Patent Publication Number: US-8988376-B2

Title: Charge compensation for capacitive touch sensor nodes

Description:
TECHNICAL FIELD 
     This disclosure relates generally to touch sensors. 
     BACKGROUND 
     A touch sensor may detect the presence and location of a touch or the proximity of an object (such as a user&#39;s finger or a stylus) within a touch-sensitive area of the touch sensor overlaid on a display screen, for example. In a touch-sensitive-display application, the touch sensor may enable a user to interact directly with what is displayed on the screen, rather than indirectly with a mouse or touch pad. A touch sensor may be attached to or provided as part of a desktop computer, laptop computer, tablet computer, personal digital assistant (PDA), smartphone, satellite navigation device, portable media player, portable game console, kiosk computer, point-of-sale device, or other suitable device. A control panel on a household or other appliance may include a touch sensor. 
     There are a number of different types of touch sensors, such as (for example) resistive touch screens, surface acoustic wave touch screens, and capacitive touch screens. Herein, reference to a touch sensor may encompass a touch screen, and vice versa, where appropriate. When an object touches or comes within proximity of the surface of the capacitive touch screen, a change in capacitance may occur within the touch screen at the location of the touch or proximity. A touch-sensor controller may process the change in capacitance to determine its position on the touch screen. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an example touch sensor with an example touch-sensor controller. 
         FIG. 2  illustrates an example system for compensating for charge present at a capacitive node of a mutual-capacitance implementation of the touch sensor of  FIG. 1 . 
         FIG. 3  illustrates an example system for compensating for charge present at a capacitive node of a self-capacitance implementation of the touch sensor of  FIG. 1 . 
         FIG. 4  illustrates an example method for compensating for charge present at a capacitive node of a touch sensor. 
     
    
    
     DESCRIPTION OF EXAMPLE EMBODIMENTS 
       FIG. 1  illustrates an example touch sensor  10  with an example touch-sensor controller  12 . Touch sensor  10  and touch-sensor controller  12  may detect the presence and location of a touch or the proximity of an object within a touch-sensitive area of touch sensor  10 . Herein, reference to a touch sensor may encompass both the touch sensor and its touch-sensor controller, where appropriate. Similarly, reference to a touch-sensor controller may encompass both the touch-sensor controller and its touch sensor, where appropriate. Touch sensor  10  may include one or more touch-sensitive areas, where appropriate. Touch sensor  10  may include an array of drive and sense electrodes (or an array of electrodes of a single type) disposed on one or more substrates, which may be made of a dielectric material. Herein, reference to a touch sensor may encompass both the electrodes of the touch sensor and the substrate(s) that they are disposed on, where appropriate. Alternatively, where appropriate, reference to a touch sensor may encompass the electrodes of the touch sensor, but not the substrate(s) that they are disposed on. 
     An electrode (whether a ground electrode, a guard electrode, a drive electrode, or a sense electrode) may be an area of conductive material forming a shape, such as for example a disc, square, rectangle, thin line, other suitable shape, or suitable combination of these. One or more cuts in one or more layers of conductive material may (at least in part) create the shape of an electrode, and the area of the shape may (at least in part) be bounded by those cuts. In particular embodiments, the conductive material of an electrode may occupy approximately 100% of the area of its shape. As an example and not by way of limitation, an electrode may be made of a transparent material such as indium tin oxide (ITO) and the ITO of the electrode may occupy approximately 100% of the area of its shape (sometimes referred to as 100% fill), where appropriate. In particular embodiments, the conductive material of an electrode may occupy substantially less than 100% of the area of its shape. As an example and not by way of limitation, an electrode may be made of one or more opaque materials such as fine lines of metal or other conductive material (FLM), such as for example copper, silver, or a copper- or silver-based material, and the fine lines of conductive material may occupy approximately 5% of the area of its shape in a hatched, mesh, or other suitable pattern. Herein, reference to FLM encompasses such material, where appropriate. Although this disclosure describes or illustrates particular electrodes made of particular conductive material forming particular shapes with particular fill percentages having particular patterns, this disclosure contemplates any suitable electrodes made of any suitable conductive material forming any suitable shapes with any suitable fill percentages having any suitable patterns. 
     Where appropriate, the shapes of the electrodes (or other elements) of a touch sensor may constitute in whole or in part one or more macro-features of the touch sensor. One or more characteristics of the implementation of those shapes (such as, for example, the conductive materials, fills, or patterns within the shapes) may constitute in whole or in part one or more micro-features of the touch sensor. One or more macro-features of a touch sensor may determine one or more characteristics of its functionality, and one or more micro-features of the touch sensor may determine one or more optical features of the touch sensor, such as transmittance, refraction, or reflection. 
     A mechanical stack may contain the substrate (or multiple substrates) and the conductive material forming the drive or sense electrodes of touch sensor  10 . As an example and not by way of limitation, the mechanical stack may include a first layer of optically clear adhesive (OCA) beneath a cover panel. The cover panel may be clear and made of a resilient material suitable for repeated touching, such as for example glass, polycarbonate, or poly(methyl methacrylate) (PMMA). This disclosure contemplates any suitable cover panel made of any suitable material. The first layer of OCA may be disposed between the cover panel and the substrate with the conductive material forming the drive or sense electrodes. The mechanical stack may also include a second layer of OCA and a dielectric layer (which may be made of PET or another suitable material, similar to the substrate with the conductive material forming the drive or sense electrodes). As an alternative, where appropriate, a thin coating of a dielectric material may be applied instead of the second layer of OCA and the dielectric layer. The second layer of OCA may be disposed between the substrate with the conductive material making up the drive or sense electrodes and the dielectric layer, and the dielectric layer may be disposed between the second layer of OCA and an air gap to a display of a device including touch sensor  10  and touch-sensor controller  12 . As an example only and not by way of limitation, the cover panel may have a thickness of approximately 1 mm; the first layer of OCA may have a thickness of approximately 0.05 mm; the substrate with the conductive material forming the drive or sense electrodes may have a thickness of approximately 0.05 mm; the second layer of OCA may have a thickness of approximately 0.05 mm; and the dielectric layer may have a thickness of approximately 0.05 mm. Although this disclosure describes a particular mechanical stack with a particular number of particular layers made of particular materials and having particular thicknesses, this disclosure contemplates any suitable mechanical stack with any suitable number of any suitable layers made of any suitable materials and having any suitable thicknesses. As an example and not by way of limitation, in particular embodiments, a layer of adhesive or dielectric may replace the dielectric layer, second layer of OCA, and air gap described above, with there being no air gap to the display. 
     In particular embodiments, the mechanical stack containing the substrate and the drive or sense electrodes may be formed within a display panel (thus forming an in-cell sensor) or on a display panel (thus forming an on-cell sensor). In an in-cell sensor, the display may be on the same substrate as the drive or sense electrodes. The display panel may be a liquid crystal display (LCD), a light-emitting diode (LED) display, an LED-backlight LCD, or other suitable electronic display and may be visible through the touch sensor  10  that provides the touch-sensitive area. Although this disclosure describes particular display types, this disclosure contemplates any suitable display types. 
     One or more portions of the substrate of touch sensor  10  may be made of polyethylene terephthalate (PET) or another suitable material. This disclosure contemplates any suitable substrate with any suitable portions made of any suitable material. In particular embodiments, the drive or sense electrodes in touch sensor  10  may be made of ITO in whole or in part. In particular embodiments, the drive or sense electrodes in touch sensor  10  may be made of fine lines of metal or other conductive material. As an example and not by way of limitation, one or more portions of the conductive material may be copper or copper-based and have a thickness of approximately 5 μm or less and a width of approximately 10 μm or less. As another example, one or more portions of the conductive material may be silver or silver-based and similarly have a thickness of approximately 5 μm or less and a width of approximately 10 μm or less. This disclosure contemplates any suitable electrodes made of any suitable material. 
     Touch sensor  10  may implement a capacitive form of touch sensing. In a mutual-capacitance implementation, touch sensor  10  may include an array of drive and sense electrodes forming an array of capacitive nodes. A drive electrode and a sense electrode may form a capacitive node. The drive and sense electrodes forming the capacitive node may come near each other, but not make electrical contact with each other. Instead, the drive and sense electrodes may be capacitively coupled to each other across a space between them. A pulsed or alternating voltage applied to the drive electrode (by touch-sensor controller  12 ) may induce a charge on the sense electrode, and the amount of charge induced may be susceptible to external influence (such as a touch or the proximity of an object). When an object touches or comes within proximity of the capacitive node, a change in capacitance may occur at the capacitive node and touch-sensor controller  12  may measure the change in capacitance. By measuring changes in capacitance throughout the array, touch-sensor controller  12  may determine the position of the touch or proximity within the touch-sensitive area(s) of touch sensor  10 . 
     In a self-capacitance implementation, touch sensor  10  may include an array of electrodes of a single type that may each form a capacitive node. When an object touches or comes within proximity of the capacitive node, a change in self-capacitance may occur at the capacitive node and touch-sensor controller  12  may measure the change in capacitance, for example, as a change in the amount of charge needed to raise the voltage at the capacitive node by a pre-determined amount. As with a mutual-capacitance implementation, by measuring changes in capacitance throughout the array, touch-sensor controller  12  may determine the position of the touch or proximity within the touch-sensitive area(s) of touch sensor  10 . This disclosure contemplates any suitable form of capacitive touch sensing, where appropriate. 
     In particular embodiments, one or more drive electrodes may together form a drive line running horizontally or vertically or in any suitable orientation. Similarly, one or more sense electrodes may together form a sense line running horizontally or vertically or in any suitable orientation. In particular embodiments, drive lines may run substantially perpendicular to sense lines. Herein, reference to a drive line may encompass one or more drive electrodes making up the drive line, and vice versa, where appropriate. Similarly, reference to a sense line may encompass one or more sense electrodes making up the sense line, and vice versa, where appropriate. 
     Touch sensor  10  may have drive and sense electrodes disposed in a pattern on one side of a single substrate. In such a configuration, a pair of drive and sense electrodes capacitively coupled to each other across a space between them may form a capacitive node. For a self-capacitance implementation, electrodes of only a single type may be disposed in a pattern on a single substrate. In addition or as an alternative to having drive and sense electrodes disposed in a pattern on one side of a single substrate, touch sensor  10  may have drive electrodes disposed in a pattern on one side of a substrate and sense electrodes disposed in a pattern on another side of the substrate. Moreover, touch sensor  10  may have drive electrodes disposed in a pattern on one side of one substrate and sense electrodes disposed in a pattern on one side of another substrate. In such configurations, an intersection of a drive electrode and a sense electrode may form a capacitive node. Such an intersection may be a location where the drive electrode and the sense electrode “cross” or come nearest each other in their respective planes. The drive and sense electrodes do not make electrical contact with each other—instead they are capacitively coupled to each other across a dielectric at the intersection. Although this disclosure describes particular configurations of particular electrodes forming particular nodes, this disclosure contemplates any suitable configuration of any suitable electrodes forming any suitable nodes. Moreover, this disclosure contemplates any suitable electrodes disposed on any suitable number of any suitable substrates in any suitable patterns. 
     As described above, a change in capacitance at a capacitive node of touch sensor  10  may indicate a touch or proximity input at the position of the capacitive node. Touch-sensor controller  12  may detect and process the change in capacitance to determine the presence and location of the touch or proximity input. Touch-sensor controller  12  may then communicate information about the touch or proximity input to one or more other components (such one or more central processing units (CPUs)) of a device that includes touch sensor  10  and touch-sensor controller  12 , which may respond to the touch or proximity input by initiating a function of the device (or an application running on the device). Although this disclosure describes a particular touch-sensor controller having particular functionality with respect to a particular device and a particular touch sensor, this disclosure contemplates any suitable touch-sensor controller having any suitable functionality with respect to any suitable device and any suitable touch sensor. 
     Touch-sensor controller  12  may be one or more integrated circuits (ICs), such as for example general-purpose microprocessors, microcontrollers, programmable logic devices or arrays, application-specific ICs (ASICs). In particular embodiments, touch-sensor controller  12  comprises analog circuitry, digital logic, and digital non-volatile memory. For example, controller  12  may include a computer-readable storage medium storing logic that when executed by a processor is operable to perform one or more functions of controller  12  described herein. In particular embodiments, touch-sensor controller  12  is disposed on a flexible printed circuit (FPC) bonded to the substrate of touch sensor  10 , as described below. The FPC may be active or passive, where appropriate. In particular embodiments, multiple touch-sensor controllers  12  are disposed on the FPC. Touch-sensor controller  12  may include a processor unit, a drive unit, a sense unit, and a storage unit. The drive unit may supply drive signals to the drive electrodes of touch sensor  10 . The sense unit may sense charge at the capacitive nodes of touch sensor  10  and provide measurement signals to the processor unit representing capacitances at the capacitive nodes. The processor unit may control the supply of drive signals to the drive electrodes by the drive unit and process measurement signals from the sense unit to detect and process the presence and location of a touch or proximity input within the touch-sensitive area(s) of touch sensor  10 . The processor unit may also track changes in the position of a touch or proximity input within the touch-sensitive area(s) of touch sensor  10 . The storage unit may store programming for execution by the processor unit, including programming for controlling the drive unit to supply drive signals to the drive electrodes, programming for processing measurement signals from the sense unit, and other suitable programming, where appropriate. Although this disclosure describes a particular touch-sensor controller having a particular implementation with particular components, this disclosure contemplates any suitable touch-sensor controller having any suitable implementation with any suitable components. 
     Tracks  14  of conductive material disposed on the substrate of touch sensor  10  may couple the drive or sense electrodes of touch sensor  10  to connection pads  16 , also disposed on the substrate of touch sensor  10 . As described below, connection pads  16  facilitate coupling of tracks  14  to touch-sensor controller  12 . Tracks  14  may extend into or around (e.g. at the edges of) the touch-sensitive area(s) of touch sensor  10 . Particular tracks  14  may provide drive connections for coupling touch-sensor controller  12  to drive electrodes of touch sensor  10 , through which the drive unit of touch-sensor controller  12  may supply drive signals to the drive electrodes. Other tracks  14  may provide sense connections for coupling touch-sensor controller  12  to sense electrodes of touch sensor  10 , through which the sense unit of touch-sensor controller  12  may sense charge at the capacitive nodes of touch sensor  10 . Tracks  14  may be made of fine lines of metal or other conductive material. As an example and not by way of limitation, the conductive material of tracks  14  may be copper or copper-based and have a width of approximately 100 μm or less. As another example, the conductive material of tracks  14  may be silver or silver-based and have a width of approximately 100 μm or less. In particular embodiments, tracks  14  may be made of ITO in whole or in part in addition or as an alternative to fine lines of metal or other conductive material. Although this disclosure describes particular tracks made of particular materials with particular widths, this disclosure contemplates any suitable tracks made of any suitable materials with any suitable widths. In addition to tracks  14 , touch sensor  10  may include one or more ground lines terminating at a ground connector (which may be a connection pad  16 ) at an edge of the substrate of touch sensor  10  (similar to tracks  14 ). 
     Connection pads  16  may be located along one or more edges of the substrate, outside the touch-sensitive area(s) of touch sensor  10 . As described above, touch-sensor controller  12  may be on an FPC. Connection pads  16  may be made of the same material as tracks  14  and may be bonded to the FPC using an anisotropic conductive film (ACF). Connection  18  may include conductive lines on the FPC coupling touch-sensor controller  12  to connection pads  16 , in turn coupling touch-sensor controller  12  to tracks  14  and to the drive or sense electrodes of touch sensor  10 . In another embodiment, connection pads  16  may be connected to an electro-mechanical connector (such as a zero insertion force wire-to-board connector); in this embodiment, connection  18  may not need to include an FPC. This disclosure contemplates any suitable connection  18  between touch-sensor controller  12  and touch sensor  10 . 
       FIG. 2  illustrates an example system  200  for compensating for charge present at a capacitive node Cx,y of a mutual-capacitance implementation of touch sensor  10  of  FIG. 1 . In the example of  FIG. 2 , system  200  includes a controller  12   a  and a touch sensor  10   a . Controller  12   a  and touch sensor  10   a  may respectively include any suitable characteristics of controller  12  and touch sensor  10  described above. In the embodiment depicted, controller  12   a  includes a compensation capacitor Ccomp, multiplexer  208 , an integration circuit  210 , an analog-to-digital converter (ADC)  212  or any other voltage-level detector, a driver  214 , an inverter  215 , and a switch  224 . Integration circuit  210  includes an operational amplifier  216 , multiplexer  218 , switch system  220 , and an integration capacitor Cint. In the embodiment depicted, Cint is coupled to a negative input terminal and the output terminal of operational amplifier  216  in parallel with switch system  220 . A positive input terminal of operational amplifier  216  is coupled to a voltage, e.g., half of a reference voltage Vref. The parallel combination of integration capacitor Cint and switch system  220  forms a feedback loop with operational amplifier  216 . Controller  12   a  may be coupled to touch sensor  10   a  through one or more drive lines x and one or more sense lines y. 
     In particular embodiments, the Ccomp and integration circuit  210  may operate to allow controller  12   a  to compensate for a base charge present at Cx,y. The base charge is an amount of charge present at Cx,y in the absence of a touch with respect to Cx,y (e.g., when object  222  is not located near Cx,y). The base charge may be induced by a drive signal sent to Cx,y by controller  12   a . When an object  222  is located in proximity to Cx,y, the object may cause a change in the effective capacitance of Cx,y by absorbing charge from Cx,y. In typical situations, the change in effective capacitance is small, e.g., about 10 percent. Accordingly, in such a situation, the charge present at Cx,y when a drive signal is sent to Cx,y may be roughly 90 percent of the base charge. The difference between the base charge of Cx,y and the actual charge present at Cx,y during measurement is referred to herein as a delta charge. In particular embodiments, a sufficient magnitude of the delta charge may indicate a touch or proximity input with respect to Cx,y. 
     In typical touch sensors, the charge present at Cx,y is analyzed to determine whether a touch has occurred with respect to Cx,y. In a noisy environment, the charge may need to be sampled many times for many successive drive signals in order to reliably detect a touch, because the difference between the base charge of Cx,y and the charge present during a touch is small. In various embodiments of the present disclosure, Ccomp and circuit  210  operate to remove the base charge from the measurement, such that only the delta charge is sampled and analyzed to determine whether a touch has occurred. Accordingly, if no touch is present, then a charge that is substantially zero will be collected by integration circuit  210 . However, if a touch is present, then a charge based on the delta charge is collected by integration circuit  210  for measurement. For example, the charge collected by integration circuit  210  may be the delta charge itself or an amplified version of the delta charge. In particular embodiments, amplification of the delta charge may reduce the number of measurement samples needed to reliably detect a touch at a capacitive node Cx,y. While amplification may also be utilized in situations where the entire charge at Cx,y is measured, saturation of the operational amplifier will occur much sooner, having negative effects on the accuracy of the measurements. Compensation for the base charge of Cx,y allows a touch to be detected by measuring less samples. Accordingly, various embodiments of the present disclosure may provide technical advantages such as one or more of a reduction in power consumption, an increase in measurement speed, and an increase in the accuracy of touch detection. System  200  and other related embodiments may also have the advantage of being largely immune to temperature drift such that the base charge may be accurately compensated for over a broad range of operating temperatures. 
     In particular embodiments, touch sensor  10   a  is a mutual-capacitance touch sensor that includes an array of drive electrodes and sense electrodes coupled to one of corresponding drive lines x and sense lines y respectively. Each intersection of a drive electrode and sense electrode forms a capacitive node Cx,y. Controller  12   a  may detect and process the change in capacitance to determine the presence and location of a touch or proximity input. Controller  12   a  may be one or more integrated circuits (ICs), such as for example general-purpose microprocessors, microcontrollers, programmable logic devices or arrays, application-specific ICs (ASICs). Although this disclosure describes and illustrates a particular controller in system  200 , this disclosure contemplates any suitable controller in system  200 . 
     Driver  214  transmits a drive signal to one or more drive electrodes through drive lines x. The drive signal may include one or more voltage transitions, such as a transition from a high voltage to a low voltage or vice versa. In particular embodiments, the high voltage is a supply voltage having a magnitude Vref that is also used as a supply voltage for any of operational amplifier  216 , inverter  215 , or ADC  212 , and the low voltage is a ground of controller  12   a . In other embodiments, any suitable high and low voltages may be used. 
     The drive signal may induce charge on the associated sense electrode through capacitive node Cx,y. As an example and not by way of limitation, driver  214  may be implemented as an inverter with p-type metal-oxide semiconductor (PMOS) transistor px and n-type metal-oxide semiconductor (NMOS) transistor nx. Driver  214  may also be realized through other circuits, such as an analog buffer providing predetermined voltage levels. Interaction between an object  222  and touch sensor  10   a  may affect an amount of charge induced on one or more sense electrodes. The induced charge is sensed as a change in capacitance by controller  12   a.    
     Driver  214  may also transmit the drive signal to inverter  215 . In particular embodiments, driver  214  simultaneously transmits the drive signal to the one or more drive electrodes and to inverter  215 . Inverter  215  receives and inverts the drive signal. For example, the inverter  215  may receive a low voltage and output a high voltage or receive a high voltage and output a low voltage. 
     The inverted drive signal is applied to compensation capacitor Ccomp. The inverted drive signal may induce charge on one or more of the electrodes of capacitor Ccomp. In particular embodiments, the capacitance of Ccomp is substantially equal to the capacitance of Cx,y. In such embodiments, the application of the inverted drive signal to Ccomp may be operable to induce a charge stored by Ccomp that is substantially equal in magnitude to the charge induced by the drive signal and stored at Cx,y in the absence of a touch. However, the charge stored by Ccomp has a polarity that is the opposite to the polarity of the charge stored by Cx,y. Accordingly, when the charges stored by Cx,y and Ccomp are released to a summing junction (e.g., the node coupling multiplexer  218  to switch  224 ), the charge present at Ccomp may cancel out the base charge present at Cx,y (e.g., the charge present at Cx,y in the absence of a touch). Accordingly, only the uncancelled charge is presented to integration circuit  210  for analysis of whether a touch has occurred. The uncancelled charge is the delta charge referred to above, that is, the difference between the base charge of Cx,y and the actual charge present at Cx,y. 
     A single capacitor system Ccomp and a single integration circuit  210  may be used to compensate charge for one or more capacitive nodes Cx,y and measure the delta charges stored by the capacitive nodes Cx,y. In particular embodiments, multiplexer  218  of integration circuit  210  selects one of sense lines y coupled to multiplexer  218 . Multiplexer  218  provides a path between the sense line y and integration circuit  210  such that charge from capacitive node Cx,y or capacitor system Ccomp may accumulate in circuit  210  for measurement. Multiplexer  218  then selects a different sense line y for measurement. In particular embodiments, multiplexer  218  selects each sense line y in accordance with a predetermined sequence. 
     In particular embodiments, Ccomp may be a tunable capacitor system comprising one or more capacitors. At any suitable time, such as during production, the value of the capacitance of Ccomp may be tuned to match the capacitance of one or more capacitive nodes Cx,y. In particular embodiments, the values of capacitive nodes Cx,y that are close together generally have the same capacitance. Accordingly, Ccomp may be tuned to this capacitance and used to compensate charge for multiple different capacitive nodes Cx,y. In some embodiments, Ccomp may be used to compensate charge for distinct capacitive nodes Cx,y that each have different capacitances. For example, a first code may set Ccomp to a capacitance of a first capacitive node Cx,y while the first capacitive node is measured. A second code may set Ccomp to the capacitance of a second capacitive node Cx,y while the second capacitive node is measured. Such embodiments may be useful for touch sensors that include an array of capacitive nodes (in which each capacitive node generally has the same capacitance) and one or more other capacitive nodes such as capacitive buttons, wheels, or sliders (which may have capacitances different from the capacitive nodes of the array). Any suitable number of codes with any suitable resolution may be used to tune the capacitance of Ccomp. In particular embodiments, the charge compensation techniques described herein may be selectively enabled. For example, in particular embodiments, the compensation of the base charge may be turned off by opening switch  224  and turned on by closing switch  224 . 
     In particular embodiments, the same capacitor system Ccomp and integration circuit  210  may be used to compensate charge for both mutual-capacitance nodes and self-capacitance nodes and measure the delta charges present at the capacitive nodes. For example, multiplexer  218  may be coupled to one or more sense lines y that are each coupled to a sense electrode of a capacitive node and one or more lines that are each coupled to an electrode of a node that utilizes self-capacitance (e.g., Cs of  FIG. 3 ). In particular embodiments, the measurement sequences used to measure the delta charge of a capacitive node Cx,y utilizing mutual-capacitance and the delta charge of a capacitive node Cs utilizing self-capacitance may be different. Example sequences are described herein in connection with  FIG. 4 . 
     Integration circuit  210  may operate in multiple different modes. For example, in a first mode, switch system  220  may implement a connection between the output terminal of operational amplifier  216  and the negative input terminal of the operation amplifier, thereby bypassing integration capacitor Cint. Bypassing integration capacitor system Cint turns operational amplifier  216  into a unity gain amplifier, which drives the voltage at a negative input terminal of the operational amplifier  216 , as well as the selected input of multiplexer  218  (e.g., sense line y) to the voltage of a positive terminal of the operational amplifier  216 . In the embodiment depicted this voltage is equal to have half of the reference voltage (Vref/2). Closing the feedback loop of operational amplifier  216  also removes charge stored by the integration capacitor system Cint. 
     As another example, in a second mode, switch system  220  coupled to the feedback loop of operational amplifier  216  is opened, forming an integrator using integration capacitor Cint and operational amplifier  216 . Opening switch system  220  couples integration capacitor Cint between the negative input terminal and the output terminal of the integrator. The integrator generates a voltage that is a function of the amount of charge transferred from the selected one of sense lines y and capacitor system Ccomp. In particular embodiments, the voltage based on the amount of charge may be transmitted to ADC  212  for conversion to a digital representation of the voltage that may be processed to determine whether a touch has occurred at Cx,y. 
     As yet another example, in a third mode, switch system  220  is opened in a manner that reverses the polarity of the integration capacitor Cint with respect to the polarity of Cint in the second mode. For example, if a first electrode of Cint is coupled to the output of operational amplifier  216  and a second electrode of Cint is coupled to the negative input terminal of the operational amplifier in the second mode, the first electrode of Cint is coupled to the negative input terminal of the operational amplifier and the second electrode of Cint is coupled to the output terminal of the operational amplifier in the third mode. 
     In particular embodiments, circuit  210  enters the second mode of operation before the drive signal is transitioned (e.g., from a high voltage to a low voltage). The drive signal is transitioned and the circuit  210  integrates the sum of the resulting charges present at Cx,y and Ccomp and stores the charge at Cint. Circuit  210  then enters the third mode of operation before the drive signal is transitioned back to its original value (e.g., from a low voltage to a high voltage). Circuit  210  again integrates the sum of the resulting charges present at Cx,y and Ccomp adds this to the charge already stored by Cint (reversing the polarity of Cint before integrating the charge induced by the second transition prevents the new charge from cancelling the charge already present at Cint). In particular embodiments, the voltage based on the amount of charge stored by Cint due to both transitions of the drive signal may be transmitted to ADC  212  for conversion to a digital representation of the voltage. Embodiments that involve integrating the charge induced by the drive signal on both transitions (i.e., from high to low and from low to high) may reduce or eliminate common mode interferences. 
     Cint may include one or more capacitors having any suitable capacitance. In particular embodiments, Cint may have substantially the same capacitance as Cx,y. In other embodiments, Cint may have a smaller capacitance than Cx,y such that the gain of operational amplifier  216  is greater than one. In such embodiments, the amplification of the delta charge may lead to more accurate measurements and power savings because less measurements are required to determine whether a touch has occurred with respect to Cx,y. In particular embodiments, the capacitance of Cint is dynamically adjustable to provide high dynamic range for the output voltage of integration circuit  210 . For example, one or more digital codes may be sent to capacitor system Cint to configure its capacitance. 
     Multiplexer  208  is operable to select an input and couple the input to ADC  212 . For example, in embodiments that include multiple integration circuits  210 , multiplexer  208  may select a particular integration circuit  210  for coupling to the ADC. The multiplexer  208  may iterate between the integration circuits  210  such that a voltage from each integration circuit  210  may be converted into a digital value by ADC  212 . 
       FIG. 3  illustrates an example system  300  for compensating for charge present at a capacitive node Cs of a self-capacitance implementation of the touch sensor of  FIG. 1 . Accordingly, capacitive node Cs of system  300  is shown with a single electrode as opposed to capacitive node Cx,y of system  200  that includes two electrodes. In the example of  FIG. 3 , system  300  includes a controller  12   b  and a touch sensor  10   b . Controller  12   b  includes a capacitor system Ccomp, multiplexer  308 , integration circuit  310 , ADC  312  or any other voltage-level detector, a driver  314 , an inverter  315 , and a switch system  324 . Integration circuit  310  includes an operational amplifier  316 , multiplexer  318 , switch system  320 , and an integration capacitor Cint. In the embodiment depicted, Cint is coupled to a negative input terminal and the output terminal of operational amplifier  316  in parallel with switch system  320 . A positive input terminal of operational amplifier  316  is coupled to a voltage, e.g., half of a reference voltage Vref. The parallel combination of integration capacitor Cint and switch system  320  forms a feedback loop with operational amplifier  316 . Controller  12   b  may be coupled to touch sensor  10   b  through one or more lines z. Each component of  FIG. 3  may have any suitable characteristics of the corresponding component of  FIG. 2 . For example, touch sensor  10   b , driver  314 , inverter  315 , Ccomp, switch system  324 , integration circuit  310 , multiplexer  308 , and ADC  312  may have any suitable characteristics described above with respect to touch sensor  10   a , driver  214 , inverter  215 , Ccomp, switch system  224 , integration circuit  210 , multiplexer  208 , and ADC  212  respectively. 
     Self-capacitance touch sensor  10   b  includes one or more electrodes coupled to an associated line z that acts as a drive line and a sense line. Self-capacitance touch sensor  110   b  detects a presence of an object  322  through an interaction between the object  322  and an electric field generated by one or more electrodes of self-capacitance touch sensor  110   b . Self-capacitance touch sensor  10   b  may include one or more capacitive node Cs that each include a single electrode. The self-capacitance implementation depicted by system  300  may operate in a manner that is similar to the operation of system  200 . That is, the base charge present at Cs induced by a drive signal in the absence of a touch is compensated for by compensation capacitor Ccomp. Accordingly, only the delta charge (e.g., the difference between the base charge and the actual charge at Cs) or an amount of charge derived from the delta charge (e.g., an amplified amount of the delta charge) is integrated by integration circuit  310  and stored in integration capacitor Cint. In various embodiments, Ccomp may be sized to cancel out charge induced by parasitic capacitance Cpx in addition to the base charge stored by Cs. In particular embodiments, a voltage based on the amount of charge stored by Cint may be transmitted to ADC  312  for conversion to a digital representation of the voltage. An example set of operations of system  300  is described in more detail in connection with  FIG. 4 . 
     Although  FIGS. 1 ,  2 , and  3  have been described above as including particular components, the systems of  FIGS. 1 ,  2 ,  3  may include any combination of any of the described components and any of the options or features described herein, as would be understood by one of ordinary skill in the art. For example, any of the options or features described herein may be utilized in combination with the illustrated embodiments of  FIG. 1 ,  2 , or  3  or any number of the other options or features also described herein as would be understood by one of ordinary skill in the art. 
       FIG. 4  illustrates an example method for compensating for charge present at a capacitive node of a touch sensor. The method is described with reference to system  200  of  FIG. 2 , though various steps of the method may be used with alternative embodiments. The method begins at step  402 , where a first voltage is applied to a first electrode of a capacitive node. For example, driver  214  may apply Vref to a drive electrode of Cx,y. At step  404 , a second voltage is applied to a first electrode of a compensation capacitor. For example, driver  214  may invert Vref to ground and apply the inverted voltage to the top electrode of Ccomp. At step  406 , a third voltage is applied to a second electrode of the capacitive node. As an example, operational amplifier  216  may be placed in unity gain mode by closing switch system  220 . Accordingly, the output terminal of the operational amplifier is set to Vref/2. This voltage is applied to the sense electrode of Cx,y through multiplexer  218 . By maintaining the voltage of the selected one of sense lines y at substantially Vref/2 during the beginning of the charge transfer and the end of the charge transfer, the effect of parasitic capacitances Cpy on the sense lines are cancelled. In particular embodiments, this voltage may also be applied to the bottom electrode of Ccomp and may cancel out parasitic capacitances Cpx on the drive line. 
     In various embodiments, two or more of steps  402 ,  404 , and  406  may be performed simultaneously. Thus, system  200  may be in a state where the drive electrode of Cx,y is set to Vref, the top electrode of Ccomp is set to ground, and the sense electrode of Cx,y is set to Vref/2. At step  408 , an integrator is cleared. For example, the charge of an integration capacitor of the integrator may be cleared. As an example, Cint may be cleared of charge by closing switch system  220 . In particular embodiments, step  408  may be performed simultaneously with step  406 . For example, steps  406  and  408  may each be performed by closing switch system  220 . In various embodiments, steps  406  and  408  are performed before steps  402  and  404 . 
     At step  410 , the second voltage is applied to the first electrode of the capacitive node. For example, the drive signal supplied by driver  214  may transition from from Vref to ground and the drive signal may be applied to the drive electrode of Cx,y. At step  412 , the first voltage is applied to the first electrode of the compensation capacitor. For example, the grounded drive signal from driver  214  may be inverted to Vref by inverter  215  and Vref may be applied to the top electrode of Ccomp. In particular embodiments, steps  410  and  412  may be performed simultaneously. 
     At step  414 , uncancelled charge may be integrated. For example, switch system  220  may close, allowing integration circuit  210  to integrate charge received from Cx,y and Ccomp. In particular embodiments, at least a portion of charge received from Cx,y by integration circuit  210  is canceled by charge received from Ccomp. As described earlier, Ccomp may be configured to store an amount of charge that is roughly equal (and of opposite polarity) to the amount of charge stored by Cx,y in the absence of a touch. Accordingly, when there is no touch present with respect to Cx,y, the charges received from Cx,y and Ccomp may cancel each other out and substantially no charge is stored by Cint. In the case of a touch, Ccomp may store a charge that is different from the charge stored by Cx,y due to effects caused by the touch. For example, a touch by a passive object such as a finger may decrease the amount of charge stored by Cx,y. As another example, a touch by an active stylus may increase or decrease the amount of charge stored by Cx,y. In a situation in which the magnitude of the charge stored by Ccomp is greater than the magnitude of charge stored by Cx,y, after all the charge from Cx,y is canceled by the opposite charge from Ccomp, the remainder of the charge from Ccomp is integrated and stored by Cint. In a situation in which the magnitude of the charge stored by Ccomp is less than the magnitude of charge stored by Cx,y, after all the charge from Ccomp is canceled by the opposite charge from Cx,y, the remainder of the charge from Cx,y is integrated and stored by Cint. In general, the integrated charge is equal to or derived from (e.g. amplification may be performed by integration circuit  210 ) the amount of the change of charge at Cx,y due to the touch (e.g., the delta charge). 
     At step  416 , the polarity of the integrator capacitor is reversed. For example, switch system  220  may open or close one or more switches such that the polarity of integrator capacitor Cint is reversed with respect to the output terminal and negative input terminal of operational amplifier  216 . At step  418 , the first voltage is again applied to the first electrode of the capacitive node. For example, driver  214  may apply Vref to the drive electrode of Cx,y. At step  420 , the second voltage is applied to the first electrode of the compensation capacitor. For example, driver  214  may invert Vref to ground and apply the inverted voltage to the top electrode of Ccomp. In various embodiments, steps  418  and  420  may be performed simultaneously. 
     In response to the application of these voltages to Cx,y and Ccomp, the charge received by integration circuit  210  from Cx,y and Ccomp may again be combined and the uncancelled charge is integrated and added to the charge already stored by Cint. At step  422 , the output of the integration circuit is converted to a digital value. For example, integration circuit  210  may output a voltage based on the charge stored by Cint. This voltage may be sent to ADC  212  and converted to a digital value. The digital value may be further processed to determine whether a touch has occurred at Cx,y. 
     Particular embodiments may repeat one or more steps of the method of  FIG. 4 , where appropriate. For example, the steps of the method may be performed multiple times to obtain multiple digital values that are processed together to determine whether a touch has occurred at Cx,y. Although this disclosure describes and illustrates particular steps of the method of  FIG. 4  as occurring in a particular order, this disclosure contemplates any suitable steps of the method of  FIG. 4  occurring in any suitable order. Moreover, although this disclosure describes and illustrates particular components, devices, or systems carrying out particular steps of the method of  FIG. 4 , this disclosure contemplates any suitable combination of any suitable components, devices, or systems carrying out any suitable steps of the method of  FIG. 4 . 
     A compensation method for a self-capacitance implementation may also include any one or more of the steps of the method of  FIG. 4 . In particular embodiments, a compensation method for a self-capacitance implementation may involve a process that is different from the process performed for a mutual-capacitance implementation. For example, in an example process for compensating for base charge in a self-capacitance implementation that is described with reference to  FIG. 3 , a first voltage (e.g., Vref) may be applied to the electrode of capacitive node Cs by driver  314 . At the same time, Ccomp and Cint may be discharged. For example, the electrodes of Ccomp may be shorted together and the electrodes of Cint may be shorted together to release charges stored therein. Driver  314  may then be disconnected from Cs and the top electrode of Ccomp is connected to a second voltage (e.g., ground) while the bottom plate is connected to Cs through multiplexer  318 . This results in the charge being balanced between Cs and Ccomp. The bottom plate of Ccomp is then disconnected from Cs while the top plate remains connected to ground. The charge from Ccomp is then integrated by integration circuit  310  and stored in Cint. This charge may result in an output voltage that may be converted into a digital value by ADC  312 . In other embodiments, this charge remains stored in Cint and further steps are performed to effect a differential measurement. For example, Cs may then be driven to the second voltage (e.g., ground) by driver  314  while Ccomp is discharged. Driver  314  is then disconnected from Cs and the top electrode of Ccomp is connected to the first voltage (e.g., Vref) while the bottom electrode is connected to Cs through multiplexer  318 . The charge is again balanced between Cs and Ccomp. The bottom electrode of Ccomp may then be disconnected from Cs while the top electrode remains connected to Vref. The bottom electrode of Ccomp is connected to integration circuit  310  and the charge stored by Ccomp is integrated by integration circuit  310  and added to the charge already stored by Cint. The resulting voltage output of integration circuit  310  may be converted into a digital value by ADC  312 . 
     Herein, reference to a computer-readable storage medium encompasses one or more non-transitory, tangible computer-readable storage media possessing structure. As an example and not by way of limitation, a computer-readable storage medium may include a semiconductor-based or other IC (such, as for example, a field-programmable gate array (FPGA) or an ASIC), a hard disk, an HDD, a hybrid hard drive (HHD), an optical disc, an optical disc drive (ODD), a magneto-optical disc, a magneto-optical drive, a floppy disk, a floppy disk drive (FDD), magnetic tape, a holographic storage medium, a solid-state drive (SSD), a RAM-drive, a SECURE DIGITAL card, a SECURE DIGITAL drive, or another suitable computer-readable storage medium or a combination of two or more of these, where appropriate. A computer-readable non-transitory storage medium may be volatile, non-volatile, or a combination of volatile and non-volatile, where appropriate. 
     Herein, “or” is inclusive and not exclusive, unless expressly indicated otherwise or indicated otherwise by context. Therefore, herein, “A or B” means “A, B, or both,” unless expressly indicated otherwise or indicated otherwise by context. Moreover, “and” is both joint and several, unless expressly indicated otherwise or indicated otherwise by context. Therefore, herein, “A and B” means “A and B, jointly or severally,” unless expressly indicated otherwise or indicated otherwise by context. 
     This disclosure encompasses all changes, substitutions, variations, alterations, and modifications to the example embodiments herein that a person having ordinary skill in the art would comprehend. Similarly, where appropriate, the appended claims encompass all changes, substitutions, variations, alterations, and modifications to the example embodiments herein that a person having ordinary skill in the art would comprehend. Moreover, reference in the appended claims to an apparatus or system or a component of an apparatus or system being adapted to, arranged to, capable of, configured to, enabled to, operable to, or operative to perform a particular function encompasses that apparatus, system, component, whether or not it or that particular function is activated, turned on, or unlocked, as long as that apparatus, system, or component is so adapted, arranged, capable, configured, enabled, operable, or operative.