Patent Publication Number: US-10768762-B2

Title: Reduced capacitive baseline shift via mixing period adjustments

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of co-pending U.S. patent application Ser. No. 15/400,681, filed Jan. 6, 2017, entitled “REDUCED CAPACITIVE BASELINE SHIFT VIA MIXING PERIOD ADJUSTMENTS”, which is herein incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     Field 
     Embodiments disclosed herein generally relate to electronic devices, and more specifically, techniques for reducing a capacitive baseline shift between different sensing frequencies through selectable mixing periods. 
     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 opaque 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). 
     SUMMARY 
     One embodiment described herein is a method comprising driving, onto a first group of a plurality of sensor electrodes, a first capacitive sensing signal having a predefined first sensing frequency, and acquiring, based on the driven first capacitive sensing signal, first capacitive measurements of resulting signals received by a second group of the plurality of sensor electrodes, wherein acquiring first capacitive measurements comprises applying a first demodulation signal having a predefined first mixing period defined within a sensing period associated with the first sensing frequency. The method further comprises driving, onto a third group of the plurality of sensor electrodes, a second capacitive sensing signal having a second sensing frequency different than the first sensing frequency, and acquiring, based on the driven second capacitive sensing signal, second capacitive measurements of resulting signals received by a fourth group of the plurality of sensor electrodes, wherein acquiring second capacitive measurements comprises applying a second demodulation signal having a predefined second mixing period within a sensing period associated with the second sensing frequency, the second mixing period different than the first mixing period. 
     Another embodiment described herein is a processing system comprising a sensing module comprising sensing circuitry and configured to drive, onto a first group of a plurality of sensor electrodes, a first capacitive sensing signal having a predefined first sensing frequency, and acquire, based on the driven first capacitive sensing signal, first capacitive measurements of resulting signals received by a second group of the plurality of sensor electrodes, wherein acquiring first capacitive measurements comprises applying a first demodulation signal having a predefined first mixing period defined within a sensing period associated with the first sensing frequency. The sensing module is further configured to drive, onto a third group of the plurality of sensor electrodes, a second capacitive sensing signal having a second sensing frequency different than the first sensing frequency, acquire, based on the driven second capacitive sensing signal, second capacitive measurements of resulting signals received by a fourth group of the plurality of sensor electrodes, wherein acquiring second capacitive measurements comprises applying a second demodulation signal having a predefined second mixing period within a sensing period associated with the second sensing frequency, the second mixing period different than the first mixing period. 
     Another embodiment described herein is input device comprising a plurality of sensor electrodes and a processing system coupled with the plurality of sensor electrodes. The processing system is configured to drive, onto a first group of a plurality of sensor electrodes, a first capacitive sensing signal having a predefined first sensing frequency and acquire, based on the driven first capacitive sensing signal, first capacitive measurements of resulting signals received by a second group of the plurality of sensor electrodes, wherein acquiring first capacitive measurements comprises applying a first demodulation signal having a predefined first mixing period defined within a sensing period associated with the first sensing frequency. The processing system is further configured to drive, onto a third group of the plurality of sensor electrodes, a second capacitive sensing signal having a second sensing frequency different than the first sensing frequency, and acquire, based on the driven second capacitive sensing signal, second capacitive measurements of resulting signals received by a fourth group of the plurality of sensor electrodes, wherein acquiring second capacitive measurements comprises applying a second demodulation signal having a predefined second mixing period within a sensing period associated with the second sensing frequency, the second mixing period different than the first mixing period. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, 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 exemplary embodiments and are therefore not to be considered limiting of its scope, may admit to other equally effective embodiments. 
         FIG. 1  is a schematic block diagram of an input device, according to embodiments described herein. 
         FIGS. 2 and 3  illustrate portions of exemplary sensor electrode implementations, according to embodiments described herein. 
         FIG. 4  illustrates an exemplary processing system for applying selected mixing periods for different sensing frequencies, according to embodiments described herein. 
         FIG. 5  is a schematic diagram of a sensing implementation for applying selected mixing periods for different sensing frequencies, according to embodiments described herein. 
         FIG. 6  illustrates a method for applying selected mixing periods for different sensing frequencies, according to embodiments described herein. 
         FIG. 7  is a diagram illustrating exemplary operation of a sensing implementation without performing adjustments of the mixing period, according to embodiments described herein. 
         FIG. 8  is a diagram illustrating exemplary operation of a sensing implementation with adjustments performed to the mixing period, according to embodiments described herein. 
         FIG. 9  is a schematic diagram of a sensing implementation for operating one or more switching elements at different sensing frequencies, according to embodiments described herein. 
         FIG. 10  illustrates a method for operating one or more switching elements at different sensing frequencies, according to embodiments described herein. 
         FIG. 11  is a diagram illustrating exemplary operation of a sensing implementation using one or more switching elements, according to embodiments described herein. 
         FIG. 12  is a graph illustrating changes in capacitance across different sensing frequencies, according to embodiments described herein. 
     
    
    
     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. The drawings referred to here should not be understood as being drawn to scale unless specifically noted. Also, the drawings are often simplified and details or components omitted for clarity of presentation and explanation. The drawings and discussion serve to explain principles discussed below, where like designations denote like elements. 
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the disclosure or the application and uses of the disclosure. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding background, summary, or the following detailed description. 
     Various embodiments of the present technology provide input devices and methods for improving usability. An input device may include electrodes that are operated as sensor electrodes to detect interaction between the input device and an input object (e.g., a stylus or a user&#39;s finger). 
     An input device generally drives a capacitive sensing signal onto sensor electrodes to acquire capacitive measurements. The sensing frequency corresponding to the capacitive sensing signal may be adaptively changed by the input device, e.g., based on detected interference. However, capacitive measurements acquired at different sensing frequencies require some compensation, as the different sensing frequencies generally correspond to different capacitive baselines for the capacitive measurements. 
     In some embodiments, a plurality of mixing periods for a demodulation signal is selected corresponding to a plurality of predefined sensing frequencies. In some cases, the mixing periods are selected such that a plurality of average current values corresponding to a plurality of acquired baseline capacitive measurements have a predefined relationship with the different sensing frequencies. Thus, for any transitions between different ones of the predefined sensing frequencies during operation of the input device, the corresponding average current values will have a predictable relationship, which simplifies any correction or compensation that may be needed for the acquired capacitive sensing measurements. 
     Exemplary Input Device Implementations 
       FIG. 1  is a schematic block diagram of an input device  100 , in accordance with embodiments of the present technology. In various embodiments, input device  100  comprises a display device integrated with a sensing device. The input device  100  may be configured to provide input to an electronic system  150 . 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 example electronic systems include composite input devices, such as physical keyboards that include input device  100  and separate joysticks or key switches. Further example 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. Examples include I 2 C, SPI, PS/2, Universal Serial Bus (USB), Bluetooth, RF, and IRDA. 
     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  170 . Example input objects include fingers and styli, as shown in  FIG. 1 . 
     Sensing region  170  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  170  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  170  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  170  has a rectangular shape when projected onto an input surface of the input device  100 . 
     The input device  100  may utilize any combination of sensor components and sensing technologies to detect user input in the sensing region  170 . The input device  100  comprises a plurality of sensor electrodes  120  for detecting user input. The input device  100  may include one or more sensor electrodes  120  that are combined to form sensor electrodes. As several non-limiting examples, the input device  100  may use capacitive, elastive, resistive, inductive, magnetic acoustic, ultrasonic, and/or optical techniques. 
     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. 
     In some resistive implementations of the input device  100 , a flexible and conductive first layer is separated by one or more spacer elements from a conductive second layer. During operation, one or more voltage gradients are created across the layers. Pressing the flexible first layer may deflect it sufficiently to create electrical contact between the layers, resulting in voltage outputs reflective of the point(s) of contact between the layers. These voltage outputs may be used to determine positional information. 
     In some inductive implementations of the input device  100 , one or more sensor electrodes  120  pickup loop currents induced by a resonating coil or pair of coils. Some combination of the magnitude, phase, and frequency of the currents may then be used to determine positional information. 
     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 sensor electrodes  120  to create electric fields. In some capacitive implementations, separate sensor electrodes  120  may be ohmically shorted together to form larger sensor electrodes. Some capacitive implementations utilize resistive sheets, which may be uniformly resistive. 
     As discussed above, some capacitive implementations utilize “self-capacitance” (or “absolute capacitance”) sensing methods based on changes in the capacitive coupling between sensor electrodes  120  and an input object. In one embodiment, processing system  110  is configured to drive a voltage with known amplitude onto the sensor electrode  120  and measure the amount of charge required to charge the sensor electrode to the driven voltage. In other embodiments, processing system  110  is configured to drive a known current and measure the resulting voltage. In various embodiments, an input object near the sensor electrodes  120  alters the electric field near the sensor electrodes  120 , thus changing the measured capacitive coupling. In one implementation, an absolute capacitance sensing method operates by modulating sensor electrodes  120  with respect to a reference voltage (e.g. system ground) using a modulated signal, and by detecting the capacitive coupling between the sensor electrodes  120  and input objects  140 . 
     Additionally as discussed above, some capacitive implementations utilize “mutual capacitance” (or “transcapacitance”) sensing methods based on changes in the capacitive coupling between sensing electrodes. In various embodiments, an input object  140  near the sensing electrodes alters the electric field between the sensing electrodes, thus changing the measured capacitive coupling. In one implementation, a transcapacitive sensing method operates by detecting the capacitive coupling between one or more transmitter sensing electrodes (also “transmitter electrodes”) and one or more receiver sensing electrodes (also “receiver electrodes”) as further described below. Transmitter sensing electrodes may be modulated relative to a reference voltage (e.g., system ground) to transmit a transmitter signals. Receiver sensing electrodes may be held substantially constant relative to the reference voltage to facilitate receipt of resulting signals. 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). Sensing electrodes may be dedicated transmitter electrodes or receiver electrodes, or may be configured to both transmit and receive. 
     In  FIG. 1 , the 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  170 . The processing system  110  comprises parts of or all of one or more integrated circuits (ICs) and/or other circuitry components. For example, a processing system for a mutual capacitance sensor device may comprise transmitter circuitry configured to transmit signals with transmitter sensor electrodes, and/or receiver circuitry configured to receive signals with receiver sensor electrodes. In some embodiments, the processing system  110  also comprises electronically-readable instructions, such as firmware code, software code, and/or the like. In some embodiments, components composing the processing system  110  are located together, such as near sensor electrode(s)  120  of the input device  100 . In other embodiments, components of processing system  110  are physically separate with one or more components close to sensor electrode(s)  120  of input device  100 , and one or more components 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 sensor electrodes  120  to detect input, identification modules configured to identify gestures such as mode changing gestures, and mode changing modules for changing operation modes. Processing system  110  may also comprise one or more controllers. 
     In some embodiments, the processing system  110  responds to user input (or lack of user input) in the sensing region  170  directly by causing one or more actions. 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 sensor electrode(s)  120  of the input device  100  to produce electrical signals indicative of input (or lack of input) in the sensing region  170 . 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  120 . 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. As yet further examples, the processing system  110  may determine positional information, recognize inputs as commands, recognize handwriting, and the like. 
     “Positional information” as 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  170 , or some other functionality.  FIG. 1  shows buttons  130  near the sensing region  170  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  170  overlaps at least part of an active area of a display screen of the display device  160 . For example, the input device  100  may comprise substantially transparent sensor electrodes  120  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 device  160  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 device  160  may be operated in part or in total by the processing system  110 . 
     It should be understood that while many embodiments of the present technology are described in the context of a fully functioning apparatus, the mechanisms of the present technology are capable of being distributed as a program product (e.g., software) in a variety of forms. For example, the mechanisms of the present technology 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 technology 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. 
     Exemplary Sensor Electrode Implementations 
       FIGS. 2 and 3  illustrate portions of exemplary sensor electrode implementations, according to embodiments described herein. Specifically, implementation  200  ( FIG. 2 ) illustrates a portion of a pattern of sensor electrodes configured to sense in a sensing region  170  associated with the pattern, according to several embodiments. For clarity of illustration and description,  FIG. 2  shows the sensor electrodes in a pattern of simple rectangles, and does not show various associated components. This pattern of sensing electrodes comprises a first plurality of sensor electrodes  205  (e.g.,  205 - 1 ,  205 - 2 ,  205 - 3 ,  205 - 4 ), and a second plurality of sensor electrodes  215  (e.g.,  215 - 1 ,  215 - 2 ,  215 - 3 ,  215 - 4 ). The sensor electrodes  205 ,  215  are each examples of the sensor electrodes  120  discussed above. In one embodiment, processing system  110  operates the first plurality of sensor electrodes  205  as a plurality of transmitter electrodes, and the second plurality of sensor electrodes  215  as a plurality of receiver electrodes. In another embodiment, processing system  110  operates the first plurality of sensor electrodes  205  and the second plurality of sensor electrodes  215  as absolute capacitive sensing electrodes. 
     The first plurality of sensor electrodes  205  and the second plurality of sensor electrodes  215  are typically ohmically isolated from each other. That is, one or more insulators separate the first plurality of sensor electrodes  205  and the second plurality of sensor electrodes  215  and prevent them from electrically shorting to each other. In some embodiments, the first plurality of sensor electrodes  205  and the second plurality of sensor electrodes  215  may be disposed on a common layer. The pluralities of sensor electrodes  205 ,  215  may be electrically separated by insulative material disposed between them at cross-over areas; in such constructions, the first plurality of sensor electrodes  205  and/or the second plurality of sensor electrodes  215  may be formed with jumpers connecting different portions of the same electrode. In some embodiments, the first plurality of sensor electrodes  205  and the second plurality of sensor electrodes  215  are separated by one or more layers of insulative material. In some embodiments, the first plurality of sensor electrodes  205  and the second plurality of sensor electrodes  215  are separated by one or more substrates; for example, they may be disposed on opposite sides of the same substrate, or on different substrates that are laminated together. 
     The pluralities of sensor electrodes  205 ,  215  may be formed into any desired shapes. Moreover, the size and/or shape of the sensor electrodes  205  may be different than the size and/or shape of the sensor electrodes  215 . Additionally, sensor electrodes  205 ,  215  located on a same side of a substrate may have different shapes and/or sizes. In one embodiment, the first plurality of sensor electrodes  205  may be larger (e.g., having a larger surface area) than the second plurality of sensor electrodes  215 , although this is not a requirement. In other embodiments, the first and second pluralities of sensor electrodes  205 ,  215  may have a similar size and/or shape. 
     In one embodiment, the first plurality of sensor electrodes  205  extends substantially in a first direction while the second plurality of sensor electrodes  215  extends substantially in a second direction. For example, and as shown in  FIG. 2 , the first plurality of sensor electrodes  205  extend in one direction, while the second plurality of sensor electrodes  215  extend in a direction substantially orthogonal to the sensor electrodes  205 . Other orientations are also possible (e.g., parallel or other relative orientations). 
     In some embodiments, both the first and second pluralities of sensor electrodes  205 ,  215  are located outside of a plurality (or display stack) of layers that together form the display device  160 . One example of a display stack may include layers such as a lens layer, a one or more polarizer layers, a color filter layer, one or more display electrodes layers, a display material layer, a thin-film transistor (TFT) glass layer, and a backlight layer. However, other implementations of a display stack are possible. In other embodiments, one or both of the first and second pluralities of sensor electrodes  205 ,  215  are located within the display stack, whether included as part of a display-related layer or a separate layer. For example, Vcom electrodes within a particular display electrode layer can be configured to perform both display updating and capacitive sensing. 
     Implementation  300  of  FIG. 3  illustrates a portion of a pattern of sensor electrodes configured to sense in sensing region  170 , according to several embodiments. For clarity of illustration and description,  FIG. 3  shows the sensor electrodes  120  in a pattern of simple rectangles and does not show other associated components. The exemplary pattern comprises an array of sensor electrodes  120   X,Y  arranged in X columns and Y rows, wherein X and Y are positive integers, although one of X and Y may be zero. It is contemplated that the pattern of sensor electrodes  120  may have other configurations, such as polar arrays, repeating patterns, non-repeating patterns, a single row or column, or other suitable implementation. Further, in various embodiments the number of sensor electrodes  120  may vary from row to row and/or column to column. In one embodiment, at least one row and/or column of sensor electrodes  120  is offset from the others, such it extends further in at least one direction than the others. The sensor electrodes  120  is coupled to the processing system  110  and utilized to determine the presence (or lack thereof) of an input object in the sensing region  170 . 
     In a first mode of operation, the implementation of sensor electrodes  120  ( 120   1,1 ,  120   2,1 ,  120   3,1 , . . . ,  120   X,Y ) may be utilized to detect the presence of an input object via absolute sensing techniques. That is, processing system  110  is configured to modulate sensor electrodes  120  to acquire measurements of changes in capacitive coupling between the modulated sensor electrodes  120  and an input object to determine the position of the input object. Processing system  110  is further configured to determine changes of absolute capacitance based on a measurement of resulting signals received with sensor electrodes  120  which are modulated. 
     In some embodiments, the implementation  300  includes one or more grid electrodes (not shown) that are disposed between at least two of the sensor electrodes  120 . The grid electrode(s) may at least partially circumscribe the plurality of sensor electrodes  120  as a group, and may also, or in the alternative, completely or partially circumscribe one or more of the sensor electrodes  120 . In one embodiment, the grid electrode is a planar body having a plurality of apertures, where each aperture circumscribes a respective one of the sensor electrodes  120 . In other embodiments, the grid electrode(s) comprise a plurality of segments that may be driven individually or in groups or two or more segments. The grid electrode(s) may be fabricated similar to the sensor electrodes  120 . The grid electrode(s), along with sensor electrodes  120 , may be coupled to the processing system  110  utilizing conductive routing traces and used for input object detection. 
     The sensor electrodes  120  are typically ohmically isolated from each other, and are also ohmically isolated from the grid electrode(s). That is, one or more insulators separate the sensor electrodes  120  and grid electrode(s) and prevent them from electrically shorting to each other. In some embodiments, the sensor electrodes  120  and grid electrode(s) are separated by an insulative gap, which may be filled with an electrically insulating material, or may be an air gap. In some embodiments, the sensor electrodes  120  and the grid electrode(s) are vertically separated by one or more layers of insulative material. In some other embodiments, the sensor electrodes  120  and the grid electrode(s) are separated by one or more substrates; for example, they may be disposed on opposite sides of the same substrate, or on different substrates. In yet other embodiments, the grid electrode(s) may be composed of multiple layers on the same substrate, or on different substrates. In one embodiment, a first grid electrode may be formed on a first substrate (or a first side of a substrate) and a second grid electrode may be formed on a second substrate (or a second side of a substrate). For example, a first grid electrode comprises one or more common electrodes disposed on a thin-film transistor (TFT) layer of the display device  160  ( FIG. 1 ) and a second grid electrode is disposed on the color filter glass of the display device  160 . The dimensions of the first and second grid electrodes can be equal or differ in at least one dimension. 
     In a second mode of operation, the sensor electrodes  120  ( 120   1,1 ,  120   2,1 ,  120   3,1 , . . . ,  120   X,Y ) may be utilized to detect the presence of an input object via transcapacitive sensing techniques when a transmitter signal is driven onto the grid electrode(s). That is, processing system  110  is configured to drive the grid electrode(s) with a transmitter signal and to receive resulting signals with each sensor electrode  120 , where a resulting signal comprising effects corresponding to the transmitter signal, which is utilized by the processing system  110  or other processor to determine the position of the input object. 
     In a third mode of operation, the sensor electrodes  120  may be split into groups of transmitter and receiver electrodes utilized to detect the presence of an input object via transcapacitive sensing techniques. That is, processing system  110  may drive a first group of sensor electrodes  120  with a transmitter signal and receive resulting signals with the second group of sensor electrodes  120 , where a resulting signal comprising effects corresponding to the transmitter signal. The resulting signal is utilized by the processing system  110  or other processor to determine the position of the input object. 
     The input device  100  may be configured to operate in any one of the modes described above. The input device  100  may also be configured to switch between any two or more of the modes described above. 
     The areas of localized capacitive sensing of capacitive couplings may be termed “capacitive pixels,” “touch pixels,” “tixels,” etc. Capacitive pixels may be formed between an individual sensor electrode  120  and a reference voltage in the first mode of operation, between the sensor electrodes  120  and grid electrode(s) in the second mode of operation, and between groups of sensor electrodes  120  used as transmitter and receiver electrodes (e.g., implementation  200  of  FIG. 2 ). The capacitive coupling changes with the proximity and motion of input objects in the sensing region  170  associated with the sensor electrodes  120 , and thus may be used as an indicator of the presence of the input object in the sensing region of the input device  100 . 
     In some embodiments, the sensor electrodes  120  are “scanned” to determine these capacitive couplings. That is, in one embodiment, one or more of the sensor electrodes  120  are driven to transmit transmitter signals. Transmitters may be operated such that one transmitter electrode transmits at one time, or such that multiple transmitter electrodes transmit at the same time. Where multiple transmitter electrodes transmit simultaneously, the multiple transmitter electrodes may transmit the same transmitter signal and thereby produce an effectively larger transmitter electrode. Alternatively, the multiple transmitter electrodes may transmit different transmitter signals. For example, multiple transmitter electrodes may transmit different transmitter signals according to one or more coding schemes that enable their combined effects on the resulting signals of receiver electrodes to be independently determined. In one embodiment, multiple transmitter electrodes may simultaneously transmit the same transmitter signal while the receiver electrodes receive the effects and are measured according to a scanning scheme. 
     The sensor electrodes  120  configured as receiver sensor electrodes 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. Processing system  110  may be configured to receive with the sensor electrodes  120  in a scanning fashion and/or a multiplexed fashion to reduce the number of simultaneous measurements to be made, as well as the size of the supporting electrical structures. In one embodiment, one or more sensor electrodes are coupled to a receiver of processing system  110  via a switching element such as a multiplexer or the like. In such an embodiment, the switching element may be internal to processing system  110  or external to processing system  110 . In one or more embodiments, the switching elements may be further configured to couple a sensor electrode  120  with a transmitter or other signal and/or voltage potential. In one embodiment, the switching element may be configured to couple more than one receiver electrode to a common receiver at the same time. 
     In other embodiments, “scanning” sensor electrodes  120  to determine these capacitive couplings comprises modulating one or more of the sensor electrodes and measuring an absolute capacitance of the one or sensor electrodes. In another embodiment, the sensor electrodes may be operated such that more than one sensor electrode is driven and received with at a time. In such embodiments, an absolute capacitive measurement may be obtained from each of the one or more sensor electrodes  120  simultaneously. In one embodiment, each of the sensor electrodes  120  are simultaneously driven and received with, obtaining an absolute capacitive measurement simultaneously from each of the sensor electrodes  120 . In various embodiments, processing system  110  may be configured to selectively modulate a portion of sensor electrodes  120 . For example, the sensor electrodes may be selected based on, but not limited to, an application running on the host processor, a status of the input device, and an operating mode of the sensing device. In various embodiments, processing system  110  may be configured to selectively shield at least a portion of sensor electrodes  120  and to selectively shield or transmit with the grid electrode(s)  122  while selectively receiving and/or transmitting with other sensor electrodes  120 . 
     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. 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 entering, exiting, and within the sensing region. 
     In any of the above embodiments, multiple sensor electrodes  120  may be ganged together such that the sensor electrodes  120  are simultaneously modulated or simultaneously received with. As compared to the methods described above, ganging together multiple sensor electrodes may produce a coarse capacitive image that may not be usable to discern precise positional information. However, a coarse capacitive image may be used to sense presence of an input object. In one embodiment, the coarse capacitive image may be used to move processing system  110  or the input device  100  out of a “doze” mode or low-power mode. In one embodiment, the coarse capacitive image may be used to move a capacitive sensing IC out of a “doze” mode or low-power mode. In another embodiment, the coarse capacitive image may be used to move at least one of a host IC and a display driver out of a “doze” mode or low-power mode. The coarse capacitive image may correspond to the entire sensor area or only to a portion of the sensor area. 
     The background capacitance of the input device  100  is the capacitive image associated with no input object in the sensing region  170 . The background capacitance changes with the environment and operating conditions, and may be estimated in various ways. For example, some embodiments take “baseline images” when no input object is determined to be in the sensing region  170 , and use those baseline images as estimates of their background capacitances. The background capacitance or the baseline capacitance may be present due to stray capacitive coupling between two sensor electrodes, where one sensor electrode is driven with a modulated signal and the other is held stationary relative to system ground, or due to stray capacitive coupling between a receiver electrode and nearby modulated electrodes. In many embodiments, the background or baseline capacitance may be relatively stationary over the time period of a user input gesture. 
     Capacitive images can be adjusted for the background capacitance of the input device  100  for more efficient processing. Some embodiments accomplish this by “baselining” measurements of the capacitive couplings at the capacitive pixels to produce a “baselined capacitive image.” That is, some embodiments compare the measurements forming a capacitance image with appropriate “baseline values” of a “baseline image” associated with those pixels, and determine changes from that baseline image. 
     In some touch screen embodiments, one or more of the sensor electrodes  120  comprise one or more display electrodes used in updating the display of the display screen. The display electrodes may comprise one or more elements of the active matrix display such as one or more segments of a segmented Vcom electrode (common electrode(s)), a source drive line, gate line, an anode sub-pixel electrode or cathode pixel electrode, or any other suitable display element. These display electrodes may be disposed on an appropriate display screen substrate. For example, the common electrodes may be disposed on the a transparent substrate (a glass substrate, TFT glass, or any other transparent material) in some display screens (e.g., In-Plane Switching (IPS), Fringe Field Switching (FFS) or Plane to Line Switching (PLS) Organic Light Emitting Diode (OLED)), on the bottom of the color filter glass of some display screens (e.g., Patterned Vertical Alignment (PVA) or Multi-domain Vertical Alignment (MVA)), over an emissive layer (OLED), etc. In such embodiments, the display electrode can also be referred to as a “combination electrode,” since it performs multiple functions. In various embodiments, each of the sensor electrodes  120  comprises one or more common electrodes. In other embodiments, at least two sensor electrodes  120  may share at least one common electrode. While the following description may describe that sensor electrodes  120  and/or grid electrode(s) comprise one or more common electrodes, various other display electrodes as describe above may also be used in conjunction with the common electrode or as an alternative to the common electrodes. In various embodiments, the sensor electrodes  120  and grid electrode(s) comprise the entire common electrode layer (Vcom electrode). 
     In various touch screen embodiments, the “capacitive frame rate” (the rate at which successive capacitive images are acquired) may be the same or be different from that of the “display frame rate” (the rate at which the display image is updated, including refreshing the screen to redisplay the same image). In various embodiments, the capacitive frame rate is an integer multiple of the display frame rate. In other embodiments, the capacitive frame rate is a fractional multiple of the display frame rate. In yet further embodiments, the capacitive frame rate may be any fraction or integer multiple of the display frame rate. In one or more embodiments, the display frame rate may change (e.g., to reduce power or to provide additional image data such as a 3D display information) while touch frame rate maintains constant. In other embodiment, the display frame rate may remain constant while the touch frame rate is increased or decreased. 
     Continuing to refer to  FIG. 3 , the processing system  110  coupled to the sensor electrodes  120  includes a sensing module  310  and optionally, a display driver module  320 . The sensing module  310  includes circuitry configured to drive at least one of the sensor electrodes  120  for capacitive sensing during periods in which input sensing is desired. In one embodiment, the sensing module  310  is configured to drive a modulated signal onto the at least one sensor electrode  120  to detect changes in absolute capacitance between the at least one sensor electrode and an input object. In another embodiment, the sensing module  310  is configured to drive a transmitter signal onto the at least one sensor electrode  120  to detect changes in a transcapacitance between the at least one sensor electrode and another sensor electrode  120 . The modulated and transmitter signals are generally varying voltage signals comprising a plurality of voltage transitions over a period of time allocated for input sensing. In various embodiments, the sensor electrodes  120  and/or grid electrode(s) may be driven differently in different modes of operation. In one embodiment, the sensor electrodes  120  and/or grid electrode(s) may be driven with signals (modulated signals, transmitter signals and/or shield signals) that may differ in any one of phase, amplitude, and/or shape. In various embodiments, the modulated signal and transmitter signal are similar in at least one shape, frequency, amplitude, and/or phase. In other embodiments, the modulated signal and the transmitter signals are different in frequency, shape, phase, amplitude, and phase. The sensing module  310  may be selectively coupled one or more of the sensor electrodes  120  and/or the grid electrode(s). For example, the sensing module  310  may be coupled selected portions of the sensor electrodes  120  and operate in either an absolute or transcapacitive sensing mode. In another example, the sensing module  310  may be a different portion of the sensor electrodes  120  and operate in either an absolute or transcapacitive sensing mode. In yet another example, the sensing module  310  may be coupled to all the sensor electrodes  120  and operate in either an absolute or transcapacitive sensing mode. 
     The sensing module  310  is configured to operate the grid electrode(s) as a shield electrode that may shield sensor electrodes  120  from the electrical effects of nearby conductors. In one embodiment, the processing system is configured to operate the grid electrode(s) as a shield electrode that may “shield” sensor electrodes  120  from the electrical effects of nearby conductors, and to guard the sensor electrodes  120  from grid electrode(s), at least partially reducing the parasitic capacitance between the grid electrode(s) and the sensor electrodes  120 . In one embodiment, a shielding signal is driven onto the grid electrode(s). The shielding signal may be a ground signal, such as the system ground or other ground, or any other constant voltage (i.e., non-modulated) signal. In another embodiment, operating the grid electrode(s) as a shield electrode may comprise electrically floating the grid electrode. In one embodiment, grid electrode(s) are able to operate as an effective shield electrode while being electrode floated due to its large coupling to the other sensor electrodes. In other embodiment, the shielding signal may be referred to as a “guarding signal” where the guarding signal is a varying voltage signal having at least one of a similar phase, frequency, and amplitude as the modulated signal driven on to the sensor electrodes. In one or more embodiment, routing traces may be shielded from responding to an input object due to routing beneath the grid electrode(s) and/or sensor electrodes  120 , and therefore may not be part of the active sensor electrodes, shown as sensor electrodes  120 . 
     In one or more embodiments, capacitive sensing (or input sensing) and display updating may occur during at least partially overlapping periods. For example, as a common electrode is driven for display updating, the common electrode may also be driven for capacitive sensing. In another embodiment, capacitive sensing and display updating may occur during non-overlapping periods, also referred to as non-display update periods. In various embodiments, the non-display update periods may occur between display line update periods for two display lines of a display frame and may be at least as long in time as the display update period. In such embodiments, the non-display update period may be referred to as a “long horizontal blanking period,” “long h-blanking period” or a “distributed blanking period,” where the blanking period occurs between two display updating periods and is at least as long as a display update period. In one embodiment, the non-display update period occurs between display line update periods of a frame and is long enough to allow for multiple transitions of the transmitter signal to be driven onto the sensor electrodes  120 . In other embodiments, the non-display update period may comprise horizontal blanking periods and vertical blanking periods. Processing system  110  may be configured to drive sensor electrodes  120  for capacitive sensing during any one or more of or any combination of the different non-display update times. Synchronization signals may be shared between sensing module  310  and display driver module  320  to provide accurate control of overlapping display updating and capacitive sensing periods with repeatably coherent frequencies and phases. In one embodiment, these synchronization signals may be configured to allow the relatively stable voltages at the beginning and end of the input sensing period to coincide with display update periods with relatively stable voltages (e.g., near the end of a input integrator reset time and near the end of a display charge share time). A modulation frequency of a modulated or transmitter signal may be at a harmonic of the display line update rate, where the phase is determined to provide a nearly constant charge coupling from the display elements to the receiver electrode, allowing this coupling to be part of the baseline image. 
     The sensing module  310  includes circuitry configured to receive resulting signals with the sensor electrodes  120  and/or grid electrode(s) comprising effects corresponding to the modulated signals or the transmitter signals during periods in which input sensing is desired. The sensing module  310  may determine a position of the input object in the sensing region  170  or may provide a signal including information indicative of the resulting signal to another module or processor, for example, a determination module  330  or a processor of an associated electronic device  150  (i.e., a host processor), for determining the position of the input object in the sensing region  170 . 
     The display driver module  320  may be included in or separate from the processing system  110 . The display driver module  320  includes circuitry configured to provide display image update information to the display of the display device  160  during non-sensing (e.g., display updating) periods. 
     In one embodiment, the processing system  110  comprises a first integrated controller comprising the display driver module  320  and at least a portion of the sensing module  310  (i.e., transmitter module and/or receiver module). In another embodiment, the processing system  110  comprises a first integrated controller comprising the display driver module  320  and a second integrated controller comprising the sensing module  310 . In yet another embodiment, the processing system comprises a first integrated controller comprising display driver module  320  and a first portion of the sensing module  310  (e.g., one of a transmitter module and a receiver module) and a second integrated controller comprising a second portion of the sensing module  310  (e.g., the other one of the transmitter and receiver modules). In those embodiments comprising multiple integrated circuits, a synchronization mechanism may be coupled between them, configured to synchronize display updating periods, sensing periods, transmitter signals, display update signals, and the like. 
     Exemplary Sensing Implementations 
       FIG. 4  illustrates an exemplary processing system for applying selected mixing periods for different sensing frequencies, according to embodiments described herein. More specifically, implementation  400  provides one possible implementation of the processing system  110  discussed above. Further, the implementation  400  is capable of use in conjunction with various embodiments discussed herein, such as the implementations  200 ,  300  of sensor electrodes discussed above with respect to  FIGS. 2 and 3 . 
     Within implementation  400 , the sensing module  310  comprises a plurality of sensor electrode groups  405 . Each group G 1 , G 2 , G 3 , G 4 , . . . , GN of the plurality of sensor electrode groups  405  corresponds to at least one sensor electrode of the plurality of sensor electrodes that are coupled with the processing system  110 . The processing system  110  operates to perform capacitive sensing by operating selected groups of the groups G 1 , G 2 , G 3 , G 4 , . . . , GN of sensor electrodes to transmit capacitive sensing signals and/or to receive resulting signals. For example, in a transcapacitive sensing implementation, a first group G 1  of one or more sensor electrodes is driven with a capacitive sensing signal, and a second group G 2  of one or more other sensor electrodes receives resulting signals. In another example, in an absolute capacitive sensing implementation, a particular group G 1  of sensor electrodes is driven with a capacitive sensing signal and is also used to receive the resulting signals. 
     The sensing module  310  further comprises a plurality of predefined sensing frequencies  415 . Capacitive sensing signals that are generated by the processing system  110  and subsequently driven onto selected sensor electrodes are generally time-varying signals having a frequency corresponding to a selected one of the sensing frequencies f SENS,1 , f SENS,2 , f SENS,3 , . . . , f SENS,K . Further, during operation of the processing system  110 , the frequency of the generated capacitive sensing signals may be transitioned between selected ones of the sensing frequencies f SENS,1 , f SENS,2 , f SENS,3 , . . . , f SENS,K  to avoid interference and thus to improve sensing performance. In some cases, the transition between sensing frequencies f SENS,1 , f SENS,2 , f SENS,3 , . . . , f SENS,K  is performed responsive to interference measurements performed by the processing system  110 . 
     In some embodiments, the predefined sensing frequencies f SENS,1 , f SENS,2 , f SENS,3 , . . . , f SENS,K  are selected based on one or more slowest sensor electrodes  410  that are identified within the plurality of sensor electrodes. The one or more slowest sensor electrodes  410  are associated with the relatively longest RC time constants, which causes the one or more slowest sensor electrodes  410  to settle more slowly when driven with various signals (such as the generated capacitive sensing signals). For example, within an input device, the slowest sensor electrodes  410  may generally correspond to those sensor electrodes that are spatially furthest from the processing system  110  and which are connected with the longest and/or most circuitous conductive traces. In such embodiments, the predefined sensing frequencies f SENS,1 , f SENS,2 , f SENS,3 , . . . , f SENS,K  are selected such that generating capacitive sensing signals with the highest sensing frequency produces acceptable sensing performance even for the slowest sensor electrodes  410 . 
     The sensing module  310  further comprises a plurality of predefined mixing periods  420  (or “mixing window pulse widths”) that are used for demodulating resulting signals received by the sensor electrodes (i.e., according to the selected sensor electrode groups G 1 , G 2 , . . . , GN). Each mixing period T MIX,1 , T MIX,2 , T MIX,3 , . . . , T MIX,K  of the plurality of predefined mixing periods  420  corresponds to a particular sensing frequency f SENS,1 , f SENS,2 , f SENS,3 , . . . , f SENS,K . 
     In some embodiments, the mixing periods T MIX,1 , T MIX,2 , T MIX,3 , . . . , T MIX,K  are selected such that a plurality of average current values  430  (i.e., average current values I AVG,1 , I AVG,2 , I AVG,3 , . . . , I AVG,K ) corresponding to a plurality of acquired baseline capacitive measurements  425  (i.e., baseline capacitive measurements B( 1 ), B( 2 ), . . . , B(K) performed with no input object present) have a predefined relationship with the different sensing frequencies f SENS,1 , f SENS,2 , f SENS,3 , . . . , f SENS,K . Thus, for any transitions between different ones of the predefined sensing frequencies f SENS,1 , f SENS,2 , f SENS,3 , . . . , f SENS,K  during operation of the processing system  110 , the corresponding average current values I AVG,1 , I AVG,2 , I AVG,3 , . . . , I AVG,K  will have a predictable relationship, which simplifies any correction or compensation that may be needed (e.g., scaling or shifting) to capacitive sensing measurements. In this way, the processing system  110  avoids the need for calibrating and storing compensation values for each individual sensor electrode or capacitive pixel, which reduces memory requirements and associated cost and size. 
     In one embodiment, the various mixing periods T MIX,1 , T MIX,2 , T MIX,3 , . . . , T MIX,K  are selected such that the corresponding average current values I AVG,1 , I AVG,2 , I AVG,3 , . . . , I AVG,K  are substantially identical. In such a case, the processing system  110  need not perform any correction or compensation to the capacitive sensing measurements when transitioning between different sensing frequencies f SENS,1 , f SENS,2 , f SENS,3 , . . . , f SENS,K . In another embodiment, the plurality of average current values  430  has a substantially linear relation to the changes to the sensing frequency f SENS,1 , f SENS,2 , f SENS,3 , . . . , f SENS,K , which requires only a relatively simple correction or compensation by the processing system  110 . 
     In some embodiments, the implementation  400  comprises one or more switching elements  435  coupled with one or more sensor electrodes corresponding to the groups G 1 , G 2 , G 3 , G 4 , . . . , GN of sensor electrodes. The processing system  110  generally operates the switching elements  435  such that the plurality of average current values  430  (i.e., average current values I AVG,1 , I AVG,2 , I AVG,3 , . . . , I AVG,K ) have a predefined relationship with the different sensing frequencies f SENS,1 , f SENS,2 , f SENS,3 , . . . f SENS,K . In some embodiments, the switching elements  435  are in a conducting (or “on”) state during mixing periods selected from the mixing periods T MIX,1 , T MIX,2 , T MIX,3 , . . . , T MIX,K , and in a non-conducting (or “off”) state during non-mixing periods. In this way, the switching elements  435  couple the associated sensor electrodes within the sensing implementation during the mixing periods, and decouple the sensor electrodes during non-mixing periods. The operation and timing of the switching elements  435  is discussed in greater detail with respect to  FIGS. 9, 10, and 11  below. Some non-limiting examples of the one or more switching elements  435  include transistors and multiplexers. Further, although depicted as external to the processing system  110 , the switching elements  435  may alternately be included within the processing system  110 . 
       FIG. 5  is a schematic diagram of a sensing implementation  500  (or “implementation”) for applying selected mixing periods for different sensing frequencies, according to embodiments described herein. The implementation  500  is capable of use in conjunction with various embodiments discussed herein, such as the implementations  200 ,  300  of sensor electrodes discussed above with respect to  FIGS. 2 and 3  and the processing system depicted in  FIG. 4 . 
     The implementation  500  comprises a voltage source  505  that generates a capacitive sensing signal having a voltage waveform V TX . The voltage waveform V TX  may have any suitable shape, and the frequency of voltage waveform V TX  is controlled based on which predefined sensing frequency (e.g., f SENS,1 , f SENS,2 , f SENS,3 , . . . f SENS,K  of  FIG. 4 ) is selected. The voltage source  505  drives the capacitive sensing signal onto a sensor electrode  510 , which is represented as a first-order model having a transmitter resistance R TX  and a transmitter capacitance C TX . Based on a transcapacitance C T  between sensor electrode  510  and sensor electrode  515 , the sensor electrode  515  receives resulting signals which are provided to receiver circuitry  520 . The sensor electrode  515  is also represented as a first-order model having a receiver resistance R RX  and a receiver capacitance C RX . However, the principles discussed herein also apply to more complex (e.g., distributed) modeling of the plurality of sensor electrodes. Further, although described in terms of a transcapacitive implementation having two separate sensor electrodes  510 ,  515 , the principles discussed herein also apply to absolute capacitive implementations in which the same sensor electrode(s) are used to transmit and receive capacitive sensing signals. 
     As shown, the receiver circuitry  520  comprises an amplifier  525  (or “op-amp”), a current conveyor (or “current mirror”)  530 , and a demodulator (or “mixer”)  535 . The input current I IN  represents the resulting signals received by the sensor electrode  515 , and is mirrored by the current conveyor  530  (with any suitable gain value A) as an output current I OUT . Based on a received demodulation signal, the demodulator  535  downconverts the output current I OUT  having higher (RF) frequencies to a demodulated current I MIX , which may be filtered by subsequent circuitry to have approximately direct current (DC) levels (i.e., having substantially no frequency component). As shown, a voltage source  540  generates the demodulation signal with a voltage waveform V MIX  that is based on which predefined mixing period (e.g., T MIX,1 , T MIX,2 , T MIX,3 , . . . , T MIX,K  of  FIG. 4 ) is selected. 
     The demodulator  535  is generally a continuous-time demodulator having any suitable implementation, such as a square-wave mixer, harmonic rejection mixer, or sinusoidal mixer. Generally, use of a continuous-time demodulator  535  allows a relatively simple hardware implementation of the receiver circuitry  520 , while avoiding the need for calibrating and storing compensation values for each individual sensor electrode or capacitive pixel. In some embodiments, the demodulator  535  receives a three-level demodulation signal having a positive level, a negative level, and a zero level. 
       FIG. 6  illustrates a method  600  for applying selected mixing periods for different sensing frequencies, according to embodiments described herein. The method  600  is capable of use in conjunction with various embodiments discussed herein, such as the processing system depicted in  FIG. 4 , or any other suitable processing system. 
     Method  600  begins at block  605 , where the processing system drives, onto a first group of a plurality of sensor electrodes, a first capacitive sensing signal having a predefined first sensing frequency. At block  615 , the processing system acquires, based on the driven first capacitive sensing signal, first capacitive measurements of resulting signals received by a second group of the plurality of sensor electrodes. Each of the first group and the second group comprises one or more sensor electrodes. In some absolute capacitive sensing implementations, the first group and the second group are the same. In some transcapacitive sensing implementations, the first group and the second group are different. During block  615 , the processing system further applies a first demodulation signal having a predefined first mixing period defined within a sensing period associated with the first sensing frequency. 
     At block  625 , the processing system drives, onto a third group of the plurality of sensor electrodes, a second capacitive sensing signal having a second sensing frequency different than the first sensing frequency. The third group may be the same as the first group, but this is not a requirement. In some cases, the transition between the first sensing frequency and the second sensing frequency is performed responsive to detected interference at the first sensing frequency. At block  635 , the processing system acquires, based on the driven second capacitive sensing signal, second capacitive measurements of resulting signals received by a fourth group of the plurality of sensor electrodes. The fourth group may be the same as the second group, but this is not a requirement. During block  635 , the processing system further applies a second demodulation signal having a predefined second mixing period different than the first mixing period. In some embodiments, the first and second mixing periods are selected such that a first average current value for a first baseline capacitive measurement at the first sensing frequency has a linear relation with a second average current value for a second baseline capacitive measurement at the second sensing frequency. Method  600  ends following completion of block  635 . 
       FIG. 7  is a diagram  700  illustrating exemplary operation of a sensing implementation without performing adjustments of the mixing period, according to embodiments described herein. Generally, the diagram  700  represents operation of the sensing implementation  500  depicted in  FIG. 5 , or any other suitable processing system. 
     Diagram  700  includes a plot  705  of voltage waveform V TX  in volts (V) over time in microseconds (us). As shown, the voltage waveform V TX  is a square wave having a sensing frequency of 300 kilohertz (kHz) and alternating between 3V and 0V levels. Diagram  700  further includes a graph  710 , which includes a plot  720  of demodulated current I MIX  (i.e., the output current from the demodulator), and a plot  725  of input current I IN  (i.e., the input current from received resulting signals). Diagram  700  further includes a plot  715  of voltage waveform V MIX , which as shown is a square wave operating at the sensing frequency of 300 kHz and alternating between two levels (i.e., 1V and −1V levels). 
     Each predefined sensing period forms a plurality of sensing cycles  740 - 1 ,  740 - 2 , and so forth. Each sensing cycle  740 - 1 ,  740 - 2  includes a positive sensing half-cycle  730  and a negative sensing half-cycle  735 . Each sensing cycle  740 - 1 ,  740 - 2  has a duration of approximately 3.33 us, which corresponds to the 300 kHz sensing frequency. As shown, during the positive sensing half-cycle  730 , the demodulator applies the positive level of the demodulation signal (i.e., 1V), and during the negative sensing half-cycle  735 , the demodulator applies the negative level of the demodulation signal (i.e., −1V) such that the multiplicative product of the input current I IN  and the voltage waveform V MIX  produces a signal having a non-zero average value over a single sensing period or an integer number of sensing periods. Within plot  715 , the mixing period T MIX  equals the length of each sensing half-cycle  730 ,  735  (here, approximately 1.67 us). 
       FIG. 8  is a diagram  800  illustrating exemplary operation of a sensing implementation with adjustments performed to the mixing period, according to embodiments described herein. 
     Diagram  800  includes the plot  705  of voltage waveform V TX , which is a square wave having a sensing frequency of 300 kilohertz (kHz) and alternating between 3V and 0V levels. Each sensing cycle  740 - 1 ,  740 - 2  has a duration of approximately 3.33 us, which corresponds to the 300 kHz sensing frequency. Diagram  800  further includes a graph  805 , which includes a plot  815  of demodulated current I MIX , and a plot  820  of input current I IN . Diagram  800  further includes a plot  810  of voltage waveform V MIX , which as shown is a three-level demodulation signal having a positive level (i.e., 1V level), a negative level (i.e., −1V), and a zero level. 
     Sensing cycle  740 - 1  comprises a positive sensing half-cycle  730  during which the demodulator applies the positive level of the demodulation signal, a negative sensing half-cycle  735  during which the demodulator applies the negative level of the demodulation signal, and predefined periods  825 - 1 ,  825 - 2  (or non-mixing periods) during which the zero level is applied. Predefined period  825 - 1  occurs between the positive sensing half-cycle  730  and the negative sensing half-cycle  735 , and predefined period  825 - 2  occurs between the negative sensing half-cycle  735  and a positive sensing half-cycle of the subsequent sensing cycle  740 - 2 . As shown, the mixing period T MIX  represents the amount of time during which the demodulation signal is at each of the positive level and the negative level at a given sensing frequency. Within plot  810 , the mixing period T MIX  is approximately 1.06 us and each predefined period  825 - 1 ,  825 - 2  is approximately 0.60 us. 
     In some embodiments, the durations of the mixing period T MIX  (or sensing half-cycles  730 ,  735 ) and the predefined periods  825 - 1 ,  825 - 2  are controlled such that, for baseline capacitive measurements acquired at the different predefined sensing frequencies, the average current values acquired during each sensing half-cycle  730 ,  735  have a predefined relationship. In one non-limiting example, the average current values may be the same for the different sensing frequencies. In another non-limiting example, the average current values may have a linear relationship based on the difference between the sensing frequencies. 
     Referring also to the sensing implementation  500  depicted in  FIG. 5 , and assuming that R TX =0 and C TX =0, the average current value during a sensing half-cycle can be represented as: 
                     I   MIX     =       C   T     ·     V   TX     ·     f   SENS     ·             [       (     1   -     e     -       T   MIX         R   RX     ·     (       C   T     +     C   RX       )               )     ·     (     1   +     tanh   ⁢     1       4   ·     f   SENS     ·     R   RX     ·     (       C   T     +     C   RX       )       ⁢                   )       ]     .                 (   1   )               
For the case where T MIX =½f SENS  (such as depicted in  FIG. 7 ), the average current value during a sensing half-cycle can be represented as:
 
                       I   MIX     =       2   ·     C   T     ·     V   TX     ·     f   SENS     ·   tanh     ⁢     1     4   ·     f   SENS     ·     R   RX     ·     (       C   T     +     C   RX       )             ,           (   2   )               
which simplifies to I MIX =2·C T ·V TX ·f SENS  (a linear result) as the term (R RX ·(C T +C RX )) approaches zero. Thus, the output current I OUT  should scale linearly with changes to sensing frequency f SENS . The measured capacitance may thus be determined as
 
               C   T     =         I   OUT       2   ·     V   TX     ·     f   SENS         .           
upon transitioning from a first sensing frequency f SENS,1  to a second sensing frequency f SENS,2 , the ratio of
 
               I     MIX   ,   1         I     MIX   ,   2             
equals the ratio of
 
                 f     SENS   ,   1         f     SENS   ,   2         .         
In other words, I MIX,1  may be linearly scaled by
 
               f     SENS   ,   2         f     SENS   ,   1             
to obtain I MIX,2 .
 
     As a baseline reference, if T MIX  is maintained constant with changes in sensing frequency f SENS , the average current value ideally remains constant for each T MIX  window. Thus, the performance improvement for the case where a constant T MIX  is applied to Equation (1), over the case where T MIX =½f SENS  shown in Equation (2) can be represented as: 
                         Δ   ⁢           ⁢     I     MIX   ⁢           ⁢     (     eq   .           ⁢   2     )             Δ   ⁢           ⁢     I     MIX   ⁢           ⁢     (     eq   .           ⁢   1     )             =     2     1   -     e     -       T   MIX         R   RX     ·     (       C   T     +     C   RX       )                   ,     
     ⁢       where   ⁢           ⁢   Δ   ⁢           ⁢     I   MIX       =         I     MIX   ,   1       ·     [       f     SENS   ,   2         f     SENS   ,   1         ]       -     I     MIX   ,   2           ,           (   3   )               
where ideally ΔI MIX =0. Thus, applying a constant T MIX  in Equation (1) provides at least a factor of 2 improvement over the case shown in Equation (2). However, this also indicates that I MIX (eq. 1)  will not remain constant for changes in sensing frequency f SENS , as the sensor electrode may not be fully settled causing the initial conditions of the sensor electrode to change with different sensing frequencies f SENS .
 
     In some embodiments, different T MIX  values are applied for different sensing frequencies f SENS . To force ΔI MIX =0, the following relationship must be satisfied: 
                       T     MIX   ,   1       =       T     MIX   ,   2       -     2   ·     R   RX     ·     C   RX     ·     ln   [     1   +     α   ·     (     1   -     e     -       T     MIX   ,   2           R   RX     ·     (       C   T     +     C   RX       )               )         ]           ,           (   4   )             where                           1   +   α     =         1   +     tanh   ⁡     (     1     4   ·     f     SENS   ,   2       ·     R   RX     ·     (       C   T     +     C   RX       )         )           1   +     tanh   ⁡     (     1     4   ·     f     SENS   ,   1       ·     R   RX     ·     (       C   T     +     C   RX       )         )           .             (   5   )               
Therefore, a desired linear relationship between I MIX,1  and I MIX,2  can be achieved by applying different T MIX  values for different sensing frequencies f SENS . While the analysis provided above is in terms of a single-pole model (i.e., assuming that R TX =0 and C TX =0), the performance of an exemplary distributed RC model in which different T MIX  values are applied for different sensing frequencies f SENS  is illustrated further in  FIG. 12 . Further, in some embodiments a desired gain value A may be applied to the input current I IN  (illustrated in plot  820 ) via the current conveyor.
 
       FIG. 9  is a schematic diagram of a sensing implementation  900  (or “implementation”) for operating one or more switching elements at different sensing frequencies, according to embodiments described herein. The implementation  900  is capable of use in conjunction with various embodiments discussed herein, such as the implementations  200 ,  300  of sensor electrodes discussed above with respect to  FIGS. 2 and 3  and the processing system depicted in  FIG. 4 . 
     The implementation  900  comprises the voltage source  505  that generates a capacitive sensing signal having a voltage waveform V TX . The voltage waveform V TX  may have any suitable shape, and the frequency of voltage waveform V TX  is controlled based on which predefined sensing frequency (e.g., f SENS,1 , f SENS,2 , f SENS,3 , . . . , f SENS,K  of  FIG. 4 ) is selected. The voltage source  505  drives the capacitive sensing signal onto a sensor electrode  510 , which is represented as a first-order model having a transmitter resistance R TX  and a transmitter capacitance C TX . Based on a transcapacitance C T  between sensor electrode  510  and sensor electrode  515 , the sensor electrode  515  receives resulting signals which are provided to receiver circuitry  905 . The sensor electrode  515  is also represented as a first-order model having a receiver resistance R RX  and a receiver capacitance C RX . However, the principles discussed herein also apply to more complex (e.g., distributed) modeling of the plurality of sensor electrodes. Further, although described in terms of a transcapacitive implementation having two separate sensor electrodes  510 ,  515 , the principles discussed herein also apply to absolute capacitive implementations in which the same sensor electrode(s) are used to transmit and receive capacitive sensing signals. 
     As shown, the receiver circuitry  905  comprises the amplifier  525  (or “op-amp”), the current conveyor (or “current mirror”)  530 , and the demodulator (or “mixer”)  535 . The input current I IN  represents the resulting signals received by the sensor electrode  515 , and is mirrored by the current conveyor  530  (with any suitable gain value A) as an output current I OUT . Based on a received demodulation signal, the demodulator  535  downconverts the output current I OUT  having higher (RF) frequencies to a demodulated current I MIX , which may be filtered by subsequent circuitry to have approximately direct current (DC) levels (i.e., having substantially no frequency component). As shown, a voltage source  540  generates the demodulation signal with a voltage waveform V MIX  that is based on a selected mixing period (e.g., T MIX,1 , T MIX,2 , T MIX,3 , . . . , T MIX,K  of  FIG. 4 ). 
     The demodulator  535  is generally a continuous-time demodulator having any suitable implementation, such as a square-wave mixer, harmonic rejection mixer, or sinusoidal mixer. In some embodiments, the demodulator  535  receives a three-level demodulation signal having a positive level, a negative level, and a zero level. 
     In implementation  900 , a first switching element S 1  is configured to selectively couple the voltage source  505  and the sensor electrode  510 , and a second switching element S 2  is configured to selectively couple the sensor electrode  515  and the receiver circuitry  905 . The first switching element S 1  is controlled by a control signal  915 - 1 , and the second switching element S 2  is controlled by a control signal  915 - 2 . In some embodiments, a calculation block  910  outputs a magnitude of the voltage waveform V MIX  as the control signals  915 - 1 ,  915 - 2 . Thus, in the case of a three-level demodulation signal, each of the positive level and the negative level corresponds to a conducting state of the switching elements S 1  and S 2 , and the zero level corresponds to a non-conducting state of the switching elements S 1  and S 2 . In this way, the receiver circuitry  905  operates the switching elements S 1 , S 2  to selectively couple the respective sensor electrodes  510 ,  515  into the sensing path beginning from the voltage source  505  and ending at the input to the receiver circuitry  905 . Beneficially, the switching elements S 1  and S 2  may be implemented within the digital domain without requiring relatively larger analog hardware. 
     In some embodiments, the switching elements S 1 , S 2  couple the respective sensor electrodes  510 ,  515  into the sensing path only during mixing periods of a sensing period, defined by a selected mixing period T MIX . In such embodiments, the value of mixing period T MIX  is constant for different sensing frequencies f SENS . Because switching elements S 1 , S 2  are closed (conducting) for a fixed amount of time (i.e., the mixing period T MIX ) during each sensing half-cycle, charge is conserved on the sensor electrodes and the amount of input current I IN  substantially remains the same across the different sensing frequencies f SENS . Assuming that R TX =0 and C TX =0, the average current value during a sensing half-cycle can be represented as: 
                     I   MIX     =           C   T     ·     V   TX         T   MIX       ·               [     1   +       e     -       T   MIX         R   RX     ·     (       C   T     +     C   RX       )             ·     (       e     -       T   MIX         R   RX     ·     (       C   T     +     C   RX       )             -   1     )         ]     ·     (     1   -     e     -       T   MIX         R   RX     ·     (       C   T     +     C   RX       )               )       ,                 (   6   )               
illustrating that to a first order, the average current value of I MIX  will be independent of the sensing frequency f SENS .
 
     While two switching elements S 1 , S 2  are depicted in implementation  900 , alternate implementations may include a single switching element within the sensing path. Further, while single sensor electrodes  510 ,  515  are shown as coupled with respective switching elements S 1 , S 2 , alternate implementations may have multiple sensor electrodes (e.g., a plurality of sensor electrodes within a predefined group) coupled with a single switching element. 
     In some embodiments, the number and location of the one or more switching elements are selected based on the time constants of the associated sensor electrodes  510 ,  515 . As is known to the person of ordinary skill in the art, the time constant associated with a particular sensor electrode depends upon both the sensor electrode resistance and the sensor electrode capacitance. For example, the time constant associated with sensor electrode  510  depends on the values of R TX  and C TX . For the case in which the time constant associated with the sensor electrode  510  is dominant (i.e., significantly larger) than that of the sensor electrode  515 , the implementation  900  operates switching element S 1  while maintaining an electrical connection between the sensor electrode  515  and the input to the receiver circuitry  905 . In some implementations, the switching element S 2  is omitted from the implementation  900  and replaced by a direct electrical connection. In other implementations, the control signals  915 - 1 ,  915 - 2  are independent and the control signal  915 - 2  causes the switching element S 2  to remain in a conducting state to form the electrical connection. 
     Similarly, in the case in which the time constant associated with the sensor electrode  515  is dominant, the implementation  900  may operate switching element S 1  independently of switching element S 2  to maintain an electrical connection between the voltage source  505  and the sensor electrode  510 , or may omit switching element S 1  entirely. In the case in which neither time constant is dominant (e.g., the time constants are on the same order), both switching elements S 1 , S 2  are included in the implementation  900  and may be operated using a same control signal for control signals  915 - 1 ,  915 - 2 . 
       FIG. 10  illustrates a method  1000  for operating one or more switching elements at different sensing frequencies, according to embodiments described herein. The method  1000  is capable of use in conjunction with various embodiments discussed herein, such as the processing system depicted in  FIG. 4  and the sensing implementation  900  of  FIG. 9 , or any other suitable processing system. 
     Method  1000  begins at block  1005 , where the processing system drives onto a first group of a plurality of sensor electrodes, a first capacitive sensing signal having a predefined first sensing frequency. At block  1015 , the processing system acquires, based on the driven first capacitive sensing signal, first capacitive measurements of resulting signals received by a second group of the plurality of sensor electrodes. The processing system applies a first demodulation signal having a predefined mixing period defined within a sensing period associated with the first sensing frequency. In some embodiments, the predefined first sensing frequency is selected based on one or more identified slowest sensor electrodes. In some cases, the predefined first sensing frequency is one of a plurality of predefined sensing frequencies, and the predefined mixing period is selected to fit within sensing half-cycles of the greatest (i.e., highest frequency) of the predefined sensing frequencies. 
     At block  1025 , the processing system operates, based on the first demodulation signal, one or more switching elements coupled with one or more sensor electrodes of the first group or the second group, wherein the one or more switching elements are in a conducting state during the mixing period. In this way, the processing system operates the one or more switching elements to selectively couple the respective sensor electrodes into the sensing path. 
     At optional block  1035 , the processing system drives, onto a third group of the plurality of sensor electrodes, a second capacitive sensing signal having a second sensing frequency different from the first sensing frequency. The third group may be the same as the first group, but this is not a requirement. In some cases, the transition between the first sensing frequency and the second sensing frequency is performed responsive to detected interference at the first sensing frequency. 
     At optional block  1045 , the processing system acquires, based on the driven second capacitive sensing signal, second capacitive measurements of resulting signals received by a fourth group of the plurality of sensor electrodes. The fourth group may be the same as the second group, but this is not a requirement. The processing system applies a second demodulation signal having the mixing period. Because the one or more switching elements are in a conducting state for a fixed amount of time (i.e., the predefined mixing period) during each sensing half-cycle, the average current value for input current to receiver circuitry of the processing system remains substantially the same for the first sensing frequency and the second sensing frequency. Method  1000  ends following completion of block  1045 . 
       FIG. 11  is a diagram  1100  illustrating exemplary operation of a sensing implementation using one or more switching elements, according to embodiments described herein. More specifically, diagram  1100  illustrates exemplary operation of the sensing implementation  900  of  FIG. 9 , or any other suitable sensing implementation. 
     Diagram  1100  includes a plot  1105  depicting a three-level demodulation signal (V MIX ) corresponding to a sensing frequency of 200 kHz. Plot  1110  depicts an input current I IN  resulting from driving one or more sensor electrodes at the 200 kHz sensing frequency. Within a sensing cycle  1120 - 1 , the positive level (i.e., 1V) of the demodulation signal is applied during a positive sensing half-cycle  1112 - 1 , and the negative level (i.e., −1V) of the demodulation signal is applied during a negative sensing half-cycle  1116 - 1 . Each of the positive sensing half-cycle  1112 - 1  and the negative sensing half-cycle  1116 - 1  correspond to a predefined mixing period T MIX . The sensing cycle  1120 - 1  also includes predefined periods  1114 - 1 ,  1118 - 1  (or “non-mixing periods”), during which the zero level of the demodulation signal is applied. As shown, the predefined periods  1114 - 1 ,  1118 - 1  have a substantially equal length, but this is not a requirement. According to embodiments discussed herein, switching element(s) that are coupled with the sensor electrode(s) are in a conducting state during each mixing period T MIX , and are in a non-conducting state during each non-mixing period  1114 - 1 ,  1118 - 1 . 
     Plot  1125  depicts the three-level demodulation signal V MIX  corresponding to a sensing frequency of 250 kHz, and plot  1130  depicts the corresponding input current I IN  at the sensing frequency. Similarly, plot  1135  depicts the three-level demodulation signal V MIX  corresponding to a sensing frequency of 300 kHz, and plot  1140  depicts the corresponding input current I IN . Within each of the sensing cycles  1120 - 2  (250 kHz),  1120 - 3  (300 kHz), the positive sensing half-cycles  1112 - 2 ,  1112 - 3  and negative sensing half-cycles  1116 - 2 ,  1116 - 3  each correspond to the predefined mixing period T MIX , while the predefined periods  1114 - 2 ,  1118 - 2 ,  1114 - 3 ,  1118 - 3  are shortened as sensing frequency increases. Because the switching element(s) are in a conducting state for a fixed amount of time (i.e., the predefined mixing period T MIX ) during each sensing half-cycle, the average current value for the input current I IN  remains substantially the same across the different sensing frequencies. And because the average current value is controlled to be substantially the same for different sensing frequencies, any shift occurring in the capacitive baseline when transitioning between different sensing frequencies is partly or fully mitigated. 
       FIG. 12  is a graph  1200  illustrating changes in capacitance across different sensing frequencies, according to embodiments described herein. More specifically, graph  1200  illustrates simulation results for a distributed RC model of the plurality of sensor electrodes, and corresponding to a transcapacitance C T  of 6.4 picofarads (pF). Within graph  1200 , a plot  1205  represents a case in which the predefined mixing period equals half of the period corresponding to the particular sensing frequency (similar to  FIG. 7 ). Plot  1205  illustrates a substantially linear change in baseline capacitance over the depicted range of sensing frequencies (160 kHz to 195 kHz), which will be compensated by the processing system when transitioning between different sensing frequencies. 
     Plot  1210  represents an implementation having a constant mixing period applied across the range of sensing frequencies. As shown, and consistent with the discussion of Equation (3) above, applying a constant mixing period offers at least a factor of 2 improvement (i.e., reduction or mitigation) of the baseline capacitance shift, when compared with plot  1205 . Plot  1215  represents an implementation having switching elements selectively coupling sensor electrodes during mixing periods (such as implementation  900  of  FIG. 9 ), which provides an additional reduction or mitigation of the baseline capacitance shift. Plot  1220  represents an implementation having a variable mixing period that depends on the selected sensing frequency (similar to FIG.  8 ). A maximum value of the plot  1220  is approximately 5 femtofarads (fF), offering an additional reduction or mitigation of the baseline capacitance shift. 
     Thus, the embodiments and examples set forth herein were presented in order to best explain the embodiments in accordance with the present technology and its particular application and to thereby enable those skilled in the art to make and use the disclosure. 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 disclosure to the precise form disclosed. 
     In view of the foregoing, the scope of the present disclosure is determined by the claims that follow.