Abstract:
Touch screen controllers and methods are presented for removing charger noise and other high frequency noise from touch screens in the presence of aliasing in which a digital low pass filter rejects the high frequency noise, a noise tracker determines whether noise is being aliased into the low pass filter pass band, and a noise shaper artificially induces or modifies aliasing in the system by adjusting an analog-to-digital converter sampling frequency and/or a panel scan frequency to try to move the aliased noise outside the low pass filter pass band.

Description:
FIELD OF THE INVENTION 
     The present disclosure relates to control apparatus and methods for operating touchscreen controllers to interface a touchscreen device with a host system. 
     BACKGROUND 
     Touchscreens have gained wide usage in a variety of applications such as smart phones, tablets, portable navigation devices, laptop and desktop computers, portable music players and other general user interfaces and consumer electronic devices, allowing users to intuitively select from prompted options and to perform other user interface operations by simply touching a display screen and a variety of ways. Different touchscreen technologies have been developed, including capacitive, surface capacitance, projected capacitance, resistive, surface acoustic wave touchscreens and others. Touchscreen controllers interface a touchscreen device and a host system, such as a microprocessor of a touchscreen-enabled tablet computer, and can be operated to detect actuations or touches of different locations on the display screen by a user. The touchscreen device is typically configured as an array of rows and columns, and user touches are detected by the controller providing excitation signals to row connections while sensing column connections of the touchscreen device in a panel scanning operation. As with other circuitry in battery-powered portable user devices, touchscreen controller performance and scanning the touchscreen panel can be affected by noise, particularly when connected to battery chargers, where chargers can couple noise into touchscreen devices thereby impacting the ability to detect user touch actuations. The frequency of touch signals are typically close to DC, such as about 10 Hz or less, and theoretically can be separated from the higher frequency charger noise by low-pass filtering. However, the separation is difficult in practice due to aliasing in a multi-rate sampling procedure involving analog-to-digital conversion sampling rates and panel scan rates. In a typical configuration, the touchscreen controller includes one or more analog-to-digital converters operating at a relatively high sampling frequency, with the panel scan operation being performed at a lower frequency. In practice, situations arise in which noise associated with charger operation is aliased into the pass band of a low pass filter, making it difficult to differentiate aliased noise from user touch occurrences. Accordingly, a need remains for improved touchscreen controllers and methods for addressing charger noise and other sources of high-frequency noise. 
     SUMMARY 
     Touchscreen controllers and methods are provided in which charger noise and other high frequency noise is selectively shaped to mitigate aliasing of the noise into the pass band of a low pass filter, thereby enhancing the ability to identify user touch occurrences. Digital processing circuitry detects whether noise is being aliased into a digital low pass filter pass band, and aliasing is modified by adjusting the analog-to-digital converter sampling frequency and/or a panel scan frequency to facilitate removal of the aliased noise from the low pass filter pass band. In this manner, if the current sampling and/or panel scan frequencies inhibit the ability of the low pass filter to effectively remove charger and other high-frequency noise, the controller can automatically reconfigure the scanning operation to facilitate user device operation while charging. 
     A touchscreen controller is provided, including an analog-to-digital converter operating at a sampling frequency to provide digital samples according to input signals received by controller sensing circuitry, and a digital low pass filter operating to filter the digital samples and to provide filtered digital samples for detecting user actuation of one or more portions of the touchscreen. The controller further includes a noise tracker which computes a first set of statistics corresponding to the digital samples output by the analog-to-digital converter and a second set of statistics corresponding to the filtered digital samples from the digital low pass filter. If the statistics indicate that noise is aliased into the pass band of the digital low pass filter, a noise shaper of the controller selectively adjusts the analog-to-digital converter sampling frequency and/or a panel scan frequency. In this manner, the controller facilitates reduction in detected high frequency noise aliased into the low pass filter pass band through selective noise shaping. 
     The statistics before and after the low pass filter can be computed by the noise tracker in a temporal fashion or in a spatial fashion in various embodiments, and may be of various forms such as standard deviations, mean values, etc. In certain embodiments, the noise tracker computes the statistics in a temporal fashion for individual nodes of the touchscreen for a plurality of panels scans both before and after low pass filtering. The noise tracker in certain embodiments employs thresholds for the first and second sets of statistics, and determines whether elements of the first set of statistics exceed a first threshold and whether elements of the second set of statistics exceed a second threshold, where the thresholds may be the same in certain embodiments, and may be adjustable by the controller. In certain implementations, the noise tracker causes the noise shaper to selectively adjust the sampling frequency and/or panel scan frequency if, for a given touchscreen node, the corresponding value of the first set of statistics exceeds the first threshold value and the corresponding value of the second set of statistics exceeds the second threshold value. Spatial statistics may be used for noise tracking in various embodiments, corresponding to a plurality of the touchscreen nodes for a single panel scan, with first and second sets of statistics being computed for the digital values and compared to thresholds before and after the low pass filter, respectively. In this manner, the noise tracker can detect situations in which the low pass filter is not effective in rejecting high-frequency noise, and initiate noise shaping adjustment by the noise shaper. 
     In certain implementations, the process is repeated multiple times until the aliased noise is moved sufficiently out of the low pass filter pass band or until a predetermined number of values for the adjusted sampling frequency and/or panel scan frequency have been tried without success, in which case the controller may notify the host system. In further embodiments, moreover, the controller may selectively adjust one or both of the threshold values during the adjustment process, and discontinue noise shaping adjustment of the sampling frequency and/or panel scan frequency if the predetermined number of frequency values have been tried for different thresholds without changing the results of the threshold comparisons. 
     Methods are provided for mitigating a high-frequency noise aliasing in a touchscreen controller, as well as non-transitory computer readable mediums with computer executable instructions for performing the methods in accordance with further aspects of the disclosure. The methods involve sequentially obtaining a series of digital sample sets of input signals from a connected touchscreen at a panel scan frequency using an analog-to-digital converter, as well as filtering the digital samples using a digital low pass filter. The methods further involve computing a first set of statistics corresponding to the samples from the analog-to-digital converter and computing a second set of statistics corresponding to the filtered digital samples from the digital low pass filter, and selectively adjusting the sampling frequency and/or the panel scan frequency if the first and second sets of statistics indicate the presence of noise aliased into the pass band of the digital low pass filter. Certain embodiments of the methods include comparing the first and second sets of statistics with corresponding first and second threshold values and selectively adjusting the sampling frequency and/or the panel scan frequency if, for a given touchscreen node, the corresponding value of the first set of statistics is greater than the first threshold value and a corresponding value of the second set of statistics is greater than the second threshold value. 
    
    
     
       DESCRIPTION OF THE VIEWS OF THE DRAWINGS 
       The following description and drawings set forth certain illustrative implementations of the disclosure in detail, which are indicative of several exemplary ways in which the various principles of the disclosure may be carried out. The illustrated examples, however, are not exhaustive of the many possible embodiments of the disclosure. Other objects, advantages and novel features of the disclosure will be set forth in the following detailed description when considered in conjunction with the drawings, in which: 
         FIG. 1  is a schematic system diagram illustrating an exemplary controller interfacing a touch screen device to a host processor, with onboard noise tracking and noise shaping to remove charger noise from a digital low pass filter pass band according to one or more aspects of the present disclosure; 
         FIG. 2  is a flow diagram illustrating an exemplary method for noise shaping high frequency aliased noise in a touch screen controller according to further aspects of the disclosure; 
         FIG. 3  illustrates standard deviation graphs for node capacitance values of the touch screen in  FIG. 1  both before and after the digital low pass filter in the presence of noise with no user touches; 
         FIG. 4  illustrates standard deviation graphs for node capacitance values of the touch screen in  FIG. 1  in the presence of noise and user touching in a third column where high frequency battery charger noise is aliased into the low pass filter pass band and exceeds threshold levels both before and after the digital low pass filter; 
         FIG. 5  illustrates pre and post filter standard deviation graphs for node capacitance values of the touch screen in  FIG. 1  in the presence of noise and user touching in the third column after adjustment of the panel scan frequency to modify the noise aliasing; 
         FIG. 6  illustrates pre and post filter standard deviation graphs subsequent to further panel scan frequency adjustment by the touch screen controller of  FIG. 1  causing the standard deviation values after the low pass filter to fall below the threshold indicating successful automatic movement of charger noise outside the pass band of the digital low pass filter; and 
         FIG. 7  illustrates frequency spectrum graphs showing high frequency charger noise aliasing initially within the low pass filter pass band and subsequent noise shifting by the noise shaper in  FIG. 1  to move the aliased charger noise outside the low pass filter pass band. 
     
    
    
     DETAILED DESCRIPTION 
     One or more embodiments or implementations are hereinafter described in conjunction with the drawings, wherein like reference numerals are used to refer to like elements throughout, and wherein the various features are not necessarily drawn to scale. The present disclosure provides techniques and touchscreen control apparatus by which the adverse effects of aliased noise caused by battery chargers and other sources of high-frequency noise can be mitigated in an automated fashion through selective noise shaping implemented in a touchscreen controller in response to detection of aliased noise in a low pass filter pass band. The various techniques of the present disclosure can be employed in connection with controllers for interfacing any type of touchscreen device with a host system, including without limitation capacitive touchscreens. Moreover, the touchscreen controllers may be implemented in any suitable form, including without limitation integrated circuit touchscreen controllers for use in consumer products, industrial user interfaces, military equipment or other applications. 
       FIG. 1  illustrates an exemplary system  2  with a touchscreen device  4  having an array of nodes or spatial sampling points or locations  6  configured in rows and columns with corresponding electrical row connections  8   a  and column connections  8   b  to electrical couple with a touchscreen controller  10  for interfacing with a host processor  12 . In the illustrated example, the touchscreen device  4  is a capacitive touchscreen with 10 rows and 6 columns, where the array of 60 nodes  6  forms a touchscreen panel to register one or more types of single or multiple touch operations that may be performed by a user by physical interaction with a display screen of the device  4 . As is known, the touchscreen device  4  may include display components operable to display a variety of visual renderings to a user, and user actuation of one or more portions of the touchscreen  4  can be selectively detected by providing driver or excitation signals to the row connections  8   a  and sensing corresponding input signals from the column connections  8   b . In a capacitive touchscreen example, for instance, the device  4  includes a transparent window layer and a touch panel layer with interleaved rows and columns separated by a dielectric material, where an LED, LCD or other type of display panel renders visual images that can be viewed through the polarizing, touch panel and window layers (not shown). In this form of touchscreen  4 , user physical interaction with the transparent window layer changes electrostatic fields created by excitation signals on the row connections  8   a , and the controller  10  senses the input signals through connections  8   b  to assess one or more locations at which the user is pressing the touchscreen  4 . The system  2  may be powered by a battery  16 , such as for a portable user device (e.g., touchscreen-enabled laptop computer, smart phone, tablet, etc.), with a power interface  14  providing power from the battery  16  to the host processor  12  and the controller  10 , and alternatively allowing power to be provided from an external source via a charger  18  for charging the battery  16  and/or for operating the host processor  12  and the touchscreen controller  10 . As previously noted, however, battery chargers  18  and other sources in or near the system  2  may couple high-frequency noise into the touchscreen device  4 . 
     As further shown in  FIG. 1 , the exemplary touchscreen controller  10  includes an analog circuit  20  or analog front end including row driver circuitry  22  and column sensing circuitry  24  for interconnection with the rows and columns of the touchscreen device  4 , respectively. The column sensing circuitry  24  may include analog multiplexing by which multiple sensed column lines can be provided through an analog signal chain for ultimate conversion to digital values or samples. In the illustrated example, the column sensing circuit  24  provides an analog signal output to an analog filter  26  (e.g., an anti-aliasing low pass filter with a cutoff frequency of about 250 KHz in one example) whose output is provided to an adjustable sense amplifier circuit  28 . The amplifier output is provided as an input to an analog-to-digital converter (ADC)  30  which operates at a sampling frequency  34  (Fs) to provide digital samples  32  according to sampled input signals received by the sensing circuit  24 . Operation of the row drivers  24  and the column sensing circuit  24  is controlled by a panel scan controller  46  to implement panel scanning to sequentially obtain a series of sets of digital samples  32  at a panel scan frequency  48  (Fp), with the individual digital sample sets corresponding to a plurality of locations on the touchscreen  4 . 
     A digital processing circuit  36  receives the digital samples  32  from the analog-to-digital converter  30 , which can be any suitable programmed processor, logic circuit, etc. The processing circuit  36  is programmed or otherwise configured to implement the panel scan control functions of a scan controller  46  to operate the row drivers  22  and column sensing circuitry  24 , as well as to perform other functions for interfacing the touchscreen  4  with a host processor  12  by which user inputs to the touchscreen device  4  are relayed to the host  12 . In certain embodiments, moreover, one or more operating parameters of the analog circuit  20  may be programmable, such as configuration of one or more low pass filters in the analog filter circuitry  26 , a programmable gain and/or offset of the sense amplifier  28  and the sampling frequency  34  of the analog-to-digital controller  30 . The controller  10  in certain embodiments is operable in various different modes under control of the host processor  12 , such as idle or sleep modes, a panel scan mode, and a monitoring scan mode for low-power touch sensing to assess changes in self-capacitance of the touch screen columns for selective transition into the panel scan mode. The digital processing circuitry  36  further implements a digital low pass filter  38  with a pass band from DC to a cutoff frequency Fc which filters the digital samples  32  from the analog-to-digital converter  30  and provides filtered digital samples  40  for use in detecting user actuation of one or more portions of the touchscreen  4 . The filtered digital samples  40  are provided to a node, panel and I/O processing component  42  which implements I 2 C or other suitable form of communications via suitable connection  44  with the host processor  12 . 
     In accordance with one or more aspects of the present disclosure, moreover, the digital processing circuit  36  includes a noise tracker  50  and a noise shaper  58  configured to detect whether high-frequency noise is aliased into the pass band of the digital low pass filter  38  and to selectively adjust one or both of the sampling frequency Fs  34  of the analog-to-digital converter  30  and the panel scan frequency Fp  48  of the panel scan circuit  46 . A panel scan refers to sampling of multiple nodes or spatial sensing locations  6  of the touchscreen device  4 , and the panel scan frequency  48  is the reciprocal of the time between panel scans. As previously noted, the illustrated analog circuit  20  provides analog multiplexing or other suitable configuration of the column sensing circuit  24  in order to utilize a single analog signal chain with a single analog-to-digital converter  30 , and thus the panel scan circuitry  46  operates to selectively excite or drive individual rows via the road drivers circuit  22  in the connections  8   a  and to sense specific ones of the touchscreen columns via the column sensing circuit  24  and the connections  8   b , which scanning can be done by any suitable scanning algorithm to detect user actuation of specific spatial locations on the touchscreen  4 . 
     The noise tracker  50  in the illustrated embodiment provides a noise flag  52  which can be any suitable signal or value in order to cause the noise shaper  58  to selectively adjust one or both of the frequencies  34 ,  48  to facilitate reduction in detected aliasing of high-frequency noise into the pass band of the digital low pass filter  38 . In this regard, certain embodiments of the noise tracker  50  compute various statistics  54  based on the original digital samples  32  from the analog-to-digital converter  30  and on the filtered digital samples  40  from the digital low pass filter  38  (a first set of before LPF statistics  54   b  and a second set of after LPF statistics  54   a  in  FIG. 1 ). The noise tracker  50  selectively provides the noise flag  52  to the noise shaper  58  if the computed statistics  54  indicate the presence of noise aliased into the pass band of the digital low pass filter  38 . In this manner, the controller  10  selectively response to particular situations in which noise from the charger  18  or other high frequency noise source, although outside the pass band defined by the cutoff frequency Fc of the digital low pass filter  38 , is nevertheless being aliased into the pass band by operation of the panel scanning at the frequency Fp and the higher rate sampling at the sampling frequency Fs of the analog-to-digital converter  30 . In this regard, the sampling frequency Fs is typically much higher than the panel scan frequency Fp, and the frequencies Fs and Fp are significantly higher than the cutoff frequency Fc of the digital low pass filter  38 , where Fc is typically set to pass only low frequencies in the typical touch signal frequency range of about 10 Hz or less, with a single analog-to-digital converter  30  operating at a sampling frequency Fs up to around 4 MHz for sampling  60  nodes  6  of the touchscreen  4  for a panel scan frequency Fp of about 50-400 Hz. In practice, chargers  18  can couple noise ranging from 50 Hz to hundreds of kHz into the touchscreen device  4 , and inexpensive chargers  18  can couple high frequency voltages of 10 V peak-to-peak or more. 
     Absent countermeasures such as the intelligent noise tracking and shaping contemplated by the inventors, aliasing is commonly introduced into the node data due to the multi-rate sampling nature of touch screen controllers. Moreover, the inventors have appreciated that aliasing becomes of concern only in the presence of noise, with touch detection being inhibited if noise is aliased into the same spectral band as a touch signal. For example, the input signal is band limited to a maximum frequency Fmax by the anti-aliasing filter  26  in the analog circuit  20 , while the samples are obtained via the converter  30  at a rate of Fs, where Fs is typically much higher than Fmax causing aliasing according to the Shannon-Nyquist Sampling Theory. 
     Moreover, a second sampling occurs due to the panel scan, where each node can be sampled in a time-multiplexed fashion with the panel scan frequency or rate Fp typically being much lower than the converter sampling frequency Fs. In this case, signals in the [0, Fs/2] range will be aliased down to the [0, Fp/2] range, and it is possible that the data is aliased twice, first to the [0, Fs/2] range, and again into the [0, Fp/2] frequency range. 
     The noise tracker  50  and the noise shaper  58  can be employed to address such multiple aliasing situations as well as cases where aliasing happens only once (either due to sampling only, or due to panel scanning only). The exemplary noise tracker  50  computes the first set of statistics  54   b  corresponding to the digital samples  32  from the converter  30  (before the low pass filter  38 ) as well as the second set of statistics  54   a  corresponding to the filtered digital samples  40  after the low pass filter  38 . If the statistics  54  indicate the presence of noise aliased into the pass band of the digital low pass filter  38 , the noise tracker  50  activates the noise flag  52  to cause the noise shaper  58  to adjust one or both of the converter sampling frequency  34  (Fs) and/or the panel scan frequency  48  (Fp) used by the panel scan control component  46  to provide noise shaping. 
     Referring also to  FIG. 2 , a process or method  60  is illustrated for mitigating high-frequency noise aliasing in a touchscreen controller, which may be implemented via the noise tracker  50  and the noise shaper  58  in the controller  10 . Although the exemplary method  60  is illustrated and described in the form of a series of acts or events, it will be appreciated that the various methods of the disclosure are not limited by the illustrated ordering of such acts or events except as specifically set forth herein. In this regard, except as specifically provided hereinafter, some acts or events may occur in different order and/or concurrently with other acts or events apart from those illustrated and described herein, and not all illustrated steps may be required to implement a process or method in accordance with the present disclosure. The illustrated methods may be implemented in hardware, processor-executed software, processor-executed firmware, FPGAs, logic circuitry, etc. or combinations thereof, for example, in the digital processing circuit  36  of the controller  10  in order to provide the noise shaping functionality described herein. 
     The controller  10  performs panel scanning and sampling at  62  in  FIG. 2  using initial panel scan and sampling frequencies  48  and  34  (Fp and Fs), respectively, with the node, panel and I/O processing circuitry  42  using the collected filtered samples  40  to identify any discernible user touches of the touchscreen  4 . Any suitable processing algorithms can be used for detecting one or more user touches, and the controller  10  reports any identified touch inputs to the host processor  12  via the communications connection  44 . In one possible implementation, the processing circuitry  42  receives the sample values  40  and may perform adjustment by subtracting calibration data and identifying one or more peaks (e.g., the highest four peaks) for interpolation processing using a linear approximation to produce X and Y coordinates for a touch, with Z values reported as the magnitudes of the peaks used to interpolate the XY location (node location  6 ) on the touchscreen  4 . 
     At  64  in  FIG. 2 , the noise tracker  50  ( FIG. 1 ) obtains the node capacitance samples  32 ,  40  before and after the digital low pass filter  38  for the current panel scan operation, and computes displayed node capacitance statistics  54  both before and after the digital low pass filter at  66 , where the computed statistics may be temporal or spatial in various implementations. In the illustrated embodiment, the tracker  50  computes the first set of statistics  54   b  for the data samples  32  preceding (before) the digital low pass filter  38 , and computes the second set of statistics  54   a  for the data samples  40  following (after) the digital low pass filter  38 . In one non-limiting example, the noise tracker  50  computes the statistics in a temporal fashion as standard deviation values for individual nodes  6  of the touchscreen  4  for a plurality of panels scans both before and after low pass filtering, and the computation at  66  is an update of the statistics computed over an integer number of scans, such as 500. Other embodiments are possible in which different forms and types of statistics are computed at  66 , such as mean or average values. In the illustrated embodiment, moreover, the noise tracker  50  stores and maintains one or more threshold values  56 , and determines whether elements of the first set of statistics  54   b  individually exceed a first threshold  56   b , and also determines whether elements of the second set of statistics  54   a  exceed a second threshold  56   a . In various embodiments the threshold values  56   b  and  56   a  can be the same value, and the thresholds  56  can be adjustable by the controller  10 , automatically and/or under control of the host processor  12 . 
     At  68  in  FIG. 2 , the noise tracker  50  compares the display node capacitance statistical values  54   b  (before low pass filtering) and  54   a  (after low pass filtering) to the corresponding threshold values  56   b  and  56   a , and determines at  70  whether, for a given touchscreen node, the corresponding value of the first set of statistics  54   b  exceeds the first threshold value  56   b  and the corresponding value of the second set of statistics  54   a  exceeds the second threshold value  56   a . If not (NO at  70 ), the noise tracker  50  turns off the noise flag  52  (if previously turned on), and the process  60  returns to obtain further samples at  64  as described above. 
     Referring also to  FIG. 3 , graphs  80  and  82  illustrate exemplary node capacitance standard deviation statistical values  54   b  and  54   a  (from 0 through 100% of a predetermined range) for each of the 60 numbered nodes  6  of the touchscreen  4 , along with the corresponding threshold values  56   b  and  56   a  shown in dashed lines, where the numbered nodes  6  are grouped into six sets of nodes corresponding to the six columns of the exemplary touchscreen  4  (COL 1 , COL 2 , COL 3 , COL 4 , COL 5  and COL 6  in  FIG. 3 ). In the situation shown in  FIG. 3 , the standard deviation values  54   b  for all nodes are all well below the corresponding pre-low pass filtering threshold  56   b , and a similar situation is seen in the post-filtering standard deviation values  54   a  which are below the corresponding second threshold  56   a . In this situation, the user is not touching the touchscreen  4 , and the noise tracker  50  does not infer presence of aliased noise in the pass band of the digital low pass filter  38 . 
     Referring to  FIGS. 2 and 4 , the noise tracker  50  again obtains node capacitance samples both before and after the digital low pass filter  38  at  64  in  FIG. 2 , computes or updates the standard deviation statistics  54  at  66 , and compares the statistics before and after the digital low pass filtering with the corresponding threshold values  56  at  68 . Graphs  84  and  86  in  FIG. 4  illustrate a different statistical situation in which a user is actuating the touchscreen  4  and noise (e.g., from the charger  18  in  FIG. 1 ) is aliased into the pass band of the low pass filter  38  (see also graph  96  in  FIG. 7  below). As seen in the graph  84  of  FIG. 4 , the standard deviation values  54   b  for the nodes  20 - 30  of the third touchscreen column (COL  3 ) exceed the first threshold value  56   b  for the pre-filtering samples  32  from the converter  30 . Also, the post-filtering statistical values  54   a  for nodes  20 - 30  exceed the second threshold value  56   a  in graph  86 . In this case, the noise tracker  50  determines (YES at  70  in  FIG. 2 ) that the threshold noise level has been reached or exceeded both before and after the digital low pass filter  38 , and accordingly concludes that high-frequency noise is aliased into the pass band of the digital low pass filter  38 . A determination is made by the noise tracker  50  at  74  in  FIG. 2  as to whether a predetermined maximum number of adjustments have been attempted. When this is not the case (NO at  74 ), the noise tracker asserts the noise flag  52  and the noise shaper  58  adjusts one or both of the sampling frequency  34  (Fs) of the analog-to-digital converter  30  and/or the panel scan frequency  48  (Fp) used by the panel scan controller  46  at  78  in  FIG. 2 . 
     Referring also to  FIG. 5 , the process  60  is then repeated at  64 - 70  as described above, with the noise tracker  50  again determining whether a threshold noise level is seen both before and after the low pass filter  38  using the updated sampling frequency  34  and/or panel scan frequency  48 .  FIG. 5  shows graphs  88  and  90  in which the standard deviation values  54   b  before the digital low pass filtering are at roughly the same levels as in  FIG. 4 , but the adjustment of the sample frequency and/or panel scan frequency by the noise shaper  58  have reduce the post-filtering standard deviation values  54   a  for the touched nodes  20 - 30 . As seen in graph  90 , for example, these value still exceed the second threshold value  56   a , but are significantly reduced compared with those seen in the graph  86  of  FIG. 4 . At this point, since the threshold amounts of noise are still seen both before and after the digital low pass filtering (YES at  70  in  FIG. 2 ), the noise tracker again determines at  74  whether the maximum number of adjustments have been attempted at  74 , and if not (NO at  74 ), again asserts the noise flag  52  to cause the noise shaper  58  to adjust one or both of the frequencies  34  and/or  48 . This processing may be repeated a number of times until the aliased noise is moved sufficiently out of the low pass filter pass band or until a predetermined number of values for the adjusted sampling frequency and/or panel scan frequency have been tried without success (YES at  74  in  FIG. 2 ), in which case the controller  10  sends a noise flag to the host processor  12  at  76  in  FIG. 2 . 
     Referring also to the graphs  92  and  94  and  FIG. 6 , the further adjustment of one or both of the sampling frequency  34  and the panel scan frequency  48  in the illustrated example results in reduction of the post-filtering standard deviation values  54   a  below the second threshold value  56   a , as seen in the graph  94 . Accordingly, the noise tracker determines at  70  that the high-frequency noise from the charger  18  has now been successfully shaped out of the pass band of the low pass filter  38  (NO at  70  and  FIG. 2 ), and accordingly turns off the noise flag  52  at  72 . In this manner, the operation of the noise tracker  50  and the noise shaper  58  have successfully adjusted the aliasing of the system such that the adjusted operation of the digital-to-analog converter  30  and the panel scan controller  46  can better identify actual user touches of the touchscreen  4  in the presence of noise during operation of the battery charger  18 . As seen in  FIG. 2 , moreover, the controller  10  proceeds in this operating mode, making selective adjustments as needed via the noise tracker  50  and the noise shaper  58  to automatically adapt to changing noise conditions in the system  2 . 
     The noise shaper  58  can implement any suitable form of adjustment to one or both of the operating frequencies  34  and  48  of the converter  30  and the panel scan controller  46 . In one non-limiting example, the noise shaper  58  causes the panel scan controller  46  to adjust the panel scanning frequency  48  by a certain amount (ΔFp), which is calculated based on Fp and Fs. Alternatively or in combination, the noise shaper  58  can send a certain change amount (e.g., ΔFs) to the converter  30 , and may calculate the change amount based on one or more operating parameters, the statistics  54 , or other variables. In another possible implementation, the noise shaper  58  is configured with an integer number of predetermined values for one or both of the frequencies  34  and/or  48 , and selectively makes adjustments (at  78  in  FIG. 2 ) in response to assertion of the noise flag  52  by the noise tracker  50 , and once a certain number of the values (or combinations of a set of frequency values  34  with a set of multiple frequency values  48 ) have been attempted without success (YES at  74  in  FIG. 2 ), the controller  10  may discontinue the selective noise shaping and take one or more reporting and/or remedial actions, such as sending a noise flag at  76  to the host processor  12 . The host  12 , in turn, may take one or more actions, such as notifying the user to disconnect the battery charger  18 , etc. 
     In certain embodiments, moreover, the controller  10  may selectively adjust (e.g., raise or reduce) one or both of the threshold values  56   b  and/or  56   a  as part of the selective noise shaping if the predetermined number of values for one or both of the sampling frequency  34  and the panel scan frequency  48  have been tried without changing the threshold comparison results, in order to further attempt to find a combination which successfully moves all or at least part of the high-frequency aliased noise from the pass band of the digital low pass filter  38 . Also, the noise tracker  50  may discontinue causing the noise shaper  58  to adjust one or both of the frequencies  34 ,  48  if the predetermined number of frequency values  34 ,  48  have been tried for different thresholds  56  without changing the results of the threshold comparisons. 
     Referring also to  FIG. 7 , a graph  96  illustrates a frequency spectrum of high frequency charger noise  97  aliasing initially within a pass band  98  of the digital low pass filter  38 . As noted above, the noise tracker  50  performs statistical analysis and threshold comparisons to detect whether the noise  97  is aliased inside the pass band  98  of the filter  38 , and if so causes the noise shaper  58  to adjust one or both of the frequencies  34 ,  48  to modify the system aliasing. Graph  99  in  FIG. 7  illustrates subsequent noise shifting by the noise shaper  58  leading to movement or shifting of the aliased charger noise  97  outside the low pass filter pass band  98 . 
     As previously noted, the noise tracker  50  may alternatively compute and employ spatial statistics  54  for noise tracking in various embodiments. In one possible example, the noise tracker  50  computes one or more mean values, standard deviation values, or other suitable statistical values  54  corresponding to a plurality of the touchscreen nodes  6  for a single panel scan, with first and second sets of statistics  54   b  and  54   a  being computed for the digital values  32  and  40 , respectively before and after the low pass filter  38 . For example, the noise tracker  50  may compute a single statistical value  54   b  for all the nodes  6  of the touchscreen  4  scanned in a single panel scan operation based on the pre-filtering samples  32  from the analog-to-digital converter  30 , and may similarly compute a single statistical value  54   a  for all the touchscreen nodes  6  in the same panel scan operation based on the post-filtering samples  40 . In this case, the noise tracker  50  compares the first value  54   b  with the first threshold value  56   b , compares the second statistical value  54   a  with the second threshold  56   a , and selectively causes the noise shaper  58  to adjust one or both of the frequencies  34 ,  48  if both the statistical values  54  exceed their corresponding threshold  56 . 
     The above examples are merely illustrative of several possible embodiments of various aspects of the present disclosure, wherein equivalent alterations and/or modifications will occur to others skilled in the art upon reading and understanding this specification and the annexed drawings. In addition, although a particular feature of the disclosure may have been disclosed with respect to only one of multiple implementations, such feature may be combined with one or more other features of other embodiments as may be desired and advantageous for any given or particular application. Also, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in the detailed description and/or in the claims, such terms are intended to be inclusive in a manner similar to the term “comprising”.