Mitigating spatially correlated noise in data from capacitive sensors

This disclosure generally provides an input device with a matrix sensor that includes a plurality of sensor electrodes arranged on a common surface or plane. Moreover, the input device includes routing traces that electrically couple the sensor electrodes to analog front ends (AFEs). Because of the spatial relationships between these electrical components, the sensor electrodes can be categorized into groups where each of the sensors in the group is affected by a common noise source. The capacitive measurements for the sensor electrodes in each of the groups are compared to a touch threshold to determine if an input object (e.g., finger or stylus) is proximate to the sensor electrodes. If the capacitive measurements are below the touch threshold, the input device calculates an offset value that compensates for the noise. After the offset is applied, the compensated capacitive measurements are used to generate a capacitive image.

FIELD OF THE INVENTION

This invention generally relates to electronic devices and mitigating noise in these devices.

BACKGROUND

BRIEF SUMMARY OF THE INVENTION

One embodiment described herein includes an input device that includes a plurality of sensor electrodes in a sensing region of the input device where the plurality of sensor electrodes is categorized into respective groups of sensor electrodes where sensor electrodes in each of the groups are affected by a common noise source. The input device also includes a processing system coupled to the plurality of sensor electrodes configured to determine resulting signals for the sensor electrodes in a first group of the groups of sensor electrodes and compare the resulting signals of the first group to a predefined touch threshold to determine whether an input object is proximate to the sensor electrodes in the first group. The processing system is configured to calculate at least one offset using the resulting signals based on a determination that the input object is not proximate to the sensor electrodes in the first group and compensate for an effect of the common noise source by adjusting the resulting signals using the at least one offset.

Another embodiment described herein is a processing system for performing capacitive sensing that includes an interface configured to couple to a plurality of sensor electrodes in a sensing region of an input device, where the plurality of sensor electrodes are categorized into a plurality of groups of sensor electrodes where sensor electrodes in each of the groups are affected by a common noise source. The processing system also includes at least one sensor module configured to determine resulting signals for the sensor electrodes in a first group of the groups of sensor electrodes and compare the resulting signals to a predefined touch threshold to determine whether an input object is proximate to the sensor electrodes in the first group. Moreover, the sensor module is configured to calculate at least one offset using the resulting signals based on a determination that the input object is not proximate to the sensor electrodes in the first group and compensate for an effect of the common noise source by adjusting the resulting signals using the at least one offset.

Another embodiment described herein is a method for performing capacitive sensing using a plurality of sensor electrodes in a sensing region of an input device. The method includes determining resulting signals for sensor electrodes in a first group of the plurality of sensor electrodes where the sensor electrodes in the first group are affected by a common noise source. The method includes comparing the resulting signals of the first group to a predefined touch threshold to determine whether an input object is proximate to the sensor electrodes in the first group and calculating at least one offset using the resulting signals based on a determination that the input object is not proximate to the sensor electrodes in the first group. The method also includes compensating for an effect of the common noise source by adjusting the resulting signals using the at least one offset.

DETAILED DESCRIPTION

The following detailed description is merely exemplary in nature and is not intended to limit the disclosure or its application and uses. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description.

Various embodiments of the present invention provide input devices and methods that facilitate improved usability. In one embodiment, the input device includes a matrix sensor with a plurality of sensor electrodes arranged on a common surface or plane. Moreover, the input device includes routing traces that electrically couple the sensor electrodes to one or more integrated circuits in a processing system. In turn, the integrated circuits include internal traces that couple the sensor electrodes to analog front ends (AFEs) for determining capacitive sensing measurements corresponding to the sensor electrodes.

Because of the spatial relationships between the routing traces, the internal traces in the integrated circuits, and/or the AFEs, the sensor electrodes can be categorized into groups where each of the sensors in the group are affected by a common noise source. In one example, the routing traces for a column of sensor electrodes in the matrix may be cross coupled (e.g., capacitively coupled). If noise is introduced onto one of the routing traces in the column, this noise is transferred to all the traces because of the capacitive coupling. In another example, the sensor electrodes in the same row (or half of a row) may be selectively coupled to the same AFE. For example, the input device may use a multiplexer to selectively couple each of the sensor electrodes to the same AFE during different time periods. If a circuit in or near the AFE introduces noise when measuring the capacitive sensing measurements, this noise is introduced into each of the sensor electrodes coupled to the AFE, albeit at different times. By understanding the topology and the noise sources of the input device, the sensor electrodes can be logically divided (i.e., categorized) into groups—e.g., a column, row, or half row—where each sensor electrode in a particular group is affected by a common noise source.

In one embodiment, the capacitive sensing measurements for the sensor electrodes in each of the groups are compared to a touch threshold to determine if an input object (e.g., finger or stylus) is proximate to the sensor electrodes. Typically, the capacitive measurements caused by a finger or stylus are much larger than measurements caused by noise sources in a display panel or integrated circuit. Thus, the touch threshold can be set at a level that ensures measurements exceeding the threshold are caused by an input device while measurements below the threshold are caused by noise, and not by the input object. Assuming the capacitive measurements for a group of sensor electrodes are below the touch threshold (i.e., the sensor electrodes are not proximate to an input object), the input device calculates an offset value that compensates for the noise. For example, the input device may average the measurements and then subtract this average (i.e., the offset) from the capacitive measurements of the sensor electrodes in the group in order to remove the noise from the capacitive measurements. The compensated capacitive measurements can then be used to generate a capacitive image for further processing.

InFIG. 1, the input device100is 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 objects140in a sensing region120. Example input objects include fingers and styli, as shown inFIG. 1.

The input device100may utilize any combination of sensor components and sensing technologies to detect user input in the sensing region120. The input device100comprises one or more sensing elements for detecting user input. As several non-limiting examples, the input device100may 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 device100, 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 capacitive implementations of the input device100, voltage or current is applied to create an electric field. Nearby input objects140cause 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 “mutual capacitance” (or “transcapacitance”) sensing methods based on changes in the capacitive coupling between sensor electrodes. In various embodiments, an input object140near the sensor electrodes alters the electric field between the sensor 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 sensor electrodes (also “transmitter electrodes” or “transmitters”) and one or more receiver sensor electrodes (also “receiver electrodes” or “receivers”). Transmitter sensor electrodes may be modulated relative to a reference voltage (e.g., system ground) to transmit transmitter signals. Receiver sensor electrodes may be held substantially constant relative to the reference voltage to facilitate receipt of resulting signals. A resulting signal may comprise effect(s) corresponding to one or more transmitter signals, and/or to one or more sources of environmental interference (e.g. other electromagnetic signals). Sensor electrodes may be dedicated transmitters or receivers, or may be configured to both transmit and receive.

InFIG. 1, a processing system110is shown as part of the input device100. The processing system110is configured to operate the hardware of the input device100to detect input in the sensing region120. The processing system110comprises parts of or all of one or more integrated circuits (ICs) and/or other circuitry components. For example, a processing system110for 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 system110also comprises electronically-readable instructions, such as firmware code, software code, and/or the like. In some embodiments, components composing the processing system110are located together, such as near sensing element(s) of the input device100. In other embodiments, components of processing system110are physically separate with one or more components close to sensing element(s) of input device100, and one or more components elsewhere. For example, the input device100may be a peripheral coupled to a desktop computer, and the processing system110may 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 device100may be physically integrated in a phone, and the processing system110may comprise circuits and firmware that are part of a main processor of the phone. In some embodiments, the processing system110is dedicated to implementing the input device100. In other embodiments, the processing system110also performs other functions, such as operating display screens, driving haptic actuators, etc.

For example, in some embodiments, the processing system110operates the sensing element(s) of the input device100to produce electrical signals indicative of input (or lack of input) in the sensing region120. The processing system110may perform any appropriate amount of processing on the electrical signals in producing the information provided to the electronic system. For example, the processing system110may digitize analog electrical signals obtained from the sensor electrodes. As another example, the processing system110may perform filtering or other signal conditioning. As yet another example, the processing system110may 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 system110may determine positional information, recognize inputs as commands, recognize handwriting, and the like.

FIG. 2shows a portion of an exemplary pattern of capacitive sensing pixels205(also referred to herein as capacitive pixels or sensing pixels) configured to sense in the sensing region120associated with a pattern, according to some embodiments. Each capacitive pixel205may include one of more of the sensing elements described above. For clarity of illustration and description,FIG. 2presents the regions of the capacitive pixels205in a pattern of simple rectangles and does not show various other components within the capacitive pixels205. In one embodiment, the capacitive sensing pixels205are areas of localized capacitance (capacitive coupling). Capacitive pixels205may be formed between an individual sensor electrode and ground in a first mode of operation and between groups of sensor electrodes used as transmitter and receiver electrodes in a second mode of operation. The capacitive coupling changes with the proximity and motion of input objects in the sensing region120associated with the capacitive pixels205, and thus may be used as an indicator of the presence of the input object in the sensing region120of the input device.

The exemplary pattern comprises an array of capacitive sensing pixels205X,Y (referred collectively as pixels205) arranged in X columns and Y rows in a common plane, wherein X and Y are positive integers, although one of X and Y may be zero. It is contemplated that the pattern of sensing pixels205may comprises a plurality of sensing pixels205having other configurations, such as polar arrays, repeating patterns, non-repeating patterns, non-uniform arrays a single row or column, or other suitable arrangement. Further, as will be discussed in more detail below, the sensor electrodes in the sensing pixels205may be any shape such as circular, rectangular, diamond, star, square, noncovex, convex, nonconcave concave, etc. As shown here, the sensing pixels205are coupled to the processing system110and utilized to determine the presence (or lack thereof) of an input object in the sensing region120.

In a first mode of operation, at least one sensor electrode within the capacitive sensing pixels205may be utilized to detect the presence of an input object via absolute sensing techniques. A sensor module204in processing system110is configured to drive a sensor electrode using a trace240in each pixel205with a modulated signal (i.e., a capacitive sensing signal) and measure a capacitance between the sensor electrode and the input object (e.g., free space or earth ground) based on the modulated signal, which is utilized by the processing system110or other processor to determine the position of the input object.

The various electrodes of capacitive pixels205are typically ohmically isolated from the electrodes of other capacitive pixels205. Additionally, where a pixel205includes multiple electrodes, the electrodes may be ohmically isolated from each other. That is, one or more insulators separate the sensor electrodes and prevent them from electrically shorting to each other.

In a second mode of operation, sensor electrodes in the capacitive pixels205are utilized to detect the presence of an input object via transcapacitance sensing techniques. That is, processing system110may drive at least one sensor electrode in a pixel205with a transmitter signal and receive resulting signals using one or more of the other sensor electrodes in the pixel205, where a resulting signal comprising effects corresponding to the transmitter signal. The resulting signal is utilized by the processing system110or other processor to determine the position of the input object.

The input device100may be configured to operate in any one of the modes described above. The input device100may also be configured to switch between any two or more of the modes described above.

In some embodiments, the capacitive pixels205are “scanned” to determine these capacitive couplings. That is, in one embodiment, one or more of the sensor electrodes are driven to transmit transmitter signals. Transmitters may be operated such that one transmitter electrode transmits at one time, or 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 effectively 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.

The sensor electrodes 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 pixels205.

In other embodiments, “scanning” pixels205to determine these capacitive coupling includes driving with a modulated signal and measuring the absolute capacitance of one or more of the sensor electrodes. In another embodiment, the sensor electrodes may be operated such that the modulated signal is driven on a sensor electrode in multiple capacitive pixels205at the same time. In such embodiments, an absolute capacitive measurement may be obtained from each of the one or more pixels205simultaneously. In one embodiment, the input device100simultaneously drives a sensor electrode in a plurality of capacitive pixels205and measures an absolute capacitive measurement for each of the pixels205in the same sensing cycle. In various embodiments, processing system110may configured to selectively drive and receive with a portion of sensor electrodes. 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, an operating mode of the sensing device and a determined location of an input device.

A set of measurements from the capacitive pixels205form a capacitive image (also capacitive frame) representative of the capacitive couplings at the pixels205as discussed above. 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 some embodiments, one or more of the sensor electrodes in the capacitive pixels205include one or more display electrodes used in updating the display of the display screen. In one or more embodiment, the display electrodes comprise one or more segments of a Vcom electrode (common electrodes), a source drive line, gate line, an anode electrode or cathode electrode, or any other display element. These display electrodes may be disposed on an appropriate display screen substrate. For example, the 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) 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, an electrode that is used as both a sensor and a display electrode can also be referred to as a combination electrode, since it performs multiple functions.

Continuing to refer toFIG. 2, the processing system110coupled to the sensing electrodes includes a sensor module204and optionally, a display driver module208. In one embodiment the sensor module comprises circuitry configured to drive a transmitter signal or a modulated signal onto and receive resulting signals with the resulting signals the sensing electrodes during periods in which input sensing is desired. In one embodiment the sensor module204includes a transmitter module including circuitry configured to drive a transmitter signal onto the sensing electrodes during periods in which input sensing is desired. The transmitter signal is generally modulated and contains one or more bursts over a period of time allocated for input sensing. The transmitter signal may have an amplitude, frequency and voltage which may be changed to obtain more robust location information of the input object in the sensing region. The modulated signal used in absolute capacitive sensing may be the same or different from the transmitter signal used in transcapacitance sensing. The sensor module204may be selectively coupled to one or more of the sensor electrodes in the capacitive pixels205. For example, the sensor module204may be coupled to selected portions of the sensor electrodes and operate in either an absolute or transcapacitance sensing mode. In another example, the sensor module204may be coupled to different sensor electrodes when operating in the absolute sensing mode than when operating in the transcapacitance sensing mode.

In various embodiments the sensor module204may comprise a receiver module that includes circuitry configured to receive a resulting signal with the sensing electrodes comprising effects corresponding to the transmitter signal during periods in which input sensing is desired. In one or more embodiments, the receiver module is configured to drive a modulated signal onto a first sensor electrode in one of the pixels205and receive a resulting signal corresponding to the modulated signal to determine changes in absolute capacitance of the sensor electrode. The receiver module may determine a position of the input object in the sensing region120or may provide a signal including information indicative of the resulting signal to another module or processor, for example, a determination module or a processor of the electronic device (i.e., a host processor), for determining the position of the input object in the sensing region120. In one or more embodiments, the receiver module comprises a plurality of receivers, where each receiver may be an analog front ends (AFEs).

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 combination electrode is driven for display updating, the combination electrode may also be driven for capacitive sensing. Or overlapping capacitive sensing and display updating may include modulating the reference voltage(s) of the display device and/or modulating at least one display electrode for a display in a time period that at least partially overlaps with when the sensor electrodes are configured 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 embodiment, the non-display update period may be referred to as a long horizontal blanking period, long h-blanking period or a distributed blanking period. In other embodiments, the non-display update period may comprise horizontal blanking periods and vertical blanking periods. Processing system110may be configured to drive sensor electrodes for capacitive sensing during any one or more of or any combination of the different non-display update times.

The display driver module208includes circuitry confirmed to provide display image update information to the display of the display device during non-sensing (e.g., display updating) periods. The display driver module208may be included with or separate from the sensor module204. In one embodiment, the processing system comprises a first integrated controller comprising the display driver module208and at least a portion of the sensor module204(i.e., transmitter module and/or receiver module). In another embodiment, the processing system comprises a first integrated controller comprising the display driver module208and a second integrated controller comprising the sensor module204. In yet another embodiment, the processing system comprises a first integrated controller comprising a display driver module208and one of a transmitter module or a receiver module and a second integrated controller comprising the other one of the transmitter module and receiver module.

FIG. 3is a schematic of an input device300illustrating the connection between processing system110and sensor electrodes310using traces315, according to one embodiment described herein. The input device300includes an electrode matrix array that establishes the capacitive sensing region120. In one embodiment, the sensor electrodes used to establish the sensing region120are integrated into a display. However, for simplicity, the source lines (and other display components) are not shown inFIG. 3.

The sensor electrodes310are each coupled to the processing system110(which may include one or more integrated circuited) using one of the traces315. The processing system110includes one or more interfaces coupled to the traces315which permit the processing system110to receive signals from, and transmit signals to, the sensor electrodes310. In this embodiment, each of the traces315is coupled to exactly one of the sensor electrodes310. Moreover, the traces315are illustrated as dotted lines to represent that the traces315are routed on a plane different from a plane on which the sensor electrodes310are disposed. For example, the traces315may be disposed on a different substrate (or on a different side of the same substrate) as the sensor electrodes310. Although not shown, the input device300includes respective vias that electrically connect the sensor electrodes310on a first plane to one of the traces315in a second plane. Moreover, the processing system110includes internals traces in an integrated circuit that couple the traces315to an AFE305. Stated differently, the processing system uses both internal traces as well as the external traces315to couple the sensor electrodes310to one of the AFEs305.

The AFEs305can be any circuitry configured to receive capacitive sensing signals and generate a capacitive sensing measurement. In one embodiment, each AFE305is a sensing module (e.g., sensor module204) that measures absolute capacitance (if performing absolute capacitance sensing) or receives resulting signals (if performing transcapacitance sensing) and generates the capacitive sensing measurement. In one example, the AFEs305include integrators that measure the change in capacitive sensing signals due to capacitive coupling between the input device300and an input object in the sensing region120. Each AFE305may include an analog-to-digital converter (ADC) for converting the analog signal outputted by the integrator into a digital capacitive sensing measurement. While the embodiments here specifically mention using an AFE to generate the capacitive sensing measurements, other hardware could be included, such as transmitters/drivers that drive signals for transcapacitive sensing. The drivers could also be used for absolute capacitance sensing.

Moreover, multiple sensor electrodes310can be coupled to the same AFE305. In one embodiment, all the sensor electrodes310in a column are coupled to the same AFE305using a multiplexer (MUX). Using four different time periods, the same AFE305can derive a capacitive sensing measurement for each of the sensor electrodes310in the column using, for example, either absolute capacitive sensing or transcapacitive sensing. In one embodiment, the AFE305coupled to the first column of sensor electrodes310(e.g., Group A) can determine a capacitive sensing measurement for one of the sensor electrodes310at the same time other AFEs305determine capacitive sensing measurements for sensor electrodes310in other columns (e.g., Groups B-D). The sensor electrodes310in a column not currently being sensed can be driven with a shield signal. The shield signals may comprise a substantially constant voltage or modulated voltage. In cases where the shield signal comprises a modulated voltage, the shield signal may be referred to as a guard signal. When performing transcapacitive sensing, a transmitter signal can be driven on three of the electrodes310in a column while the AFE305is coupled to the remaining sensor electrode310to measure resulting signals generated by the transmitter signals. Using a plurality of non-overlapping time periods, the AFE305can determine capacitive sensing measurements for each of the sensor electrodes310in the group by changing which sensor electrodes310is electrically coupled to the AFE305.

When routing the traces315between the sensor electrodes310and the processing system110, the traces315are within close proximity to each other as shown by trace group320. Because of this spatial relationship, the traces315are cross coupled (i.e., capacitively coupled) such that noise introduced on one of the traces315is transferred to the remaining traces315in the group320. For example, if switching a gate line in the display introduces noise onto the left most trace315, this noise is also transferred to the other three traces315in group320. As a result, each time the gate line is switched, noise is introduced onto the traces315which may be measured by the AFE305when performing capacitive sensing. In one embodiment, the noise source is transient or periodic such that the source affects all the capacitive sensing measurements for the sensor electrodes310in Group A. For example, a transient signal in a LCD display panel may generate noise in the topmost sensor electrode310in Group A each time the processing system110captures a capacitive sensing measurement for the sensor electrodes310in the group. Because the topmost sensor electrode is capacitively coupled to the other sensor electrodes in Group A as a result of the close proximity of the traces315in a routing plane or layer, the noise introduced onto the topmost sensor electrodes310affects the capacitive measurements measured when performing capacitive sensing using the other three sensor electrodes310in Group A. Thus, by identifying transient noise sources, a system designer can identify groups of sensor electrodes310in the sensing region120(e.g., Group A, Group B, Group C, etc.) that are affected by the noise source as a result of the spatial arrangement of the components in the device300. For example, while all the sensor electrodes310in Group A are affected by a transient noise source that affects one of the sensor electrodes310in the leftmost column, the sensor electrodes310in Groups B, C, and D are not.

In another embodiment, each of the traces315and sensor electrodes310in a column may be coupled to different AFEs305. As a result, the AFEs305can derive capacitive sensing measurement for the electrodes310in the column in parallel. Since the electrodes310in a column are sensed in parallel and the traces315are cross coupled, any noise introduced into one of the traces will be reflected in the capacitive sensing measurements for all the electrodes310in the column. As a result, the columns of sensor electrodes310can be grouped as shown.

In another embodiment, the sensor electrodes310may be grouped according to routing in the processing system110. For example, an integrated circuit in the processing system110may route traces in close proximity such that the traces are cross coupled similar to the traces315in group320. Thus, if one of the traces in the integrated circuit is affected by a transient noise source, this noise is transferred to the remaining cross coupled traces. For example, even if the traces315are not cross coupled (i.e., are not capacitively coupled) in the routing layer, the internal routing in the processing system110coupling the traces315to the AFEs305can be cross coupled. Because of this spatial relationship, the sensor electrodes310electrically coupled to the internal traces in the processing system can be grouped together. Thus, any periodic noise source (e.g., a circuit in the integrating circuit, an AFE, power supply, display driver, other capacitive sensing circuitry, and the like) affecting one of the internal traces can affect the capacitive measurements taken using all of the sensor electrodes310in the group because of the cross coupling between the internal traces. By identifying cross coupled traces in the processing system110, the sensor electrodes310can be categorized in groups in order to compensate for periodic (or transient) noise sources as described in detail below.

In another embodiment, the sensor electrodes310are grouped according to capacitive coupling between the electrodes310themselves. For example, the spacing between neighboring electrodes (i.e., electrodes that are directly adjacent) may mean that these electrodes are capacitive coupled such that noise introduced onto one of the sensor electrodes310is transferred to all its neighboring electrodes310. If the noise source is periodic and occurs when deriving capacitive sensing measurements for all the neighboring electrodes, the noise can be identified and removed as described below.

FIG. 3is just one exemplary configuration of the input device300. In other embodiments, the device300may include more or less sensor electrodes310in a column or row than the example shown. For example, a column may include eight sensor electrodes310arranged in a column in which case the processing system110may use two 4:1 muxes to couple four of the eight sensor electrodes310to one AFE305and the other four sensor electrodes310to another AFE305.

FIG. 4is a schematic of an input device400illustrating the connection between AFEs305to the sensor electrodes310, according to one embodiment described herein. In device400, sensor electrodes310in a row (rather than in a column) are selectively coupled to the same AFE305. In this example, the sensor electrodes310A and310B (e.g., the left half of the top row) are coupled via traces315A and315B to AFE305A. In contrast, sensor electrodes310C and310D (i.e., the right half of the top row) are coupled via traces315C and315D to AFE305C. The bottom row is arranged in a similar manner where the sensor electrodes310in the left half are coupled to AFE305B and the electrodes310in the right half are coupled to AFE305D. Using multiplexers405, the processing system110can control which of the sensor electrodes310are currently coupled to the AFEs305.

In one embodiment, the AFEs305coupled to sensor electrodes310in the left sides of the rows (i.e., AFEs305A and305B) are disposed in a left side of the processing system110—e.g., on a left side of an integrated circuit—while the AFEs305coupled to sensor electrodes310in the right sides of the rows (i.e., AFES305C and305D) are disposed in a right side of the processing system110. In one embodiment, AFEs305A and305B may be disposed in a different integrated circuit than AFEs305C and305D. Furthermore, although traces315are shown as routing in a vertical direction, in one example, AFEs305A and305B are disposed to the left of the sensor electrodes310, while AFEs305C and305D are disposed to the right of the electrodes310. Instead of routing vertically, the traces315can route horizontally to couple the electrodes310to the AFEs305. For example, the traces315coupled to the sensor electrodes310in the left sides of the rows route out to the left side of the input device400while the traces315coupled to the sensor electrodes310in the right sides of the row route out to the right side of the input device400. Like in input device300, the traces315may be routed on a separate layer or plane than the layer containing the sensor electrodes310.

As shown, the sensor electrodes310are categorized into logical groups (e.g., Groups A, B, C, and D) according to which AFE305the electrodes310are coupled. For example, when performing capacitive sensing, each AFE305may introduce noise into the capacitive sensing measurement. Because the amplitude or frequency of the noise may be unique to each AFE305, the sensor electrodes310are grouped according to which AFE305the sensor electrodes310are connected. For example, each time AFE305A derives capacitive sensing measurements for electrodes310A and310B, a circuit in AFE305A introduces the same noise into the measurements. Because this noise is periodic, the processing system110can compensate for the noise (i.e., remove the noise from the capacitive sensing measurements) as described below.

As shown inFIG. 4, the electrodes310can be grouped according to which circuits (e.g., AFEs305) they are electrically coupled to, in addition to the factors described inFIG. 3such as cross coupled electrical components in the sensing region and/or in the processing system110. In this example, the electrodes are grouped into half rows since each of the sensors electrodes310in a half of a row are selectively coupled to the same AFE305. However, if the entire row is selectively coupled to the same AFE305, then the electrodes310may be grouped into full rows. Alternatively, if a third of the sensor electrodes310in each row are coupled to the same AFE305, then each row may be subdivided into three groups.

Generally,FIGS. 3 and 4illustrate that the sensor electrodes310can be categorized into logical groupings by identifying periodic noise sources that affect the capacitance sensing measurements for all the sensor electrodes310in that group. However, the periodic noise source does not need to affect the capacitance sensing measurements equally. For example, referring back toFIG. 3, a noise source coupled to the traces315in group320may affect the capacitive sensing measurement of the sensor electrodes in Group A differently because of the different capacitances associated with the traces315in group320. Put differently, because the traces315in group320have different lengths, each trace315has a corresponding capacitance and/or resistance value which changes how the noise affects the capacitance sensing measurement for the corresponding sensor electrode310. As a result, the noise source may have a different effect on the capacitance sensing measurement for the topmost sensor electrode310than the bottommost sensor electrode310in Group A because of the different lengths of the respective traces315. In contrast, in other arrangements, the noise source may have substantially the same effect on the capacitive sensing measurements for the sensor electrodes310in a group. For example, inFIG. 4, the electrical characteristics of the electrical connections between the AFE305A and the sensor electrodes310A and310B may be substantially the same. Thus, if the AFE305A (or a circuit close by in the processing system110) is a noise source, the resulting noise affects the capacitive sensing measurements for electrodes310A and310B in substantially the same manner.

FIG. 5is a flowchart of a method500for compensating for noise introduced into a group of sensor electrodes, according to one embodiment described herein. At block505, the processing system identifies a group of spatially correlated sensor electrodes that are affected by a common noise source. The sensor electrodes may grouped using any of the techniques described above such as cross coupled traces in a display panel, cross coupled routes internal to an integrated circuit, cross coupled sensor electrodes, couplings to the same circuit (e.g., AFE), and the like. In one embodiment, the groupings of the sensor electrodes may be provided by a system designer. For example, by testing the input device, the system designer may identify a periodic noise source such as a transient signal in a LCD panel or a driver circuit in the processing system which injects noise into the capacitive sensing measurements. Using the topology of the input device, the system designer can identify which sensor electrodes are directly or indirectly coupled (e.g., via capacitive coupling) to the noise source. The system designer can inform the processing system of the noise source and the group of sensor electrodes affected by the source.

At block510, the processing system determines resulting signals for the sensor electrodes in the group. For example, using absolute or transcapacitive sensing, one or more AFEs in the processing system generate capacitive sensing measurements which are used as the resulting signals. In one embodiment, the resulting signals are digital values that represent a change in capacitance relative to a baseline measurement. In one example, Resulting signals indicate a change from the baseline measurement captured when no input object is interacting with the input device. Moreover, the resulting signals can be affected by the noise source. In another embodiment, the resulting signals are processed to determine an amount of interference caused by noise in the input device and may not be used to track input objects.

At block515, the processing system compares the resulting signals (e.g., capacitive sensing signals) to a touch threshold to determine if an input object (e.g., finger or stylus) is proximate to the sensor electrodes. Typically, the capacitive measurements caused by a finger are much larger than measurements affected only by noise sources in a display panel or an integrated circuit. Thus, in one embodiment, the touch threshold is set at a value that ensures resulting signals exceeding the threshold are caused by an input device while signals below the threshold are caused by noise and not by any input object.

If any one of the resulting signals for the group of sensor electrodes are above the threshold, method500proceeds to block520where the processing system determines that an input object is interacting with one of the sensor electrodes in the group. For example, referring toFIG. 3, the resulting signals for the topmost electrode in Group A may satisfy the touch threshold while the measurements for the other electrodes in Group A do not. Because at least one of the electrodes exceeds the threshold, the processing system skips the noise compensation process described below. That is, method500proceeds to block540where processing system uses the resulting signals for the group of electrodes to generate a capacitive image without compensating for the noise that may have been introduced by the common noise source. However, the noise compensation technique described below can also be used to alter the resulting signals measured on sensor electrodes that are above the touch threshold.

If the resulting signals are below the threshold, method500proceeds from block515to block525where the processing system determines that an input object is not interacting with any of the sensor electrodes in the group. Stated differently, at block525, the processing system determines that an input object is not proximate to the sensor electrodes in the group since the resulting signals do not exceed the predetermined touch threshold.

At block530, the processing system calculates at least one offset using the resulting signals of the group of sensor electrodes. Because the processing system has determined that an input object is not interacting with the sensor electrodes, then much of the difference between the baseline measurement and the resulting signals is attributable to the periodic noise source. That is, but for the noise source, the resulting signals should be close to zero, indicating no or very little change from the baseline measurement. However, the input object may still be the cause of some change in the resulting signals from the baseline measurement. Thus, in one embodiment, the offset indicates how much of the change from the baseline measurement is attributable to the noise source.

In one embodiment, the processing system averages resulting signals for the sensor electrodes in the group to determine the offset. For example, if the noise source affects the resulting signals equally, then averaging the resulting signals identifies the offset which estimates the amount of noise introduced into the resulting signals. Averaging the offset may be used in the arrangement shown inFIG. 4where noise introduced by the AFE305coupled to the electrodes310in the same group has substantially the same effect on the resulting signals. Generally, taking the average of the resulting signals estimates the amount of power injecting into the AFE from the noise source over time.

Other techniques for measuring the offset may be used if the noise source does not affect the resulting signals equally. For example, noise introduced on the traces315shown inFIG. 3(which have varying lengths) may have varying effects on the resulting signals for the sensor electrodes in the column. That is, even if the resulting signals were captured simultaneously, the noise source may inject more power into one of the resulting signals than another. As a result, the processing system may use a linear or non-linear function to calculate a customized offset for each of the sensor electrodes in the group. The function may model how the noise source affects the resulting signals depending on the particular electrical properties of the electrical connections between the sensor electrodes and the AFE (or AFEs) in the group. Thus, instead of having one offset, the processing system calculates multiples offsets depending on the particular capacitance or resistance corresponding to the sensor electrodes.

In one embodiment, the offset may be used to alter the resulting signals for sensor electrodes outside of the group. For example, the common noise source may affect a plurality of sensor electrodes, but the group of electrodes used to generate the offset may only be a subset of the plurality of sensor electrodes. Nonetheless, the input device can use the offset to adjust the resulting signals for all the sensor electrodes affected by the same noise source.

Moreover, in one embodiment, the offset is applied to the resulting signals that satisfy (e.g., are above) the touch threshold used at block515. That is, the sensor electrode may also be affected by the same noise source as the group of electrodes used to generate the offset. Thus, the input device can use the offset to adjust the resulting signals for electrodes outside the group that may currently be proximate to the input object.

At block535, the processing system compensates for the noise by subtracting the offset from the resulting signals of each of the sensor electrodes. If the offset is the same for all the electrodes in the group, the processing system subtracts the same offset from all the resulting signals. If, however, the offset is different for different sensor electrodes, the processing system subtracts the customized offsets from the corresponding resulting signals. After compensating for the effects of the noise source, the remaining value of the resulting signals represents the effect of the input object at the locations corresponding to the sensor electrodes in the group.

Method500then proceeds to block540where the processing system uses the compensated (and uncompensated if any) resulting signals to generate the capacitive image. Removing or mitigating the noise at the locations of the sensor electrodes that are not interacting with the input object provide a cleaner capacitive image relative to using solely uncompensated measurements. For example, rather than only looking for locations where the input object is proximate to the sensor electrodes, some capacitive sensing algorithms evaluate the amount of force applied by the input object or identify spacing between multiple input objects interacting with the sensing region. These algorithms may process the capacitive image to identify locations which sensor electrodes are not proximate to the input object. Put differently, it may be important to these algorithms to determine where the input object is, and where the input object is not, in the sensing region. Moreover, the algorithms may process the capacitive images to determine peaks and valleys of the resulting signals.

Using method500, the resulting capacitive images provide more accurate resulting signals for the locations in the sensing region that are not proximate to the input object as well as locations in the sensing region that are proximate to the input object. For example, if the group of electrodes is between two locations in the sensing region that are being simultaneously contacted by two fingers, removing the noise from the resulting signals derived from the group of electrodes provides a more accurate indicator that the input device is being contacted by at two different points rather than being contacted by one large input object (e.g., the palm of the hand). Similarly, if the processing system (or other hardware or software application) uses the difference between capacitive measurements in the image to determine the amount of force used to press the input object against the input device, this force measurement may be improved by using method500to remove the noise from the resulting signals where the input object is not touching, thereby providing a more accurate contrast between the resulting signals captured at the locations where the input object is touching and the resulting signals captured at the locations where the input object is not touching.

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 invention. However, those skilled in the art will recognize that the foregoing description and examples have been presented for the purposes of illustration and example only. The description as set forth is not intended to be exhaustive or to limit the invention to the precise form disclosed.