be the same as the stimulation signal used to stimulate the touch node electrodes on the touch sensor panel. In this way, the voltages on both sides of the above-described parasitic or stray capacitances can mirror each other, causing those capacitances to fall out of the touch sensing measurements performed by the touch sensing circuitry in the touch sensing chip. Therefore, the signal portion (out of sense amplifier  522 ) associated with the undesired stray capacitances can be largely reduced, therefore improving the touch dynamic range and the touch sensing performance of the touch sensor panel. It should be understood that “guard plane” need not refer to a planar element or electrode; rather, the guard planes of the disclosure can be implemented in any number of manners, including being non-planar, being composed of one or more portions of the device that are driven/maintained at a guard potential, and being implemented in different ways in different parts of the device (e.g., as part of a flex circuit in one portion of the device, as part of the touch sensor panel in another portion of the device, etc.). 
       FIGS. 6A-6C  illustrate an exemplary touch sensor panel configuration  600  in which the touch sensing circuitry of the touch sensor panel is included on an electronic chip (e.g., an integrated circuit, etc.) that is referenced to a guard ground rather than a chassis or earth ground according to examples of the disclosure. Specifically, in configuration  600  of  FIG. 6A , touch sensing chip  604  (e.g., corresponding to touch sensing chip  504 ) is disposed or fabricated on guard plane  620 , which can represent a virtual ground plane of touch sensing chip  604  that is different from chassis or earth ground  606 . In particular, stimulation source  626  (“guard source”) can be referenced to chassis or earth ground  606 , and can output a guard voltage (e.g., a guard stimulation signal, such as a square wave) that can establish the voltage at guard plane  620 . Stimulation source  626  can be included on a chip, separate from touch sensing chip  604 . Because touch sensing chip  604  can be built on guard plane  620 , the circuitry (e.g., touch sensing circuitry) included in touch sensing chip  604  can be referenced to the guard signal, and can be isolated from chassis or earth ground  606  by guard plane  620 . In other words, touch sensing chip  604  and the chip in which guard source  626  is included can operate in different “power domains”: touch sensing chip  604  can operate in the guard power domain, and guard source  626  can operate in the chassis or earth power domain. 
     Additionally, guard plane  624  can be disposed between touch node electrode  608  and chassis  602  (or, more generally, earth ground  606 ), and guard plane  628  can be disposed between traces that couple touch node electrode  608  to touch sensing chip  604  and chassis  602  (or, more generally, earth ground  606 ). Guard plane  624  and guard plane  628  can also be stimulated by the same guard voltage as is guard plane  620 . These guard planes  624  and  628  can similarly isolate touch node electrode  608  and traces that couple touch node electrode  608  to touch sensing chip  604  from chassis or earth ground  606 . One or more of guard planes  620 ,  624  and  628  can reduce or eliminate parasitic or stray capacitances that may exist between touch node electrode  608  and chassis or earth ground  606 , as will be described below. 
       FIG. 6B  illustrates an exemplary structural configuration  601  for implementing the guarding of  FIG. 6A  according to examples of the disclosure. Specifically, as previously described, guard source  626  can provide a guard stimulus voltage to guard plane  620 , on which touch sensing chip  604  can be disposed or fabricated and to which touch sensing chip  604  can be referenced. Guard plane  620  can be any conductive material on which touch sensing chip  604  can be disposed or fabricated (e.g., silver, copper, gold, etc.). For example, touch sensing chip  604  may be assembled on a flex circuit or printed circuit board (PCB), and may be referenced to the flex circuit or PCB ground layer  620  driven by guard source  626 . Guard source  626  can be fabricated in an electronic chip that is distinct and separate from touch sensing chip  604 , and the electronic chip in which guard source  626  is fabricated can be referenced to chassis or earth ground  606 . Touch sensing chip  604  (e.g., touch sensing circuitry in touch sensing chip  604 ) can be coupled to touch node electrodes  608 A,  608 B,  608 C and  608 D (and other touch node electrodes included in touch electrode and routing layer  634  of touch sensor panel  630 , referred to collectively as  608 ) in touch sensor panel  630  via traces  632  included on a flex circuit that couples touch sensing chip  604  to touch sensor panel  630 . The flex circuit can include top  628 A and bottom  628 B shields that sandwich traces  632  on two sides, and that can also be coupled to guard source  626 . Finally, touch sensor panel  630  can also include top  624 A and bottom  624 B shields that sandwich touch node electrodes  608  on touch electrode and routing layer  634  on two sides, and that can also be coupled to guard source  626 . In some examples, the material(s) out of which shields  628  are made in the flex circuit can be different than the material(s) out of which shields  624  are made in touch sensor panel  630 . For example, shields  624  in touch sensor panel  630  can be made of the same material that touch node electrodes  608  are made of (e.g., ITO, or another substantially transparent conductor), and shields  628  in the flex circuit can be made of a different conductor, such as copper, aluminum, or other conductor that may or may not be transparent. Top shield  624 A can include openings  609 A (e.g., corresponding to touch node electrode  608 A),  609 B (e.g., corresponding to touch node electrode  608 B),  609 C (e.g., corresponding to touch node electrode  608 C) and  609 D (e.g., corresponding to touch node electrode  608 D) that allow touch node electrodes  608 A,  608 B,  608 C and  608 D, respectively, to detect, from above, touch activity on touch sensor panel  630  while guarding the routing on touch electrode and routing layer  634  from stray capacitances that can form due to a touch or other stray capacitances. Accordingly, the touch signal path from touch sensor panel  630  to the flex circuit to the touch sensing chip  604  can be referenced to a guard potential provided by guard source  626 , and can be isolated from chassis or earth ground  606 . 
       FIG. 6C  illustrates various capacitances associated with proximity detection using touch sensor panel configuration  600  of  FIG. 6A  and/or touch sensor panel configuration  601  of  FIG. 6B  according to examples of the disclosure. Configuration  602  of  FIG. 6C  can be the same as configuration  501  of  FIG. 5B , except as otherwise described below. Specifically, finger (or object)  610  can be in proximity to touch node electrode  608 . Finger  610  can be grounded to earth ground  606  through capacitance  612  (e.g., C body ), which can represent a capacitance from finger  610  through a user&#39;s body to earth ground  606 . Capacitance  614  (e.g., C touch ) can represent a capacitance between finger  610  and touch node electrode  608 , and can be the capacitance of interest in determining how close finger  610  is to touch node electrode  608 . Typically, C body    612  can be significantly larger than C touch    614  such that the equivalent series capacitance seen at touch node electrode  608  through finger  610  can be approximately C touch    614 . Capacitance  616  can represent a capacitance between touch node electrode  608  and shield  624  (e.g., shields  624 A and  624 B in  FIG. 6B ), and capacitance  618  can represent a capacitance between trace(s)  632  and shield  628  (e.g., shields  628 A and  628 B in  FIG. 6B ). Touch sensing chip  604  can be mounted on (e.g., disposed or fabricated on) a printed circuit board  623 , which can have a ground layer  620  to which the touch sensing chip  604  and the touch sensor panel can be referenced. In some examples, ground layer  620  can be included in printed circuit board  623  (e.g., as a conductive layer in the printed circuit board layers), or ground layer  620  can be a separate conductive plate on which printed circuit board  623  can be mounted. In the case that ground layer  620  is included in printed circuit board  623 , the output of guard source  626  can be inputted directly into ground layer  620  within the printed circuit board  623 . Guard plane  620  and shields  624  and  628  can be stimulated by guard source  626  at a guard voltage, as described with reference to  FIGS. 6A-6B . Guard source  626  can also be used to drive touch node electrode  608  through touch sensing circuitry  622  (e.g., because the virtual ground and/or touch node electrode  608  coupled to the inverting input of sense amplifier  622  can follow the voltage reference for sense amplifier  622 , which can be the guard voltage provided by guard source  626  coupled to the non-inverting input of sense amplifier  622 ) to detect touch at touch node electrode  608  (e.g., as described with reference to  FIG. 3A ), as shown in  FIG. 6C . Because the voltage at touch node electrode  608  and trace(s)  632  can mirror or follow the voltage at shields  624  and  628 , capacitances  616  and  618  can be reduced or eliminated from the touch measurements performed by touch sensing circuitry  622 . As such, touch sensing circuitry  622  can simply detect C touch    614 , which can appear as a virtual mutual capacitance between finger  610  and touch node electrode  608  as shown in  FIG. 6D . Without stray capacitances  616  and  618  affecting the touch measurements performed by touch sensing circuitry  622 , the offset in the output signal of sense circuitry  622  (e.g., when no touch is detected at touch node electrode  608 ) can be greatly reduced or eliminated, which can increase the signal to noise ratio and/or the dynamic range of sense circuitry  622 . This, in turn, can improve the ability of touch sensing circuitry  622  to detect a greater range of touch at touch node electrode  608 , and to accurately detect smaller capacitances C touch    614  (and, thus, to accurately detect hover activity at touch node electrode  608  at larger distances). Additionally, with a near-zero offset output signal from touch sensing circuitry  622 , the effects of drift due to environmental changes (e.g., temperature changes) can be greatly reduced. For example, if the signal out of sense amplifier  622  consumes 50% of its dynamic range due to undesirable/un-guarded stray capacitances in the system, and the analog front end (AFE) gain changes by 10% due to temperature, the sense amplifier  622  output will drift by 5% and the effective signal-to-noise ratio (SNR) can be limited to 26 dB. By reducing the undesirable/un-guarded stray capacitances by 20 dB, the effective SNR can be improved from 26 dB to 46 dB. 
     Additionally, implementing touch sensing chip  604  as a separate chip than guard source  626 , as described in this disclosure, can confer benefits in addition to those described above. Specifically, separating touch sensing chip  604  from guard source  626  can enable higher guard signal voltage levels (which can also be used to stimulate touch node electrode  608 ) that can be beyond what may be possible for a single chip implementation (e.g., an implementation in which touch sensing chip  604  and guard source  626  are included in a single chip). 
       FIG. 6D  illustrates a diagram depicting the appearance of C touch    614  as a virtual mutual capacitance between finger  610  and touch node electrode  608 , as described above, according to example of the disclosure. Specifically, because capacitances  616  and  618  in  FIG. 6C  can be reduced or eliminated from the touch measurements performed by touch sensing circuitry  622  due to the shielding provided by shields  624  and  628 , C touch    614  can appear as a virtual mutual capacitance between finger  610  and touch node electrode  608 , which can be coupled to the inverting input of sense circuitry  622 . Specifically, finger  610  can appear to be stimulated via C body    612  by guard source  626 , and finger  610  can have C touch    614  between it and the inverting input of sense circuitry  622 . Changes in C touch    614  can, therefore, be sensed by sense circuitry  622  as changes in the virtual mutual capacitance C touch    614  between finger  610  and sense circuitry  622 . As such, the offset in the output signal of sense circuitry  622  (e.g., when no touch is detected at touch node electrode  608 ) can be greatly reduced or eliminated, as described above. As a result, sense circuitry  622  (e.g., the input stage of sense circuitry  622 ) need not support as great a dynamic input range that self-capacitance sense circuitry (e.g., sense circuitry  314  in  FIG. 3A ) might otherwise need to support in circumstances/configurations that do not exhibit the virtual mutual capacitance effect described here. 
     Because the self-capacitance measurements of touch node electrodes in the touch screen configurations of this disclosure can exhibit the virtual mutual capacitance characteristics described above, in some examples, touch sensing chip  604  need not be a chip that supports self-capacitance measurements (e.g., touch sensing chip  604  may not include sense circuitry  314  as described in  FIG. 3A ). Instead, touch sensing chip  604  may be a standard mutual capacitance touch sensing chip that only supports mutual capacitance measurements (e.g., touch sensing chip  604  may include sense circuitry  314  as described in  FIG. 3B , but not sense circuitry  314  as described in  FIG. 3A ). In such examples, guard source  626  can be appropriately designed and used with the mutual capacitance touch sensing chip in various configurations of this disclosure (e.g., configuration  600 ) to effectively achieve the guarded self-capacitance functionality of this disclosure despite touch sensing chip  604  being a mutual capacitance touch sensing chip, rather than a self-capacitance touch sensing chip. For example, referring to  FIG. 3B , stimulation source  306  (e.g., guard source  626 ) can stimulate the guard plane(s) of the disclosure, which can act as the drive electrodes in the virtual mutual capacitance configuration described here. The touch node electrodes of the touch sensor panel can then, in turn, be treated as the sense electrodes in the virtual mutual capacitance configuration described here, and can be coupled to the input of sense amplifier  308  in  FIG. 3B . Touch sensing circuitry  314  in  FIG. 3B  can then sense the mutual capacitance between the guard plane(s) and the touch node electrodes, which can be represented by the circuit configuration of  FIG. 6D . 
       FIG. 6E  illustrates an exemplary configuration for guard source  626  according to examples of the disclosure. Specifically, guard source  626  can include a direct digital synthesizer (DDS)  650  (e.g., a direct waveform synthesis generator) that can generate arbitrary waveforms, such as a square wave, and can be referenced to earth ground  606 . The output of DDS  650  can be inputted to digital-to-analog converter (DAC)  652 , which can convert the output of DDS  650  into a corresponding analog signal. The output of DAC  652  can be inputted to the non-inverting input of linear buffer  654  (e.g., a unity gain buffer, though it is understood that buffer  654  can alternatively have a non-unity gain configuration). The output of linear buffer  654  can correspond to the output of guard source  626 , as previously described, and can provide a guard signal to guard plane  620 . The inverting input of linear buffer  654  can be coupled to the output of linear buffer  654  to facilitate the feedback function of linear buffer  654 . In some examples, the inverting input of linear buffer  654  can be coupled to the output of linear buffer  654  at a location that is remote from guard source  626 ; for example, as shown in  FIG. 6E , the inverting input of linear buffer  654  can be coupled to node A at guard plane  620 . In this way, the effect of resistance that might exist between the output of linear buffer  654  and node A at guard plane  620  can be reduced (e.g., because that resistance can be included in the feedback loop of linear buffer  654 , which can, in turn, improve the accuracy of the voltage delivered by guard source  626  to guard plane  620 ). 
     It is understood that the example of  FIG. 6E  can illustrate a voltage mode DAC (e.g., DAC  652 ) and subsequent buffer (e.g., buffer  654 ). However, in some examples, DAC  652  can be a current mode DAC (iDAC) and buffer  652  can be a transimpedance amplifier (TIA) type buffer that can convert current from the iDAC to a buffer output voltage (e.g., similar to as described above with respect to  FIG. 6E ). 
     Because the guard signal provided to the touch sensing chip of the disclosure can be used as the stimulation signal for sensing touch on touch node electrodes on the touch sensor panel, it can be important for the guard signal to be known, stable and/or free of noise—noise in the guard signal can directly introduce errors into the measured value of touch at the touch node electrodes. Further, because the touch sensing chip can be separate from the guard source chip, resistances of bond pads, wires or other coupling circuitry/components between the touch sensing chip and the guard source chip can be substantial. Such substantial resistances can cause the guard signal that reaches the touch sensing chip to differ from the guard signal that is outputted by the guard source, as will be described with reference to  FIGS. 7A-7B . 
     Specifically,  FIG. 7A  illustrates an exemplary guard stimulus voltage delivery configuration  700  for delivering a guard stimulus voltage to touch sensing chip  704  according to examples of the disclosure. Touch sensing chip  704  can be referenced to, disposed and/or fabricated on guard plane  720 , as previously discussed. Guard ground  707  can represent the virtual ground provided by guard plane  720 . Guard source  726  can be a buffer or other amplifier circuit whose output is coupled to guard plane  720 , and that outputs guard signal  721 . Guard source  726  can be included in a chip that is referenced to chassis or earth ground  706 , and can be coupled to guard plane  720  via one or more of bond pads, wires, traces, etc. 
     Circuitry in touch sensing chip  704 , such as touch sensing circuitry  622  as previously described (e.g., amplifier  308 , multiplier  328 , filter  332 , etc.), can require two voltage rails to operate—a low voltage rail (e.g., provided by guard plane  720  stimulated by guard signal  721 ), and a high voltage rail (e.g., a voltage rail with a higher, or more positive, voltage than guard plane  720 ). As mentioned above, the voltage for the low voltage rail can be provided to touch sensing chip  704  by guard source  726 , which can stimulate guard plane  720  with guard signal  721 . In order to provide the high voltage rail to touch sensing chip  704 , capacitor  730  can be coupled between guard plane  720  and high voltage input  732  of touch sensing chip  704 . Capacitor  730  can be charged until a specified direct current (DC) voltage is built-up across capacitor  730 , which can provide the high voltage (referenced to guard signal  721  on guard plane  720 ) to touch sensing chip  704 . For example, one terminal of capacitor  730  can be coupled to guard plane  720  at node A, while the other terminal of capacitor  730  can be coupled to switch  734 , which can be coupled to DC voltage source  736  that is referenced to chassis or earth ground  706 . While guard signal  721  outputted by guard source  726  is low (e.g., substantially the voltage at chassis or earth ground  706 , such as 0V), switch  734  can be closed (e.g., the device can be configured to close switch  734  via a control signal when guard signal  721  is low, or switch  734  can be configured to close itself when guard signal  721  is low), and DC voltage source  736  can charge capacitor  730  to the desired DC voltage offset between the low voltage rail (e.g., guard plane  720 ) and the high voltage rail of touch sensing chip  704 . For example, this DC voltage offset can be on the order of 3V or 4V, though other voltages are also within the scope of the disclosure. In some examples, capacitor  730  can be charged in excess of the desired DC voltage offset between the low and high voltage rails of touch sensing chip  704 , and the excess DC voltage on capacitor  730  can be regulated to the final desired voltage for the high voltage rail by voltage regulator  738  (e.g., a low-dropout or LDO regulator) in touch sensing chip  704  to be used by circuitry in the touch sensing chip (e.g., touch sensing circuitry  622 ). 
     While guard signal  721  outputted by guard source  726  is high (e.g., a voltage greater, or more positive, than chassis or earth ground  706 , such as 2V, 3V, 4V, etc.), switch  734  can be open (e.g., the device can be configured to open switch  734  via a control signal when guard signal  721  is high, or switch  734  can be configured to open itself when guard signal  721  is high). In other words, when guard signal  721  transitions from low to high, switch  734  can open, and when guard signal  721  transitions from high to low, switch  734  can close. While switch  734  is open, capacitor  730  can provide its voltage to high voltage input  732 , which can be regulated by voltage regulator  738  to the final desired voltage for the high voltage rail in touch sensing chip  704 . In this way, voltage that is appropriately offset from guard signal  721  can be provided for the high voltage rail of touch sensing chip  704  during the various states of guard signal  721 . Because voltage regulator  738  can be referenced to ground  707 , which can be referenced to the guard signal, the operation of voltage regulator  738  need not be changed as the guard signal transitions between low and high—the voltage rails of voltage regulator  738  (e.g., ground  707  and the high voltage rail of touch sensing chip  704 ) can maintain their predetermined relationship, and voltage regulator  738  can operate without regard for the voltage transitions in the guard signal. In some examples, switch  734  can be current-limited or otherwise regulated to reduce or minimize inrush current into capacitor  730  (e.g., limited to only allow current below a current threshold), and/or reduce or minimize load current variations from voltage source  736 . The low and high voltage rails of touch sensing chip  704  can have a predetermined relationship with respect to the “ground”  707  of touch sensing chip  704  (e.g., provided by guard source  626 ). Thus, as the voltage outputted by guard source  626  alternates (e.g., between a low voltage and a high voltage), the voltages on the low and high voltage rails of touch sensing chip  704  can similarly alternate such that the predetermined relationship of those voltages with respect to ground  707  can be maintained. 
     In some examples, voltage source  736 , switch  734 , and guard source  726  can be included in one chip (e.g., guard source chip), and capacitor  730  can be a discrete component (e.g., separate from touch sensing chip  704  and the chip in which guard source  726  is included) coupled in the electrical pathway between the guard source chip and touch sensing chip  704 . In some examples, capacitor  730  can be included in touch sensing chip  704  if the current requirements of capacitor  730 /touch sensing chip  704  are sufficiently small. 
     In some examples, particularly when touch sensing chip  704  is separate from the chip on which guard source  726  is disposed or fabricated, the resistances between the output of guard source  726  and node A (e.g., the node on or near touch sensing chip  704  at which guard plane  720  and capacitor  730  can be coupled) can be substantial. These resistances, which can include resistances such as the output resistance of guard source  726 , trace resistances between guard source  726  and touch sensing chip  704 , bond pad resistances, etc., can be represented by resistance  728  in  FIG. 7A . Further, during the time periods when guard signal  721  is low and when switch  734  is closed to charge capacitor  730 , current can flow from DC voltage source  736 , through capacitor  730 , resistance  728  and guard source  726 , to chassis or earth ground  706  via current path  740 . This current, which can flow through resistance  728 , can cause the voltage at node A to differ from (e.g., be higher, or more positive than) the voltage at the output of guard source  726 . Additionally, this current can change as a function of time, as it can decrease as capacitor  730  becomes charged to the desired DC voltage by DC voltage source  736 . Therefore, the amount by which the voltage at node A differs from the voltage at the output of guard source  726  can also change as a function of time while guard signal  721  is low. 
       FIG. 7B  illustrates exemplary guard signal  721  at node A in  FIG. 7A  according to examples of the disclosure. Solid lines  723  can represent the ideal or desired guard signal  721  to be provided to node A in  FIG. 7A . When guard signal  721  transitions from high to low, because of resistance  728  and current path  740 , instead of reaching the desired low voltage point immediately, the guard signal at node A can gradually, over time, reach the desired low voltage point, as illustrated by dashed lines  725 . In some examples, guard signal  721  at node A may not reach the desired low voltage point by the time the guard signal transitions from low to high. As a result, guard signal  721  at node A can be unknown and can change over time, which can cause errors in touch sensing performance, as described above. In other words, because guard signal  721  at node A can be used by touch sensing circuitry in touch sensing chip  704  to sense touch on the touch sensor panel, errors in the guard signal voltage at node A can directly introduce errors into the touch signals outputted by the touch sensing circuitry. Therefore, it can be desirable to include a mechanism in configuration  700  for better regulating the voltage at node A, particularly in circumstances in which guard source  726  and touch sensing circuitry are included in separate and distinct chips. 
       FIG. 8A  illustrates an exemplary voltage delivery configuration  800  for delivering voltage to touch sensing chip  804  using voltage regulator  842  according to examples of the disclosure. Configuration  800  of  FIG. 8A  can be the same as configuration  700  of  FIG. 7A , except that configuration  800  can include voltage regulator  842  coupled between node A and guard source  826  (e.g., voltage regulator  842  can be coupled to one or more inputs of guard source  826 ). For example, switch  834 , like switch  734 , can be current-limited or otherwise regulated to reduce or minimize inrush current into capacitor  830  (e.g., limited to only allow current below a current threshold), and/or reduce or minimize load current variations from voltage source  836 . Other components of configuration  800  can similarly be the same as components of configuration  700 . Voltage regulator  842  can regulate the voltage at node A while guard signal  821  is low and/or high, as will be described in more detail below, to provide a guard signal for guard plane  820  and for use by touch sensing chip  804  that is stable and known. 
       FIG. 8B  illustrates exemplary details of guard source  826  and voltage regulator  842  according to examples of the disclosure. In some examples, guard source  826  can comprise a push-pull configuration of P-FET  856  and N-FET  858  (though other configurations of guard source  826  can also be utilized) in which the source of P-FET  856  can be coupled to high voltage rail  854 , the drain of P-FET  856  can be coupled to the drain of N-FET  858 , and the source of N-FET  858  can be coupled to chassis or earth ground  806 . Guard source  826  can have input  846  and input  848 , which can provide input voltage signals to P-FET  856  and N-FET  858 , respectively. For example, when guard source  826  is to output a high voltage, inputs  846  and  848  can be low such that P-FET  856  is on, and N-FET  858  is off. Analogously, when guard source  826  is to output a low voltage, inputs  846  and  848  can be high such that P-FET  856  is off, and N-FET  858  is on. Node B can represent the output node of guard source  826 . Node B can be coupled to node A (corresponding to node A in  FIG. 8A ). Resistances R out    828 A and R trace    828 B, which can correspond to the output resistance of guard source  826  and the resistance of traces that couple guard source  826  and node A, respectively, can represent the resistances between node A and guard source  826 . 
     Voltage regulator  842  can be coupled between node A and guard source  826 . Specifically, voltage regulator  842  can include amplifier  844  having two inputs: an input from node A coupled to the non-inverting input of amplifier  844 , and V guard-target  coupled to the inverting input of amplifier  844 , which can be the desired guard voltage at node A. The output of amplifier  844  (and thus voltage regulator  842 ) can be coupled to input  848  via summer  852 . Further, voltage regulator  842  can receive an ENABLE signal which can control whether or not amplifier  844  is operational, as will be discussed below. The ENABLE signal can be timed to guard signal  821  outputted by guard source  826  (e.g., using appropriate circuitry with provisions for precise timing control) such that when guard signal  821  is low, the ENABLE signal can be active (e.g., high), which can cause amplifier  844  to be operational (e.g., amplifier  844  can output a signal), and when guard signal  821  is high, the ENABLE signal can be inactive (e.g., low), which can cause amplifier  844  to be nonoperational (e.g., amplifier  844  can not output a signal). 
     The operation of voltage regulator  842  in conjunction with guard source  826  will now be described. Voltage regulator  842  can perform low- and/or high-side voltage regulation at node A. Low-side voltage regulation (e.g., regulating the voltage at node A when guard signal  821  is low) will be described with reference to  FIG. 8B , but it is understood that high-side voltage regulation (e.g., regulating the voltage at node A when guard signal  821  is high) can be analogously implemented. For example, voltage regulator  842  can regulate the voltage at node A when guard signal  821  outputted by guard source  826  is low. Specifically, the output of voltage regulator  842  can be coupled to input  848 , as shown in  FIG. 8B , and V guard-target  can be set to the desired voltage at node A when guard signal  821  is low. For example, V guard-target  can be a voltage slightly above chassis or earth ground  806 , such as 0.1V or 0.2V. When guard signal  821  is low, the ENABLE signal inputted to voltage regulator  842  can be active, which can cause amplifier  844  to be operational, and if the voltage at node A deviates from V guard-target , amplifier  844  can output a signal corresponding to the deviation, which can be added to input  848  via summer  852  to change the gate voltage of N-FET  858  until the voltage at node A reaches V guard-target.  As such, even though current may flow through R out    828 A and R trace    828 B, and even if that current varies as a function of time (e.g., as described with reference to current path  740  in  FIG. 7A ), voltage regulator  842  can maintain the voltage at node A at V guard-target . It should be noted that V guard-target  may not be able to be set to the voltage at chassis or earth ground  806  (e.g., 0V), because of the existence of R out    828 A and R trace    828 B. Therefore, the voltage at node A when guard signal is low may not be able to reach chassis or earth ground  806 , but rather may be maintained at a level slightly above (e.g., 0.1V, 0.2V, etc.) chassis or earth ground  806 . 
     As previously mentioned, voltage regulator  842  (or separate voltage regulation circuitry that is analogously structured and/or operates analogously to voltage regulator  842 ) can additionally or alternatively perform high-side voltage regulation at node A. For example, such voltage regulation circuitry can regulate the voltage at node A when guard signal  821  outputted by guard source  826  is high. Specifically, the output of such voltage regulation circuitry can be coupled to input  846 , and V guard-target  for that voltage regulation circuitry can be set to the desired voltage at node A when guard signal  821  is high. For example, V guard-target  can be a voltage slightly above high voltage rail  854  (e.g., above high voltage rail by 0.1V, 0.2V, etc.). If the voltage at node A deviates from V guard-target  the voltage regulation circuitry can output a signal corresponding to the deviation, which can be added to input  846  via a summer to change the gate voltage of P-FET  856  until the voltage at node A reaches V guard-target . As such, even though current may flow through R out    828 A and R trace    828 B, and even if that current varies as a function of time (e.g., as described with reference to current path  740  in  FIG. 7A ), the voltage regulation circuitry can maintain the voltage at node A at V guard-target . It should be noted that V guard-target  may not be able to be set to the voltage at high voltage rail  854 , because of the existence of R out    828 A and R trace    828 B (e.g., current may flow from node A through R out    828 A and R trace    828 B to high voltage rail  854 ). Therefore, the voltage at node A when guard signal is high may not be able to reach the voltage at high voltage rail  854 , but rather may be maintained at a level slightly above (e.g., 0.1V, 0.2V, etc. above) the voltage at high voltage rail  854 . 
       FIG. 8C  illustrates exemplary guard signal  821  at node A of  FIGS. 8A-8B  resulting from low-side voltage regulation according to examples of the disclosure. Solid lines  823  can represent the ideal or desired guard signal  821  to be provided to node A in  FIGS. 8A-8B , and dashed lines  825  can correspond to the voltage at node A maintained by voltage regulator  842 . As shown, the voltages maintained by voltage regulator  842  at node A during time periods when guard signal  821  is low can slightly deviate from the ideal; however, the voltages at node A can be stable and known, and thus may not interfere with accurate touch detection by touch sensing chip  804 . In circumstances in which high-side voltage regulation is implemented, the voltages maintained at node A during time periods when guard signal is high can similarly slightly deviate from the ideal (e.g., slightly above the ideal voltages), as described above. 
       FIG. 8D  illustrates another exemplary voltage delivery configuration  801  for delivering voltage to touch sensing chip  804  in which capacitor  830  is selectively coupled to node A according to examples of the disclosure. Configuration  801  of  FIG. 8D  can be the same as configuration  700  of  FIG. 7A , except that configuration  801  can include switch  835  coupled between capacitor  830  and node A. Specifically, as discussed above, current flowing from voltage source  836  through capacitor  830  and resistance  828  to guard source  826  when guard signal  821  is low can cause the voltage at node A to fluctuate from a desired or known guard voltage. To address this, in configuration  801 , switch  835  can be coupled between capacitor  830  and node A, as shown in  FIG. 8D . While guard signal  821  is low (e.g., at earth or chassis ground  806 , or at approximately earth or chassis ground  806 ), switch  835  can couple capacitor  830  to node B, which can be coupled to earth or chassis ground  806 , and can decouple capacitor  830  from node A. As such, capacitor  830  can continue to be coupled to the low voltage level of guard signal  821 —e.g., earth or chassis ground  806 , or approximately earth or chassis ground  806 —which can be provided by node B rather than by guard source  826 . While guard signal  821  is high, switch  835  can couple capacitor  830  to node A, and can decouple capacitor  830  from node B. As a result, while guard signal  821  is low, no pathway can exist for current to flow from voltage source  836  through capacitor  830  and resistance  828  to guard source  826 , because switch  835  can decouple capacitor  830  from node A while guard signal  821  is low. Rather, while guard signal  821  is low, current can flow from voltage source  836  through capacitor  830  directly to earth or chassis ground  806  through node B, to which switch  835  can couple capacitor  830  while guard signal  821  is low. Therefore, the effect of such current flow on the voltage at node A can be reduced or eliminated without the use of voltage regulator  842  in  FIGS. 8B-8C . More generally, the voltage at node B can be whatever the intended low voltage output of guard source  826  is (e.g., chassis ground, 1V below chassis ground, 1V above chassis ground, etc.), except that the voltage at node B can be provided, not by guard source  826 , but by another separate voltage source that outputs, to node B, the low voltage value corresponding to the low side of guard signal  821 . 
     Switch  835  can be implemented as a physical switch, a solid state switch (e.g., comprised of one or more transistors), or any other design for performing the above-described functions. In some examples, switch  835  can be current-limited or otherwise regulated to reduce or minimize inrush current into/through capacitor  830  (e.g., limited to only allow current below a current threshold), and/or reduce or minimize load current variations from voltage source  836 . Further, in some examples, switch  835  can switch between nodes A and B in synchronization with guard signal  821  transitioning between low and high (e.g., switch  835  can switch from node A to node B at the same time that guard signal  821  transitions from high to low, etc.). However, in other examples, switch  835  can switch between nodes A and B in synchronization with, but with a slight delay with respect to, guard signal  821  transitioning between low and high (e.g., switch  835  can switch from node A to node B some predetermined time after guard signal  821  transitions from high to low, etc.) in order to reduce spikes or other fluctuations in voltage at node A that might occur when switch  835  decouples capacitor  830  from node A or B, and couples capacitor  830  to node B or A, respectively. In such examples, the timing delay of switch  835  switching between nodes A and B can be predetermined (e.g., a predetermined time, a predetermined change in voltage of guard signal  821  during the transitions between low and high, etc.). In some examples, circuitry external to switch  835  can control the switching of switch  835  between nodes A and B as described above (e.g., by monitoring the passage of time since guard signal transitioned from low to high, by monitoring the voltage of guard signal  821  and/or the voltage at node A since guard signal transitioned from low to high, etc.). 
     As shown above, various configurations for the guard buffer (e.g., guard source  626  in  FIG. 6E  or guard source  826  in  FIG. 8B ) of the disclosure have been provided. However, in some examples, the guard buffer of the disclosure can be a hybrid of the push-pull buffer (e.g., in  FIG. 8B ) and the linear buffer (e.g., in  FIG. 6E ) disclosed in this application so as to exhibit one or more of the benefits of both. For example, the guard buffer can operate as a linear buffer during the time periods when the guard signal is LOW and during the transitions of the guard signal from LOW to HIGH and/or HIGH to LOW when high linearity and low noise can be required (e.g., a linear buffer can have good linearity but can require more power than a push-pull buffer due to the required output headroom and/or bias currents needed for internal structures, such as input/output stages). In contrast, the guard buffer can operate as a push-pull buffer during the time periods when the guard signal is HIGH (e.g. the PMOS in the output stage of the guard buffer can be fully turned on as opposed to being regulated). A push-pull buffer can have worse linearity than a linear buffer, but can require less power than a linear buffer because it may not require the output headroom of the linear buffer and/or the quiescent current budget of a linear buffer. A benefit of the above-described operation can be that during the guard signal HIGH phase, output headroom (e.g., headroom needed relative to the guard signal LOW and/or HIGH voltages) for the guard buffer may not have to be allocated, because the guard buffer can operate as a push-pull buffer. As such, the positive supply of the guard buffer can be lowered, therefore saving power. 
     In some examples, the touch sensing system of the disclosure can configure the touch sensing circuitry to operate in different modes of operation for sensing touch at different times. For example, the touch sensing circuitry can be capable of operating in one of two or more modes of operation, and can switch between those modes of operation in response to various criteria being satisfied. In some examples, the touch sensing circuitry can be capable of operating in one of three modes: 1) a RESET off mode, 2) a DC restore mode, and 3) a scan step RESET mode, as will now be described with reference to  FIGS. 3A-3B . In particular, in the RESET off mode, the RESET control signal can maintain switch  315  in an open position throughout a touch scan step, and in the DC restore mode and the scan step RESET mode, the RESET control signal can close switch  315  at different times in a touch scan step, as discussed below. 
     With respect to the RESET off mode, as described above, switch  315  can remain open throughout a touch scan step of the touch screen (e.g., which can extend for one or more periods of guard signal  721  and/or  821 ). In this mode, amplifier  308  in sense circuit  314  can be configured as a trans-impedance amplifier with a feedback impedance comprising resistor  312  and/or capacitor  310 . In such a configuration, resistor  312  can dominate the overall feedback impedance of amplifier  308 . As a result, the output of amplifier  308  (e.g., Vo) can settle relatively quickly. The touch sensing system can configure sensing circuit  314  to operate in the RESET off mode in circumstances in which sensing circuit  314  is sensing touch across greater than a threshold number of touch electrodes simultaneously (e.g., greater than two, five or ten touch node electrodes  302 ), and may configure sensing circuit  314  to operate in a different mode (e.g., the DC restore mode or the scan step RESET mode) in circumstances in which sensing circuit  314  is sensing touch across less than or equal to the threshold number of touch electrodes simultaneously. A reason for configuring sensing circuit  314  in the RESET off mode when sensing circuit  314  is sensing touch across greater than the threshold number of touch electrodes simultaneously can be that in such circumstances, the input signal from those touch electrodes to amplifier  308  can be more than capacitor  310  may be able to absorb without amplifier  308  saturating. As such, in order to reduce the feedback impedance of amplifier  308 , resistor  312  can be maintained in the feedback loop of amplifier  308  (e.g., by maintaining switch  315  in an open position) such that the feedback impedance can be dominated by resistor  312 , which can enable relatively fast settling of the output of sensing circuit  314 . 
     With respect to the DC restore mode, amplifier  308  in sense circuit  314  can be configured as a charge amplifier, and switch  315  can remain open throughout a touch scan step of the touch screen, except that switch  315  can be closed one or more times during a period of guard signal  721  and/or  821  (e.g., multiple times during the touch scan step of the touch screen) to facilitate maintenance of the desired common mode voltage of the output Vo of amplifier  308  throughout the touch scan step of the touch screen. For example,  FIG. 8E  illustrates an exemplary timing diagram that relates guard signal  821 , RESET signal  862 , and the output  864  of sensing circuit  314  during the DC restore mode. As shown, RESET signal  862  can be asserted (and thus close switch  315 ) before each transition of guard signal  821  (e.g., low to high transitions, and high to low transitions), such as during time t 0  before the transition of guard signal  821  from low to high. RESET signal  862  can then be de-asserted time t 1  before the low to high transition of guard signal  821 . In some examples, such assertion/de-assertion can continue before each transition of guard signal  821 , as shown in  FIG. 8E . While RESET signal  862  is asserted, switch  315  can be closed, which can facilitate the output of sensing circuit  314  to return to the desired common mode voltage of sensing circuit (e.g., V BIAS  in  FIG. 8E ) before the next transition of guard signal  821  occurs. 
     With respect to the scan step RESET mode, amplifier  308  in sense circuit  314  can be configured as a charge amplifier, and switch  315  can remain open throughout a touch scan step of the touch screen, except that switch  315  can be closed once at the beginning of the touch scan step (e.g., which can extend for one or more periods of guard signal  721  and/or  821 ) to facilitate fast settling of amplifier  308  before touch sensing begins that might otherwise be limited by the RC time constant attributable to resistor  312  and capacitor  310  in sensing circuit  314 . For example,  FIG. 8F  illustrates an exemplary timing diagram that relates guard signal  821 , RESET signal  862 , and the output  864  of sensing circuit  314  during the scan step RESET mode. As shown, RESET signal  862  can be asserted (and thus close switch  315 ) at point  1  for time t 0 . During time t 0 , at point  2 , guard signal  821  can transition from low to a programmed common mode level V MID , and can reach V MID  at point  3  according to a programmed rise time. Guard signal  821  can remain at V MID  from point  3  to point  5 . At point  4 , between points  3  and  5 , RESET signal  862  can be de-asserted, and time t 1  later, at point  5 , guard signal  821  can transition from V MID  to high, and can reach high at point  6  according to a programmed rise time. RESET signal  862  can remain de-asserted for the remainder of touch scan step  866 , as shown. In this way, settling of output  864  of amplifier  308  to V BIAS  before touch sensing is performed can be facilitated. If output  864  were not settled to V BIAS  before the start of touch sensing at the beginning of scan step  866 , that unsettled portion of output  864  could consume dynamic range of sensing circuit  314 , which could degrade interference/noise rejection of the touch sensing system. 
     Whether the touch sensing system configures sensing circuit  314  to operate in the DC restore mode or the scan step RESET mode can depend on the results of a spectral analysis step performed by the touch sensing system (e.g., a touch scan step that is performed during which the touch sensing system senses signals from the touch electrodes on the touch screen during a time when there is no touch activity on the touch screen, thus capturing and identifying noise or other interference sources). For example, the touch sensing system can perform a spectral analysis step, and can identify a noise profile that is present at the touch screen. Based on that noise profile, the touch sensing system can configure sensing circuit  314  to operate in the DC restore mode or the scan step RESET mode (when it is not operating in the RESET off mode). DC restore mode can provide better low frequency interference rejection than scan step RESET mode for certain guard signal stimulation frequencies (e.g., 60 Hz or 120 Hz); thus, if the spectral analysis indicates noise or interferers present at low frequencies (e.g., lower than a threshold frequency), the touch sensing system can select the DC restore mode. However, the DC restore mode can induce aliasing at amplifier  308 , which can degrade interference rejection at other frequencies. As such, the touch sensing system can, based on these and/or other factors, determine in which of the DC restore mode and the scan step RESET mode to configure sensing circuit  314 . For example, if the spectral analysis indicates noise or interferers present at high frequencies (e.g., higher than the threshold frequency), the touch sensing system can select the scan step RESET mode. The touch system operations illustrated by  FIGS. 8E-8F  can be utilized with any of the touch sensing system configurations described with reference to  FIGS. 1-9  of the disclosure. 
     With reference to  FIGS. 8E-8F , in some examples, one or more touch node electrodes on the touch sensor panel may be driven at a reference voltage (e.g., at the guard signal voltage) while one or more other touch node electrodes are being sensed for touch. The touch node electrodes being sensed can be coupled to touch sensing circuitry (e.g., as described with reference to  FIGS. 3A-3B ), which can operate in accordance with RESET  862  in  FIGS. 8E and/or 8F . The touch node electrodes being driven at the reference voltage but not sensed can be coupled, not to touch sensing circuitry, but to separate driving circuitry (e.g., buffered drive amplifiers)—the separate driving circuitry may not include a RESET switch  315 , and thus may operate independently of RESET  862  in  FIGS. 8E and/or 8F . 
     As previously described, in some examples, the touch sensing chip of the disclosure can be a separate and distinct chip than other chips in the touch sensing system (e.g., guard source chip, touch processor chip, host system chip, etc.). Further, the touch sensing chip can be guarded by a guard plane reference voltage that can be different than chassis or earth ground to which the other chips in the touch sensing system can be grounded. In some examples, the other chips in the touch sensing system may need to communicate with the touch sensing chip, in which case level shifters may need to be included in the communication link(s) between the touch sensing chip and the other chips to account for the different power domains in which the touch sensing chip and the other chips in the touch sensing system operate. 
       FIG. 9  illustrates an exemplary level shifter configuration  900  according to examples of the disclosure. Touch sensing chip  904  can be referenced to guard reference  907 , as described in this disclosure (e.g., operated in the guard power domain). Touch sensing chip  904  can also include touch microcontroller  912 , which can correspond to touch controller  206  and/or touch processor  202  in  FIG. 2  for controlling and/or processing the touch sensing performed by touch sensing chip  904 . Touch microcontroller  912  can include various logic, memory, touch sensing circuitry, etc. for sensing touch on a touch sensor panel. Touch microcontroller  912  can be communicatively coupled to host  902 , which can be referenced to chassis or earth ground  906  (e.g., operated in the chassis or earth ground power domain). Host  902  can correspond to the host processor and/or system of the device in which configuration  900  is included (e.g., devices  136 ,  140 ,  144  and  148  in  FIGS. 1A-1D ), and can include host processor  228  and/or program storage  232  in  FIG. 2 . Host  902  can be a distinct and separate chip from touch sensing chip  904  and/or guard signal generation chip  926 , though in some examples, host  902  can be on the same chip as guard signal generation chip  926 . 
     Host  902  may need to communicate data to/from touch microcontroller  912 . For example, when the device in which configuration  900  is included is first powered on, host  902  can transmit information about touch scanning to touch microcontroller  912  (e.g., information about how touch microcontroller  912  should sense touch on the touch sensor panel, such as which touch node electrodes to sense, ground, bias, etc. at any given moment in time) to properly configure touch microcontroller  912 . Because host  902  and touch sensing chip  904  (and thus touch microcontroller  912 ) can be in different power domains (e.g., grounded to chassis/earth ground  906  vs. grounded to guard ground  907 ), communications between host  902  and touch microcontroller  912  can be appropriately level-shifted between the power domains by level shifter(s)  908 . In some examples, host  902  can be communicatively coupled to touch microcontroller  904  via guard signal chip  926 , which can also be referenced to chassis or earth ground  906 . Thus, in some examples, level shifter(s)  908 , which can be included in the communication link between host  902  and touch sensing chip  904 , can be included in guard signal chip  926 . 
     In some examples, the amount of data that host  902  needs to communicate to/from touch sensing chip  904  can be substantial, and level-shifting the data between the power domains of host  902  and touch sensing chip  904  can slow down the speed of such communication between the two chips. Therefore, in some examples, guard signal chip  926  can include bypass switch  910 , which can selectively bypass level shifter(s)  908  on the communication link between host  902  and touch sensing chip  904 , when appropriate. For example, when guard signal chip  926  is not generating a guard signal for the guard plane of touch sensing chip  904 , or when the guard signal is in the low state (and thus, the voltage of the guard signal is substantially the same as the voltage at chassis or earth ground  906 ), switch  910  can be closed to bypass level shifter(s)  908  and increase the rate of communication possible between host  902  and touch sensing chip  904 . Otherwise (e.g., when the guard signal is in the high state), switch  910  can be open, and host  902  can communicate with touch sensing chip  904  via level shifter(s)  908  (i.e., level shifter(s)  908  may not be bypassed). 
     Thus, the examples of the disclosure provide various configurations for operating a touch sensing chip in a different power domain than other chips in a touch sensing system, which can improve the touch sensing performance of the system. 
     Therefore, according to the above, some examples of the disclosure are directed to a touch sensing system comprising: a guard signal generation chip operating in a first power domain referenced to a first voltage, the guard signal generation chip configured to generate a guard signal; a touch sensing chip operating in a second power domain, different from the first power domain, referenced to the guard signal, the touch sensing chip configured to sense touch at one or more touch electrodes included in a touch sensor panel operating in the second power domain referenced to the guard signal, and the touch sensing chip a different chip than the guard signal generation chip; and a voltage regulator configured to selectively regulate a voltage of the guard signal at the touch sensing chip. Additionally or alternatively to one or more of the examples disclosed above, in some examples the first power domain is referenced to a chassis ground of an electronic device in which the touch sensing system is included, or to earth ground. Additionally or alternatively to one or more of the examples disclosed above, in some examples the guard signal comprises an AC voltage. Additionally or alternatively to one or more of the examples disclosed above, in some examples the touch sensing chip is disposed on a guard plane electrically connected to the guard signal generation chip and configured to be driven with the guard signal. Additionally or alternatively to one or more of the examples disclosed above, in some examples the touch sensing chip includes touch sensing circuitry configured to sense touch at the one or more touch electrodes using the guard signal. Additionally or alternatively to one or more of the examples disclosed above, in some examples the touch sensing system further comprises: a flex circuit including: one or more traces configured to electrically couple the touch sensing chip to the touch sensor panel; and one or more shields configured to isolate the one or more traces from the first power domain, wherein: the touch sensing chip is disposed on a guard plane configured to isolate the touch sensing chip from the first power domain, the touch sensor panel includes one or more shields configured to isolate the one or more touch electrodes from the first power domain, and the guard plane, the one or more shields included in the flex circuit, and the one or more shields included in the touch sensor panel are electrically connected to the guard signal generation chip and are configured to be driven by the guard signal. Additionally or alternatively to one or more of the examples disclosed above, in some examples the voltage regulator is configured to: while the guard signal is in a first state, regulate the voltage of the guard signal at the touch sensing chip to a respective voltage, and while the guard signal is in a second state, different from the first state, forgo regulating the voltage of the guard signal at the touch sensing chip to the respective voltage. Additionally or alternatively to one or more of the examples disclosed above, in some examples the guard signal is in the first state when the guard signal is in a low voltage state, and the guard signal is in the second state when the guard signal is in a high voltage state. Additionally or alternatively to one or more of the examples disclosed above, in some examples the respective voltage is a low voltage. Additionally or alternatively to one or more of the examples disclosed above, in some examples the guard signal is in the first state when the guard signal is in a high voltage state, and the guard signal is in the second state when the guard signal is in a low voltage state. Additionally or alternatively to one or more of the examples disclosed above, in some examples the respective voltage is a high voltage. Additionally or alternatively to one or more of the examples disclosed above, in some examples the voltage regulator comprises an amplifier including: an input electrically coupled to the guard signal at the touch sensing chip; and an output electrically coupled to the guard signal generation chip. Additionally or alternatively to one or more of the examples disclosed above, in some examples the voltage regulator is configured to adjust the output of the amplifier based on a deviation of the guard signal at the touch sensing chip from a target voltage, the adjustment of the output of the amplifier causing the guard signal generation chip to adjust the guard signal. Additionally or alternatively to one or more of the examples disclosed above, in some examples the touch sensing system further comprises: a host chip operating in the first power domain, the host chip communicatively coupled to the touch sensing chip via a communication link, and the host chip configured to communicate data to the touch sensing chip via the communication link. Additionally or alternatively to one or more of the examples disclosed above, in some examples the touch sensing system further comprises one or more level shifters included in the communication link, the one or more level shifters configured to adjust a level of the data from the first power domain to the second power domain. Additionally or alternatively to one or more of the examples disclosed above, in some examples the one or more level shifters are configured to be selectively bypassed during the communication of the data from the host chip to the touch sensing chip. Additionally or alternatively to one or more of the examples disclosed above, in some examples during the communication of the data from the host chip to the touch sensing chip, the one or more level shifters are configured to be: bypassed when the guard signal generation chip is not generating the guard signal, and not bypassed when the guard signal generation chip is generating the guard signal. Additionally or alternatively to one or more of the examples disclosed above, in some examples during the communication of the data from the host chip to the touch sensing chip, the one or more level shifters are configured to be: bypassed when the guard signal is in a low state, and not bypassed when the guard signal is in a high state. Additionally or alternatively to one or more of the examples disclosed above, in some examples the guard signal generation chip comprises a direct digital synthesizer, a digital-to-analog converter and a buffer, an output of the direct digital synthesizer coupled to an input of the digital-to-analog converter, an output of the digital-to-analog converter coupled to an input of the buffer, and an output of the buffer outputting the guard signal generated by the guard signal generation chip. Additionally or alternatively to one or more of the examples disclosed above, in some examples the touch sensing chip is configured to receive a low voltage and a high voltage, the low voltage corresponding to the guard signal, and the high voltage based on the low voltage and generated using a capacitor and a switch, the switch configured to limit an amount of current flowing into the capacitor to less than a threshold amount. Additionally or alternatively to one or more of the examples disclosed above, in some examples the guard signal generation chip includes circuitry configured to generate the guard signal, the circuitry is configured to operate as a linear buffer when the guard signal is in a low state, and the circuitry is configured to operate as a push-pull buffer when the guard signal is in a high state. Additionally or alternatively to one or more of the examples disclosed above, in some examples the touch sensing chip is configured to selectively reset a sense amplifier that is configured to sense the touch at the one or more touch electrodes based on one or more of a number of touch electrodes the sense amplifier is sensing simultaneously and a spectral scan of the touch sensor panel 
     Some examples of the disclosure are directed to an electronic device comprising: a touch sensor panel including one or more touch electrodes; a guard signal generation chip operating in a first power domain referenced to a first voltage, the guard signal generation chip configured to generate a guard signal; a touch sensing chip operating in a second power domain, different from the first power domain, referenced to the guard signal, the touch sensing chip configured to sense touch at the one or more touch electrodes included in the touch sensor panel operating in the second power domain referenced to the guard signal, and the touch sensing chip a different chip than the guard signal generation chip; and a voltage regulator configured to selectively regulate a voltage of the guard signal at the touch sensing chip. 
     Some examples of the disclosure are directed to a method for operating a touch sensing system, the method comprising: operating a guard signal generation chip in a first power domain referenced to a first voltage, the guard signal generation chip configured to generate a guard signal; operating a touch sensing chip in a second power domain, different from the first power domain, referenced to the guard signal, the touch sensing chip configured to sense touch at one or more touch electrodes included in a touch sensor panel operating in the second power domain referenced to the guard signal, and the touch sensing chip a different chip than the guard signal generation chip; and selectively regulating a voltage of the guard signal at the touch sensing chip. 
     Some examples of the disclosure are directed to a touch sensing system comprising: a guard signal generation chip operating in a first power domain referenced to a first voltage, the guard signal generation chip configured to generate a guard signal; a touch sensing chip operating in a second power domain, different from the first power domain, referenced to the guard signal, the touch sensing chip configured to sense touch at one or more touch electrodes included in a touch sensor panel operating in the second power domain referenced to the guard signal, and the touch sensing chip a different chip than the guard signal generation chip; and switching circuitry configured to selectively couple a voltage input of the touch sensing chip to the guard signal generation chip based on a state of the guard signal. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the switching circuitry is configured to: couple the voltage input of the touch sensing chip to the guard signal when the state of the guard signal is a first state, and couple the voltage input of the touch sensing chip to the first voltage when the state of the guard signal is a second state, different from the first state. 
     Although examples of this disclosure have been fully described with reference to the accompanying drawings, it is to be noted that various changes and modifications will become apparent to those skilled in the art. Such changes and modifications are to be understood as being included within the scope of examples of this disclosure as defined by the appended claims.

Metadata:
Filing Date: 20170728
Publication Date: 20181106
Grant Date: 20181106
Priority Date: 20160729
Inventors: KRAH, CHRISTOPH H.
SAUER, Christian M.
Assignee: APPLE INC
CPC Classifications: [{"code": "G02F1/1343", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/1343", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/0418", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02F1/13338", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/13338", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/044", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/0412", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/0412", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F2203/04107", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F3/044", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/0418", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/0412", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02F1/13338", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/1343", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/13338", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/0412", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/044", "inventive": true, "first": true, "tree": "[]"}, {"code": "G06F3/0418", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F2203/04107", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F3/0418", "inventive": true, "first": true, "tree": "[]"}, {"code": "G06F3/044", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F2203/04107", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F3/044", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/0418", "inventive": true, "first": true, "tree": "[]"}]
Family ID: 59564251