Patent Publication Number: US-9891763-B2

Title: Current feedback techniques for capacitive sensing

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     Embodiments of the present invention generally relate to a current feedback technique for capacitive sensing with an input device. 
     Description of the Related Art 
     Input devices including proximity sensor devices (also commonly called touchpads or touch sensor devices) are widely used in a variety of electronic systems. A proximity sensor device typically includes a sensing region, often demarked by a surface, in which the proximity sensor device determines the presence, location, and/or motion of one or more input objects. Proximity sensor devices may be used to provide interfaces for the electronic system. For example, proximity sensor devices are often used as input devices for larger computing systems, such as touchpads integrated in, or peripheral to, notebook or desktop computers. Proximity sensor devices are also often used in smaller computing systems, such as touch screens integrated in cellular phones or tablet computers. 
     Proximity sensor devices may include one or more types of electrodes that are configured for transmitting and/or receiving input sensing signals. The electrodes may be coupled to a processing system that determines various characteristics of acquired input sensing signals in order to detect the presence and/or location of an input object. Processing input sensing signals received via the electrodes generally requires large components, such as a number of large capacitors and/or resistors. For example, receiver components may be coupled to each individual electrode (or to each set of electrodes), requiring tens or hundreds of relatively large capacitors and/or resistors to be included in the proximity sensor device. In addition, as the feature size of proximity sensor devices is decreased, the number of receiver components generally must be increased, further increasing the size of the proximity sensor device. 
     Accordingly, there is a need for reducing the size of components used to perform input sensing in proximity sensor devices. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention generally provide a processing system for an input device. The processing system includes an operational amplifier having a non-inverting input, an inverting input, and an output. A voltage source is coupled to the non-inverting input. The processing system further includes a first voltage-controlled current source, a second voltage-controlled current source, and a determination module. The first voltage-controlled current source is coupled between the output and the inverting input to form a feedback path. The second voltage-controlled current source is coupled to the output and configured for modifying a charge on a capacitor based on a control voltage at the output. The determination module is coupled to the capacitor and configured for determining a capacitance measurement at the inverting input based on the charge on the capacitor. 
     Embodiments of the present invention may further provide an input device for capacitive sensing. The input device includes a plurality of sensor electrodes and a processing system. The processing system is coupled to the plurality of sensor electrodes. The processing system includes an operational amplifier having a non-inverting input, an inverting input, and an output. A voltage source is coupled to the non-inverting input. The processing system further includes a first voltage-controlled current source, a second voltage-controlled current source, and a determination module. The first voltage-controlled current source is coupled between the output and the inverting input to form a feedback path. The second voltage-controlled current source is coupled to the output and configured to modify a charge on a capacitor based on a control voltage at the output. The determination module is coupled to the capacitor and configured to determine a capacitance measurement associated with a sensor electrode included in the plurality of sensor electrodes and coupled to the inverting input based on the charge on the capacitor. The determination module is further configured to determine positional information based on the capacitance measurement. 
     Embodiments of the present invention may further provide a method of input sensing with a sensor electrode. The method includes driving an input voltage onto a non-inverting input of an operational amplifier. A first voltage-controlled current source is coupled between an output of the operational amplifier and an inverting input of the operational amplifier to form a feedback path. The method further includes modifying, via a second voltage-controlled current source coupled to the output, a charge on a capacitor based on a control voltage at the output. The method further includes determining a capacitance measurement associated with a sensor electrode coupled to the inverting input based on the charge on the capacitor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features can be understood in detail, a more particular description, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only embodiments of the invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1  is a block diagram of an exemplary input device in accordance with embodiments of the invention. 
         FIG. 2  is a partial schematic plan view of the input device of  FIG. 1  in accordance with embodiments of the invention. 
         FIG. 3  is a partial schematic view of a sensing circuit included in the processing system of  FIG. 1  in accordance with embodiments of the invention. 
         FIG. 4  is a partial schematic view in which the sensing circuit of  FIG. 3  is configured for transcapacitive sensing in accordance with embodiments of the invention. 
         FIG. 5  is a partial schematic view of a processing system that includes two sensing circuits configured to perform transcapacitive sensing and/or absolute capacitance sensing in accordance with embodiments of the invention. 
         FIG. 6  is a flow chart of a method for performing input sensing using the sensing circuit of  FIG. 3  in accordance with embodiments of the invention. 
         FIG. 7  is a flow chart of a method for performing input sensing using the sensing circuits of  FIG. 5  in accordance with embodiments of the invention. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one embodiment may be beneficially utilized on other embodiments without specific recitation. 
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
     Various embodiments of the present invention generally provide a method and system for transmitting, receiving, and/or processing input sensing signals using a voltage-controlled current source (VCCS). Using a VCCS to perform input sensing allows the size of a processing system to be reduced. For example, instead of using large capacitors and/or resistors in a processing system, a VCCS may be implemented in a flexible manner to transmit, receive, and/or process various types of input sensing signals. Accordingly, the overall size of the processing system may be reduced. 
     Turning now to the figures,  FIG. 1  is a block diagram of an exemplary input device  100 , in accordance with embodiments of the invention. The input device  100  may be configured to provide input to an electronic system (not shown). As used in this document, the term “electronic system” (or “electronic device”) broadly refers to any system capable of electronically processing information. Some non-limiting examples of electronic systems include personal computers of all sizes and shapes, such as desktop computers, laptop computers, netbook computers, tablets, web browsers, e-book readers, and personal digital assistants (PDAs). Additional examples of electronic systems include composite input devices, such as physical keyboards that include the input device  100  and separate joysticks or key switches. Further exemplary electronic systems include peripherals, such as data input devices (including remote controls and mice) and data output devices (including display screens and printers). Other examples include remote terminals, kiosks, and video game machines (e.g., video game consoles, portable gaming devices, and the like). Other examples include communication devices (including cellular phones, such as smart phones) and media devices (including recorders, editors, and players such as televisions, set-top boxes, music players, digital photo frames, and digital cameras). Additionally, the electronic system could be a host or a slave to the input device. 
     The input device  100  can be implemented as a physical part of the electronic system or can be physically separate from the electronic system. As appropriate, the input device  100  may communicate with parts of the electronic system using any one or more of the following: buses, networks, and other wired or wireless interconnections (including serial and/or parallel connections). Examples include I 2 C, SPI, PS/2, Universal Serial Bus (USB), Bluetooth, RF, and IRDA. 
     In the embodiment depicted in  FIG. 1 , the input device  100  is shown as a proximity sensor device (also often referred to as a “touchpad” or a “touch sensor device”) configured to sense input provided by one or more input objects  140  in a sensing region  120 . Examples of input objects  140  include fingers and styli, as shown in  FIG. 1 . 
     Sensing region  120  encompasses any space above, around, in and/or near the input device  100  in which the input device  100  is able to detect user input (e.g., user input provided by one or more input objects  140 ). The sizes, shapes, and locations of particular sensing regions may vary widely from embodiment to embodiment. In some embodiments, the sensing region  120  extends from a surface of the input device  100  in one or more directions into space until signal-to-noise ratios prevent sufficiently accurate object detection. The distance to which this sensing region  120  extends in a particular direction, in various embodiments, may be on the order of less than a millimeter, millimeters, centimeters, or more, and may vary significantly with the type of sensing technology used and the accuracy desired. Thus, some embodiments sense input that comprises no contact with any surfaces of the input device  100 , contact with an input surface (e.g., a touch surface) of the input device  100 , contact with an input surface of the input device  100  coupled with some amount of applied force or pressure, and/or a combination thereof. In various embodiments, input surfaces may be provided by surfaces of casings within which the sensor electrodes reside, by face sheets applied over the sensor electrodes or any casings, etc. In some embodiments, the sensing region  120  has a rectangular shape when projected onto an input surface of the input device  100 . The face sheet (e.g., an LCD lens) may provide a useful contact surface for an input object. 
     The input device  100  may utilize any combination of sensor components and sensing technologies to detect user input in the sensing region  120 . The input device  100  comprises one or more sensing elements for detecting user input. Some implementations are configured to provide images that span one, two, three, or higher dimensional spaces. Some implementations are configured to provide projections of input along particular axes or planes. Cursors, menus, lists, and items may be displayed as part of a graphical user interface and may be scaled, positioned, selected scrolled, or moved. 
     In some capacitive implementations of the input device  100 , voltage or current is applied to create an electric field. Nearby input objects cause changes in the electric field, and produce detectable changes in capacitive coupling that may be detected as changes in voltage, current, or the like. 
     Some capacitive implementations utilize arrays or other regular or irregular patterns of capacitive sensing elements to create electric fields. In some capacitive implementations, separate sensing elements may be ohmically shorted together to form larger sensor electrodes. Some capacitive implementations utilize resistive sheets, which may be uniformly resistive. 
     Some capacitive implementations utilize “self capacitance” (or “absolute capacitance”) sensing methods based on changes in the capacitive coupling between sensor electrodes and an input object. In various embodiments, an input object near the sensor electrodes alters the electric field near the sensor electrodes, changing the measured capacitive coupling. In some implementations, an absolute capacitance sensing method operates by modulating sensor electrodes with respect to a reference voltage (e.g. system ground) and by detecting the capacitive coupling between the sensor electrodes and input objects. 
     Some capacitive implementations utilize “mutual capacitance” (or “transcapacitance”) sensing methods based on changes in the capacitive coupling between sensor electrodes. In various embodiments, an input object near the sensor electrodes alters the electric field between the sensor electrodes, changing the measured capacitive coupling. In one implementation, a transcapacitive sensing method operates by detecting the capacitive coupling between one or more transmitter sensor electrodes (also “transmitter electrodes” or “transmitters”) and one or more receiver sensor electrodes (also “receiver electrodes” or “receivers”). Transmitter sensor electrodes may be modulated relative to a reference voltage (e.g., system ground) to transmit transmitter signals. Receiver sensor electrodes may be held substantially constant relative to the reference voltage to facilitate receipt of resulting signals. In other embodiments, receiver sensor electrodes may be modulated while resulting signals are received. A resulting signal may comprise effect(s) corresponding to one or more transmitter signals, and/or to one or more sources of environmental interference (e.g. other electromagnetic signals). Sensor electrodes may be dedicated transmitters or receivers, or may be configured to both transmit and receive. Additionally, sensor electrodes may be configured to perform absolute capacitance sensing and/or transcapacitance sensing during distinct time periods, or sensor electrodes may be configured to perform both absolute capacitance sensing and transcapacitance sensing simultaneously, during the same time period(s). 
     In  FIG. 1 , a processing system  110  is shown as part of the input device  100 . The processing system  110  is configured to operate the hardware of the input device  100  to detect input in the sensing region  120 . The processing system  110  comprises parts of or all of one or more integrated circuits (ICs) and/or other circuitry components. In some embodiments, the processing system  110  includes sensing circuitry that may include driver circuitry and/or receiver circuitry. For example, a processing system  110  for a mutual capacitance sensor device may comprise driver circuitry configured to drive input sensing signals onto the transmitter electrodes and/or receiver circuitry configured to receive signals with receiver electrodes. In other embodiments, the same sensing circuitry is used both to drive input sensing signals onto the sensing elements  150  and to receive resulting signals with the sensing elements  150 . Additionally, the processing system  110  may also include electronically-readable instructions, such as firmware code, software code, and/or the like. 
     In some embodiments, components included in the processing system  110  are located together, such as near sensing elements  150  of the input device  100 . In other embodiments, components of processing system  110  are physically separate with one or more components being located close to sensing elements  150  of the input device  100 , and one or more components being located elsewhere. For example, the input device  100  may be a peripheral coupled to a desktop computer, and the processing system  110  may comprise software configured to run on a central processing unit of the desktop computer and one or more ICs (perhaps with associated firmware) separate from the central processing unit. As another example, the input device  100  may be physically integrated in a phone, and the processing system  110  may comprise circuits and firmware that are part of a main processor of the phone. In some embodiments, the processing system  110  is dedicated to implementing the input device  100 . In other embodiments, the processing system  110  also performs other functions, such as operating display screens, driving haptic actuators, etc. 
     The processing system  110  may be implemented as a set of modules that handle different functions of the processing system  110 . Each module may comprise circuitry that is a part of the processing system  110 , firmware, software, or a combination thereof. In various embodiments, different combinations of modules may be used. Example modules include hardware operation modules for operating hardware such as sensor electrodes and display screens, data processing modules for processing data such as sensor signals and positional information, and reporting modules for reporting information. Further example modules include sensor operation modules configured to operate sensing elements  150  to detect input, identification modules configured to identify gestures such as mode changing gestures, and mode changing modules for changing operation modes. 
     In some embodiments, the processing system  110  responds to user input (or lack of user input) in the sensing region  120  directly by causing one or more actions to be performed. Example actions include changing operation modes, as well as GUI actions such as cursor movement, selection, menu navigation, and other functions. In some embodiments, the processing system  110  provides information about the input (or lack of input) to some part of the electronic system (e.g., to a central processing system of the electronic system that is separate from the processing system  110 , if such a separate central processing system exists). In some embodiments, some part of the electronic system processes information received from the processing system  110  to act on user input, such as to facilitate a full range of actions, including mode changing actions and GUI actions. 
     For example, in some embodiments, the processing system  110  operates the sensing elements  150  of the input device  100  to produce electrical signals indicative of input (or lack of input) in the sensing region  120 . The processing system  110  may perform any appropriate amount of processing on the electrical signals in producing the information provided to the electronic system. For example, the processing system  110  may digitize analog electrical signals obtained from the sensor electrodes. As another example, the processing system  110  may perform filtering or other signal conditioning. As yet another example, the processing system  110  may subtract or otherwise account for a baseline, such that the information reflects a difference between the electrical signals and the baseline. In additional examples, the processing system  110  may determine positional information, recognize inputs as commands, recognize handwriting, and the like. 
     “Positional information,” as the term is used herein, broadly encompasses absolute position, relative position, velocity, acceleration, and other types of spatial information. Exemplary “zero-dimensional” positional information includes near/far or contact/no contact information. Exemplary “one-dimensional” positional information includes positions along an axis. Exemplary “two-dimensional” positional information includes motions in a plane. Exemplary “three-dimensional” positional information includes instantaneous or average velocities in space. Further examples include other representations of spatial information. Historical data regarding one or more types of positional information may also be determined and/or stored, including, for example, historical data that tracks position, motion, or instantaneous velocity over time. 
     In some embodiments, the input device  100  is implemented with additional input components that are operated by the processing system  110  or by some other processing system. These additional input components may provide redundant functionality for input in the sensing region  120 , or some other functionality.  FIG. 1  shows buttons  130  near the sensing region  120  that can be used to facilitate selection of items using the input device  100 . Other types of additional input components include sliders, balls, wheels, switches, and the like. Conversely, in some embodiments, the input device  100  may be implemented with no other input components. 
     In some embodiments, the input device  100  comprises a touch screen interface, and the sensing region  120  overlaps at least part of an active area of a display screen. For example, the input device  100  may comprise substantially transparent sensor electrodes overlaying the display screen and provide a touch screen interface for the associated electronic system. The display screen may be any type of dynamic display capable of displaying a visual interface to a user, and may include any type of light emitting diode (LED), organic LED (OLED), cathode ray tube (CRT), liquid crystal display (LCD), plasma, electroluminescence (EL), or other display technology. The input device  100  and the display screen may share physical elements. For example, some embodiments may utilize some of the same electrical components for displaying and sensing. As another example, the display screen may be operated in part or in whole by the processing system  110 . 
     It should be understood that while many embodiments of the invention are described in the context of a fully functioning apparatus, the mechanisms of the present invention are capable of being distributed as a program product (e.g., software) in a variety of forms. For example, the mechanisms of the present invention may be implemented and distributed as a software program on information bearing media that are readable by electronic processors (e.g., non-transitory computer-readable and/or recordable/writable information bearing media readable by the processing system  110 ). Additionally, the embodiments of the present invention apply equally regardless of the particular type of medium used to carry out the distribution. Examples of non-transitory, electronically readable media include various discs, memory sticks, memory cards, memory modules, and the like. Electronically readable media may be based on flash, optical, magnetic, holographic, or any other storage technology. 
       FIG. 2  is a partial schematic plan view of the input device  100  of  FIG. 1  in accordance with embodiments of the invention. The input device  100  includes an array of sensing elements  150  and processing system  110 . The array of sensing elements  150  includes sensor electrodes, such as a plurality of transmitter electrodes  210  (e.g.,  210 - 1 ,  210 - 2 ,  210 - 3 , etc.) and a plurality of receiver electrodes  220  (e.g.,  220 - 1 ,  220 - 2 ,  220 - 3 , etc.). Although the transmitter electrodes  210  and receiver electrodes  220  are illustrated as being rectangular, in other embodiments, the transmitter electrodes  210  and receiver electrodes  220  may be any practical geometric shape. The processing system  110  is coupled to the array of sensing elements  150 , for example, through one or more routing traces (not shown in  FIG. 2 ). 
     The sensing elements  150  may be formed on a substrate that is external to the display device  160 . For example, the receiver electrodes  220  may be disposed on the outer surface of a lens of the input device  100 , between the color filter glass of the display device  160  and the lens of the input device  100 , or between a thin film transistor substrate (TFT substrate) and the color filter glass of the display device  160 . In other embodiments, the transmitter electrodes  210  and/or the receiver electrodes  220  include one or more common electrodes that are used to perform input sensing and to update the display device  160 . 
     Although the processing system  110  is illustrated in  FIG. 2  embodied as a single integrated circuit (IC) (e.g., an integrated controller), the processing system  110  may include any appropriate number of ICs. For example, the functions of the processing system  110  may be implemented in more than one IC to drive input sensing signals (e.g., via driver circuitry) and/or receive resulting signals (e.g., via receiver circuitry) with the array of sensing elements  150 . In some embodiments, the processing system  110  includes a determination module  250  that receives resulting signals (e.g., via receiver circuitry) and determines the presence of an input object  140  based on the resulting signals. For example, in some embodiments, the determination module  250  determines a capacitance measurement associated with one or more sensing elements  150  based on voltages, currents, charge, etc. received by receiver circuitry. Additionally, the processing system  110  may be configured to drive (e.g., via driver circuitry) a modulated signal onto the at least one sensing element  150  to detect changes in absolute capacitance between the at least one sensor electrode and an input object  140 . 
     In some embodiments, one or more ICs of the processing system  110  may be implemented to control display device  160  elements. For example, one IC may be configured to perform input sensing, and another IC may be configured to perform display updating. In some embodiments, one IC may be configured to operate the transmitter electrodes  210 , and another IC may be configured to operate the receiver electrodes  220 . In embodiments where there is more than one IC, communications between separate ICs of the processing system  110  may be achieved through a synchronization mechanism, which sequences the signals provided to the common electrodes. Alternatively the synchronization mechanism may be internal to any one of the ICs. 
     Transmitter electrodes  210  and receiver electrodes  220  are ohmically isolated from each other by one or more insulators which separate the transmitter electrodes  210  from the receiver electrodes  220  and prevent them from electrically shorting to each other. The electrically insulative material separates the transmitter electrodes  210  and the receiver electrodes  220  at cross-over areas at which the electrodes intersect. In one such configuration, the transmitter electrodes  210  and/or receiver electrodes  220  are formed with jumpers connecting different portions of the same electrode. In other configurations, the transmitter electrodes  210  and the receiver electrodes  220  are separated by one or more layers of electrically insulative material or by one or more substrates, as described in further detail below. In still other configurations, the transmitter electrodes  210  and the receiver electrodes  220  are optionally disposed on a single layer of the input device  100 . Accordingly, in such embodiments, no cross-over areas may exist between the transmitter electrodes  210  and the receiver electrodes  220 . 
     The areas of localized capacitive coupling between transmitter electrodes  210  and receiver electrodes  220  may be termed “capacitive pixels.” The capacitive coupling between the transmitter electrodes  210  and receiver electrodes  220  changes with the proximity and motion of input objects in the sensing region  120  associated with the transmitter electrodes  210  and the receiver electrodes  220 . In other embodiments, such as embodiments that include matrix sensors, the term “capacitive pixels” may refer to the localized capacitance (e.g., absolute capacitance) between a sensing element  150  and an input object  140 . 
     In some embodiments, the sensor pattern is “scanned” to determine these capacitive couplings. That is, the transmitter electrodes  210  are driven to transmit transmitter signals. Transmitters may be operated such that one transmitter electrode  210  transmits at one time, or multiple transmitter electrodes  210  transmit at the same time. Where multiple transmitter electrodes  210  transmit simultaneously, these multiple transmitter electrodes  210  may transmit the same transmitter signal and effectively produce an effectively larger transmitter electrode  210 , or these multiple transmitter electrodes  210  may transmit different transmitter signals. For example, multiple transmitter electrodes  210  may transmit different transmitter signals according to one or more coding schemes that enable their combined effects on the resulting signals of receiver electrodes  220  to be independently determined. Additionally, in embodiments that implement matrix sensing techniques, the sensing elements  150  may be scanned to sense changes to absolute capacitance on the electrodes. 
     The receiver electrodes  220  may be operated singly or multiply to acquire resulting signals. The resulting signals may be used to determine measurements of the capacitive couplings at the capacitive pixels. 
     A set of measurements from the capacitive pixels form a “capacitive image” (also “capacitive frame”) representative of the capacitive couplings at the pixels. Multiple capacitive images may be acquired over multiple time periods, and differences between them used to derive information about input in the sensing region  120 . For example, successive capacitive images acquired over successive periods of time can be used to track the motion(s) of one or more input objects  140  entering, exiting, and within the sensing region  120 . 
     As described above, the processing system  110  may include sensing circuitry that is coupled to the sensor electrodes. In some embodiments, separate sensing circuitry may be coupled to each sensor electrode. In other embodiments, the sensor electrodes may be grouped into electrode sets that include any number of sensor electrodes (e.g., 2, 4, 8, 16, 32, etc. sensor electrodes per electrode set), and separate sensing circuitry may be coupled to each electrode set. Consequently, the processing system  110  may include tens, hundreds, thousands, etc. of separate sensing circuits. 
     Typically, sensing circuits included in a processing system implement relatively large components, such as large capacitors and/or resistors. For example, in a processing system that includes 32 separate sensing circuits—each coupled to one or more sensor electrodes—each sensing circuit may include a capacitor that is approximately 30 picofarads (pF). As a result, the total surface area required by all of the sensing circuits may be quite large. 
     Accordingly, in order to reduce the size requirements of sensing circuitry (e.g., driver circuitry and/or receiver circuitry) included in the processing system  110 , one or more components within the sensing circuitry may be replaced with a voltage-controlled current source (VCCS). Replacing one or more sensing circuitry components with a VCCS may allow the surface area of the processing system  110  to be reduced while still enabling input sensing signals to be accurately transmitted, received, and/or processed by the processing system  110 . Such techniques are described in further detail below in conjunction with  FIGS. 3-7 . 
     Current Feedback Techniques for Capacitive Sensing 
       FIG. 3  is a partial schematic view of a sensing circuit  310  included in the processing system  110  of  FIG. 1  in accordance with embodiments of the invention. The sensing circuit  310  may include an operational amplifier  320 , a first voltage-controlled current source (VCCS)  330 , a second VCCS  335 , a voltage source  340 , and a known capacitor  350  having a known capacitance value. The sensing circuit  310  may be coupled to an unknown capacitor  355  having an unknown capacitance value. In some embodiments, the unknown capacitor  355  may include a sensor electrode (e.g., a transmitter electrode  210  or a receiver electrode  220 ) or a set of sensor electrodes. The operational amplifier  320  includes an inverting input  322 , a non-inverting input  324 , and an output  326 . 
     As shown, the first VCCS  330  is coupled between the output  326  of the operational amplifier  320  and the inverting input  322  of the operational amplifier  320  to form a feedback path  332 . The second VCCS  335  is coupled between the output  326  of the operational amplifier  320  and the known capacitor  350 . The voltage source  340  is coupled to the non-inverting input  324  of the operational amplifier  320 . 
     During operation of the sensing circuit  310 , an input voltage is applied to the non-inverting input  324  by the voltage source  340 , producing a control voltage V C  at the output  326  of the operational amplifier  320 . The control voltage V C  at the output  326  of the operational amplifier  320  then causes the first VCCS  330  to generate a current, which modifies the charge stored by the unknown capacitor  355 . Similarly, the control voltage V C  at the output  326  of the operational amplifier  320  causes the second VCCS  335  to generate a current, which modifies the charge stored by the known capacitor  350 . 
     In various embodiments, there is a known relationship between the current at the output of the first VCCS  330  at a given control voltage V C  and the current at the output of the second VCCS  335  at that same control voltage V C . Accordingly, by monitoring changes to a voltage V C  across the known capacitor  350 , an amount of charge that has been transferred to or received from the unknown capacitor  355  can be determined. Additionally, the operational amplifier  320  will respond to changes in the amount of charge stored by the unknown capacitor  355  by modifying the control voltage V C . Then, in response to changes to the control voltage V C , the second VCCS  335  may add charge to or remove charge from the known capacitor  350 . Consequently, changes in the amount of charge stored by the unknown capacitor  355  can be determined based on changes to the voltage V C  across the known capacitor  350 . 
     In some embodiments, a scaling factor K is used to establish a relationship between the voltage/current behavior of the first VCCS  330  and the voltage/current behavior of the second VCCS  335 . For example, the second VCCS  335  may be designed to produce a known fraction (e.g., one-half, one-quarter, one-tenth, etc.) of the current produced by the first VCCS  330  at a given voltage. By designing the sensing circuit  310  such that the second VCCS  335  produces less current than the first VCCS  330  at a given voltage, a smaller known capacitor  350  may be used, decreasing the overall size of the sensing circuit  310 . That is, when the second VCCS  335  is designed to produce less current than the first VCCS  330  at a given voltage, the known capacitor  350  does not need to have the same capacitance as the unknown capacitor. Instead, to determine the capacitance of the unknown capacitor  355 , a scaling factor K may be applied to the voltage V O  across the known capacitor  350 . In other embodiments, however, the first VCCS  330  and the second VCCS  335  may have substantially the same voltage/current characteristics, enabling characteristics of the unknown capacitor  355  to be inferred directly from characteristics of the known capacitor  350 , without applying a scaling factor. 
     In addition to, or instead of, using a scaling factor K to compensate for differences between the first VCCS  330  and the second VCCS  335 , the second VCCS  335  may use a charge cancellation technique and/or offset technique in order to allow the size of the known capacitance  350  to be reduced. For example, a charge cancellation technique used by the second VCCS  335  may remove a fixed amount of charge from known capacitor  350  when certain criteria are met. One type of charge cancellation technique that may be used is coarse baseline correction (CBC). In order to perform CBC, one side of an adjustable capacitor may be coupled to an input (e.g., V −  and V − ′) of each sensing circuit, while the other side of the adjustable capacitor is driven with a voltage source. Thus, the adjustable capacitor may cancel an amount of charge that is equal to the ΔV of the voltage source multiplied by the capacitance of the adjustable capacitor. In some embodiments, CBC may be performed by generating a square wave with the voltage source. The square wave may have a polarity that is opposite to the polarity of an input sensing signal that is being transmitted to the sensor electrode. 
     Additionally, one type of charge subtraction technique that may be used is a current conveyor (CC). The CC includes a reference capacitor and a voltage source. The CC measures an amount of charge that is delivered by the voltage source to the reference capacitor, multiplies the amount of charge by a factor (e.g., 2, 5, 10, etc.), and then delivers the multiplied charge to an unknown capacitance (e.g., unknown capacitance  355 ). Accordingly, in any of the techniques described above, the charge on the known capacitor (e.g., known capacitor  350 ) will be the difference between the amount of charge that is needed to shift the voltage at the inverting input (e.g., inverting input  322 ) to the voltage at the non-inverting input (e.g., non-inverting input  324 ) and the amount of charge that was cancelled, offset, subtracted, etc. Thus, the amount of charge transferred to or from the unknown capacitor  355  may be tracked while allowing the size of the known capacitor  350  to be reduced. 
     In various embodiments, the capacitance of the unknown capacitor  355  may be determined using Equations 1-4, where Q unknown  is the charge on the unknown capacitor  355 , C unknown  is the capacitance of the unknown capacitor  355 , V− is the voltage applied to the unknown capacitor  355 , Q known  is the charge on the known capacitor  350 , C known  is the capacitance of the known capacitor  350 , V O  is the voltage applied to the known capacitor  350 , and K is the scaling factor. 
     
       
         
           
             
               
                 
                   
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     As described above, the unknown capacitor  355  may include one or more sensor electrodes. For example, in an absolute capacitance sensing configuration, one or more sensor electrodes are coupled to the inverting input  322  and first VCCS  330  of the sensing circuit  310 , and the voltage source  340  is configured to drive an input sensing signal (e.g., a square wave, sinusoidal voltage, constant voltage, etc.) onto the non-inverting input  324  of the operational amplifier  320 . In response, the operational amplifier  320  outputs a control voltage V C , causing the first VCCS  330  to add charge to and/or remove charge from the sensor electrode(s). Additionally, outputting the control voltage V C  causes the second VCCS  335  to add charge to and/or remove charge from the known capacitor  350 . Further, changes to the unknown capacitor  355  may be detected by tracking the voltage V O  across the known capacitor  350  using Equation 4, described above. Accordingly, by tracking the amount of charge added to and/or removed from the known capacitor  350 , changes to the capacitance of the sensor electrode(s) (e.g., due to the presence or absence of an input object  140 ) may be determined. 
     Thus, including the first VCCS  330  in the feedback path  332  of the sensing circuit  310  enables the size of the sensing circuit  310  to be reduced while still enabling the sensing circuit  310  to accurately determine an amount of charge transferred to and/or received from an unknown capacitor. Additional techniques for utilizing a VCCS to perform input sensing are described below. 
       FIG. 4  is a partial schematic view in which the sensing circuit  310  of  FIG. 3  is configured for transcapacitive sensing in accordance with embodiments of the invention. As shown, the sensing circuit  310  may be coupled to an unknown capacitance  455  that is configured for transcapacitive sensing. For example, in some embodiments, the inverting input  322  of the operational amplifier is electrically coupled to one or more sensor electrodes (e.g., one or more receiver electrodes), and the one or more sensor electrodes are capacitively coupled to one or more transmitter electrodes  210  included in the input device  100 . 
     When performing transcapacitive sensing with the sensing circuit  310 , the voltage source  340  coupled to the non-inverting input  324  may be held at a substantially constant voltage. Then, while receiving an input sensing signal from one or more transmitter electrodes  210 , changes to the voltage V− at the inverting input  322  will cause the second VCCS  335  to modify the charge on the known capacitor  350 . Changes to the unknown capacitance  455  (e.g., due to the input sensing signal and/or due to the presence or absence of an input object  140 ) may then be determined (e.g., via determination module  250 ) based on the voltage V O  across the known capacitor  350 , for example, using Equation 4, described above. 
       FIG. 5  is a partial schematic view of a processing system  110  that includes two sensing circuits  310  configured to perform transcapacitive sensing and/or absolute capacitance sensing in accordance with embodiments of the invention. In some embodiments, sensing circuit  310 - 1  is coupled to one or more first sensor electrodes, and sensing circuit  310 - 2  is coupled to one or more second sensor electrodes. Thus, a first unknown capacitor  355 - 1  may correspond to the absolute capacitance of the first sensor electrode(s), and a second unknown capacitor  355 - 2  may correspond to the absolute capacitance of the second sensor electrode(s). Additionally, a third unknown capacitor  555  may correspond to the capacitive coupling between the first sensor electrode(s) and the second sensor electrode(s). 
     When performing transcapacitive sensing, the first sensor electrode(s) may be operated as transmitter electrode(s)  210 , and the second sensor electrode(s) may be operated as receiver electrode(s)  220 . In such embodiments, the voltage source  340 - 2  included in sensing circuit  310 - 2  may apply a constant voltage to the non-inverting input of operational amplifier  320 - 2 . Accordingly, because V−′ is being held at a substantially constant voltage, substantially no current will flow through the second unknown capacitor  355 - 2 . Then, the voltage source  340 - 1  included in sensing circuit  310 - 1  may apply a voltage (e.g., a square wave, sinusoidal voltage, etc.) onto the non-inverting input of the operational amplifier  320 - 1 , causing an input sensing signal to be driven onto the transmitter electrode(s)  210 . A total amount of current driven onto the transmitter electrode(s)  210  (I TX ) may be determined (e.g., via determination module  250 ) by the sensing circuit  310 - 1  based on the voltage V C  across the known capacitor  350 - 1 . Additionally, while sensing circuit  310 - 1  is driving the input sensing signal onto the transmitter electrode(s), a total amount of current received by the receiver electrode(s)  220  (I RX ) from the transmitter electrode(s)  210  may be determined based on the voltage V O ′ across the known capacitor  350 - 2 . The total amount of current driven from the transmitter electrode(s)  210  to the receiver electrode(s)  220  (I RX ) may then be used to determine the capacitance of the third unknown capacitor  555 . Additionally, because there is substantially no current through the second unknown capacitor  355 - 2  (while V−′ is being held at a substantially constant voltage), the amount of current received by the first unknown capacitor  355 - 1  (I CTX ) may be determined by subtracting the total amount of current received by the receiver electrode(s)  220  (I RX ) from the total amount of current driven onto the transmitter electrode(s)  210  (I TX ), as shown below in Equation 5.
 
 I   CTX   =I   TX   −I   RX   (Eq. 5)
 
     In the same or other embodiments, the voltage source  340 - 2  included in sensing circuit  310 - 2  also may be operated to drive an input sensing signal onto the second sensor electrode(s), while the voltage source  340 - 1  included in sensing circuit  310 - 1  applies a substantially constant voltage to the non-inverting input of operational amplifier  320 - 1 . That is, the roles of the first sensor electrode(s) and second sensor electrode(s) may be reversed, such that the first sensor electrode(s) are operated as receiver electrode(s)  220 , and the second sensor electrode(s) are operated as transmitter electrode(s)  210 . The absolute capacitance of the second unknown capacitor  355 - 2  may then be determined by subtracting the amount of current received by the first sensor electrode(s) from the total amount of current transmitted to the second sensor electrode(s), using Equation 5. Thus, each of the first sensor electrode(s) and the second sensor electrode(s) may be operated for absolute sensing during different time periods. In addition, transcapacitance measurements (e.g., of the unknown capacitor  555 ) may be acquired while either of the first sensor electrode(s) or second sensor electrode(s) are being operated for absolute sensing. 
     Accordingly, the sensing circuits  310 - 1  and  310 - 2  may be operated to perform transcapacitive sensing in conjunction with absolute capacitance sensing using one or both of the first sensor electrode(s) and the second sensor electrode(s). In order to ensure that the absolute capacitance of each of the first sensor electrode(s) and second sensor electrode(s) (e.g., the first unknown capacitance  355 - 1  and the second unknown capacitance  355 - 2 , respectively) can be effectively isolated from other unknown capacitances (e.g., the third unknown capacitance  555 ), the first sensor electrode(s) and second sensor electrode(s) may be operated for absolute sensing during non-overlapping time periods. For example, the roles (e.g., the role of transmitter electrode  210  and the role of receiver electrode  220 ) of the first sensor electrode(s) and the second sensor electrode(s) may be reversed after each sensing cycle, after multiple sensing cycles, or based on some other input sensing pattern. 
       FIG. 6  is a flow chart of a method  600  for performing input sensing using the sensing circuit  310  of  FIG. 3  in accordance with embodiments of the invention. Although the method  600  is described in conjunction with  FIGS. 1-5 , persons skilled in the art will understand that any system configured to perform the method, in any appropriate order, falls within the scope of the present invention. 
     The method  600  begins at step  610 , where the voltage source  340  drives an input voltage onto the non-inverting input  324  of the operational amplifier  320 . As described above, a first VCCS  330  is coupled between the output  326  of the operational amplifier  320  and the inverting input  322  of the operational amplifier  320  to form a feedback path  332 . Next, at step  620 , the control voltage V C  at the output  326  of the operational amplifier  320  causes the first VCCS  330  to drive an input sensing signal onto one or more sensor electrode(s) coupled to the inverting input  322  of the operational amplifier  320 . 
     At step  630 , the second VCCS  335  modifies the charge on the known capacitor  350  based on a control voltage V C  at the output  326  of the operational amplifier. Then, at step  640 , the determination module  250  determines a capacitance measurement associated with the one or more sensor electrodes based on the charge on the known capacitor  350 . For example, the capacitance of the one or more sensor electrodes may be determined based on the voltage V O  across the known capacitor  350 . 
       FIG. 7  is a flow chart of a method  700  for performing input sensing using the sensing circuits  310 - 1 ,  310 - 2  of  FIG. 5  in accordance with embodiments of the invention. Although the method  700  is described in conjunction with  FIGS. 1-5 , persons skilled in the art will understand that any system configured to perform the method, in any appropriate order, falls within the scope of the present invention. 
     The method  700  begins at step  710 , where the voltage source  340 - 1  of sensing circuit  310 - 1  drives an input voltage onto the non-inverting input of operational amplifier  320 - 1 . At step  712 , the voltage source  340 - 2  of sensing circuit  310 - 2  drives a substantially constant voltage onto the non-inverting input of operational amplifier  320 - 2 . Next, at step  720 , the control voltage V C  at the output  326  of operational amplifier  320 - 1  causes VCCS  330 - 1  to drive an input sensing signal onto the first sensor electrode(s). At step  722 , the second sensor electrode(s) receive charge from the first sensor electrode(s), for example, via a transcapacitive coupling between the first sensor electrode(s) and the second sensor electrode(s). 
     At step  730 , VCCS  335 - 1  modifies the charge on the first known capacitor  350 - 1  based on the control voltage V C  at the output of operational amplifier  320 - 1 . At step  732 , VCCS  335 - 2  modifies the charge on the second known capacitor  350 - 2  based on the control voltage V C ′ at the output of operational amplifier  320 - 2 . Then, at step  740 , the determination module  250  determines an absolute capacitance of the first sensor electrode(s) and/or a transcapacitance between the first sensor electrode(s) and the second sensor electrode(s). The absolute capacitance and transcapacitance may be determined, for example, based on Equations 4 and 5, described above. Then, at step  750 , the roles of the first sensor electrode(s) and the second sensor electrode(s) are switched, such that the first sensor electrode(s) are operated as receiver electrode(s)  220 , and the second sensor electrode(s) are operated as transmitter electrode(s)  210 . 
     Accordingly, at step  760 , the voltage source  340 - 2  of sensing circuit  310 - 2  drives an input voltage onto the non-inverting input of operational amplifier  320 - 2 . At step  762 , the voltage source  340 - 1  of sensing circuit  310 - 1  drives a substantially constant voltage onto the non-inverting input of operational amplifier  320 - 1 . Next, at step  770 , the control voltage V C ′ at the output of operational amplifier  320 - 2  causes VCCS  330 - 2  to drive an input sensing signal onto the second sensor electrode(s). At step  772 , the first sensor electrode(s) receive charge from the second sensor electrode(s), for example, via a transcapacitive coupling between the first sensor electrode(s) and the second sensor electrode(s). 
     At step  780 , VCCS  335 - 2  modifies the charge on the second known capacitor  350 - 2  based on the control voltage V C ′ at the output of operational amplifier  320 - 2 . At step  782 , VCCS  335 - 1  modifies the charge on the first known capacitor  350 - 1  based on the control voltage V C  at the output of operational amplifier  320 - 1 . Then, at step  790 , the determination module  250  determines an absolute capacitance of the second sensor electrode(s) and/or a transcapacitance between the first sensor electrode(s) and the second sensor electrode(s), for example, based on Equations 4 and 5, described above. Then, the roles of the first sensor electrode(s) and the second sensor electrode(s) may be switched again, and the method  700  may return to step  710 . 
     In some embodiments, step  710  and step  712  are performed at substantially the same time, and step  720  and step  722  are performed at substantially the same time. Additionally, in some embodiments, step  760  and step  762  are performed at substantially the same time, and step  770  and step  772  are performed at substantially the same time. 
     Thus, the embodiments and examples set forth herein were presented in order to best explain the present invention and its particular application and to thereby enable those skilled in the art to make and use the invention. However, those skilled in the art will recognize that the foregoing description and examples have been presented for the purposes of illustration and example only. The description as set forth is not intended to be exhaustive or to limit the invention to the precise form disclosed.