PATENT DOCUMENT

Publication Number: US-9377907-B2
Application Number: US-201213624718-A
Country: US
Kind Code: B2

Title: Self capacitance implementation method

Abstract:
A circuit for detecting a touch or proximity event on a touch input device is provided. The circuit is able to mitigate the effects that parasitic capacitance has on a self-capacitance touch sensor panel by injecting a signal into the sensing circuitry. The signal is adjusted until it calibrates the circuitry for the effects that parasitic capacitance imparts on the detection of touch or proximity events on a touch sensor panel.

Claims:
What is claimed is: 
     
       1. A system for detecting changes in self-capacitance indicative of a touch or proximity event on or near a touch electrode, the system comprising:
 a first circuit configured to receive a first stimulation signal and supply a first output signal in response to the first stimulation signal; 
 a touch electrode configured to experience a change in self-capacitance in response to the touch or proximity event; 
 a second circuit configured to receive both a signal indicative of the change in self-capacitance from the touch electrode and a second stimulation signal, and supply a second output signal in response to the signal indicative of the change in self-capacitance from the touch electrode and the second stimulation signal; and 
 a processor capable of adjusting the second stimulation signal when no touch or proximity event is detected on the touch electrode such that the first output signal and the second output signal are substantially identical. 
 
     
     
       2. The system of  claim 1 , wherein the processor is further capable of measuring a difference between the first output signal and the second output signal created by the touch or proximity event occurring on the touch electrode. 
     
     
       3. The system of  claim 1 , wherein the second stimulation signal is a voltage and wherein the processor is capable of adjusting the voltage of the second stimulation signal when no touch or proximity event is detected on the touch electrode, such that the first output signal and the second output signal are substantially identical, and then maintaining the adjusted voltage of second stimulation signal during touch or proximity event detection. 
     
     
       4. The system of  claim 1 , wherein the second stimulation signal is a current and wherein the processor is capable of adjusting the current of the second stimulation signal when no touch or proximity event is detected on the touch electrode, such that the first output signal and the second output signal are substantially identical, and then maintaining the adjusted current of second stimulation signal during touch or proximity event detection. 
     
     
       5. A touch sensor panel that includes a plurality of touch electrodes, each touch electrode of the plurality of touch electrodes incorporating the system of  claim 1 . 
     
     
       6. The system of  claim 1 , wherein the processor determines that no touch or proximity event is detected on the touch electrode based on a difference between the first output signal and the second output signal, the difference detected by the processor, and adjusts the second stimulation signal in response to the determination. 
     
     
       7. The system of  claim 1 , wherein the first circuit comprises a first amplifier configured to receive the first stimulation signal and supply the first output signal and the second circuit comprises a second amplifier configured to receive the signal indicative of the change in self-capacitance and the second stimulation signal and supply the second output signal. 
     
     
       8. The system of  claim 1 , wherein the first stimulation signal is a first type of signal, and the second stimulation signal is a second type of signal, different from the first type of signal. 
     
     
       9. The system of  claim 8 , wherein the first stimulation signal is a voltage signal and the second stimulation signal is a current signal. 
     
     
       10. A method for detecting a touch or proximity event on a touch electrode, the method comprising:
 stimulating a touch electrode such that a self-capacitance of the touch electrode changes in response to the touch or proximity event occurring on or near the touch electrode; 
 receiving a first stimulation signal at a first circuit, and generating a first output signal from the first circuit in response to the first stimulation signal; 
 receiving a second stimulation signal and a signal indicative of the self-capacitance of the touch electrode at a second circuit, and generating a second output from the second circuit in response to the second stimulation signal and the signal indicative of the self-capacitance of the touch electrode; and 
 adjusting the second stimulation signal when no touch or proximity event is occurring on the touch electrode such that the first output signal and the second output signal are substantially identical. 
 
     
     
       11. The method of  claim 10 , wherein the method further comprises measuring a difference between the first output signal and the second output signals created by the touch or proximity event occurring on the touch electrode. 
     
     
       12. The method of  claim 10 , wherein the second stimulation signal is a voltage and wherein adjusting the second stimulation signal when no touch or proximity event is occurring on the touch electrode includes adjusting the voltage of the second stimulation signal. 
     
     
       13. The method of  claim 10 , wherein the second stimulation signal is a current and wherein adjusting the second stimulation signal when no touch or proximity event is occurring on the panel includes adjusting the current of the second stimulation signal. 
     
     
       14. The method of  claim 10 , wherein adjusting the second stimulation signal is in response to a determination that no touch or proximity event is detected on the touch electrode based on a difference between the first output signal and the second output signal. 
     
     
       15. The method of  claim 10 , wherein the first circuit comprises a first amplifier configured to receive the first stimulation signal and supply the first output signal and the second circuit comprises a second amplifier configured to receive the signal indicative of the change in self-capacitance and the second stimulation signal and supply the second output signal. 
     
     
       16. The method of  claim 10 , wherein the first stimulation signal isa first type of signal, and the second stimulation signal is a second type of signal, different from the first type of signal. 
     
     
       17. The method of  claim 16 , wherein the first stimulation signal is a voltage signal and the second stimulation signal is a current signal. 
     
     
       18. A non-transitory computer readable storage medium having stored thereon a set of instructions for detecting a change in self-capacitance of a touch electrode in a touch sensor panel that when executed by a processor causes the processor to:
 stimulate a touch electrode such that a self-capacitance of the touch electrode changes in response to a touch or proximity event occurring on or near the touch electrode; 
 receive a first stimulation signal at a first circuit, and generate a first output signal from the first circuit in response to the first stimulation signal; 
 receive a second stimulation signal and a signal indicative of the self-capacitance of the touch electrode at a second circuit, and generate a second output from the second circuit in response to the second stimulation signal and the signal indicative of the self-capacitance of the touch electrode; and 
 adjust the second stimulation signal when no touch or proximity event is occurring on or near the touch electrode such that the first output signal and the second output signal are substantially identical. 
 
     
     
       19. The non-transitory computer readable storage medium of  claim 18 , that further causes the processor to measure a difference between the first output signal and the second output signal created by the touch or proximity event occurring on the touch electrode. 
     
     
       20. The non-transitory computer readable storage medium of  claim 18 , wherein the second stimulation signal is a voltage and wherein the processor further adjusts the voltage of the second stimulation signal when no touch or proximity event is occurring on the touch electrode. 
     
     
       21. The non-transitory computer readable storage medium of  claim 18 , wherein the second stimulation signal is a current and wherein the processor further adjusts the current of the second stimulation signal when no touch or proximity event is occurring on or near the touch electrode. 
     
     
       22. The non-transitory computer readable storage medium of  claim 18 , wherein the processor determines that no touch or proximity event is detected on the touch electrode based on a difference between the first output signal and the second output signal, the difference detected by the processor, and adjusts the second stimulation signal in response to the determination. 
     
     
       23. The non-transitory computer readable storage medium of  claim 18 , wherein the first circuit comprises a first amplifier configured to receive the first stimulation signal and supply the first output signal and the second circuit comprises a second amplifier configured to receive the signal indicative of the change in self-capacitance and the second stimulation signal and supply the second output signal. 
     
     
       24. The non-transitory computer readable storage medium of  claim 18 , wherein the first stimulation signal is a first type of signal and the second stimulation signal is a second type of signal, different from the first type of signal. 
     
     
       25. The non-transitory computer readable storage medium of  claim 24 , wherein the first stimulation signal is a voltage signal and the second stimulation signal is a current signal.

Description:
FIELD OF THE DISCLOSURE 
     This relates generally to the mitigation of a parasitic capacitance on a capacitive touch sensing panel to facilitate a greater reliability in the ability of the touch sensor panel to detect touch and proximity events over a wide range of distances. 
     BACKGROUND OF THE DISCLOSURE 
     Many types of input devices are available for performing operations in a computing system, such as buttons or keys, mice, trackballs, joysticks, touch sensor panels, touch screens, and the like. Touch screens, in particular, are becoming increasingly popular because of their ease and versatility of operation as well as their declining price. Touch screens can include a touch sensor panel, which can be a clear panel with a touch-sensitive surface, and a display device such as a liquid crystal display (LCD) that can be positioned partially or fully behind the panel so that the touch-sensitive surface can cover at least a portion of the viewable area of the display device. Touch screens generally allow a user to perform various functions by touching (e.g., physical contact or near-field proximity) the touch sensor panel using a finger, stylus or other object at a location often dictated by a user interface (UI) being displayed by the display device. In general, touch screens can recognize a touch event and the position of the touch event on the touch sensor panel, and the computing system can then interpret the touch event in accordance with the display appearing at the time of the touch event, and thereafter can perform one or more actions based on the touch event. 
     Mutual capacitance touch sensor panels can be formed from a matrix of drive and sense lines of a substantially transparent conductive material such as Indium Tin Oxide (ITO). The lines are often arranged orthogonally on a substantially transparent substrate. Mutual capacitance touch sensor panels not only have the ability to detect touch events on the touch sensor panels, but also have the ability to detect proximity events, in which an object is not touching the panel but is in close proximity to the panel. However, mutual capacitance touch pads are constrained in their ability to sense proximity events, and thus only provide proximity detection over a limited range of distances from the touch sensor panel. 
     SUMMARY OF THE DISCLOSURE 
     This relates to a touch sensor panel configured to mitigate the effect of a parasitic capacitance on a touch sensor panel and its ability to reliably detect touch and proximity events. The panel can be configured to include circuitry that is capable of mitigating parasitic capacitance by employing an analog front end that is configured to calibrate out the effects of parasitic capacitance on a self-capacitance touch input device by injecting a charge into the sensing circuitry so that the effects of parasitic capacitance are substantially mitigated. The analog front end can settings can be configured during a calibration mode such that a signal-to-noise ratio of the touch sensor is substantially unaffected by parasitic capacitance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 a    illustrates an exemplary mutual capacitance touch sensor circuit according to one disclosed example. 
         FIG. 1 b    illustrates an exemplary touch node and the effect that a finger touching the node has on coupled charge according to one disclosed example. 
         FIG. 1 c    illustrates another exemplary touch node and the effect that the absence of a finger has on coupled charge according to one disclosed example. 
         FIG. 2  illustrates an exemplary self-capacitance touch sensor circuit according to one disclosed example. 
         FIG. 3 a    illustrates an exemplary electrical circuit corresponding to a self-capacitance touch sensor electrode according to one disclosed example. 
         FIG. 3 b    illustrates an exemplary electrical circuit corresponding to a self-capacitance touch sensor electrode when a parasitic capacitance is present on the touch electrode according to one disclosed example. 
         FIG. 4 a    illustrates an exemplary relationship between Cself and parasitic capacitance when a hand or object is near the self-capacitance touch sensor panel according to one disclosed example. 
         FIG. 4 b    illustrates an exemplary relationship between Cself and parasitic capacitance when a hand or object is far from the self-capacitance touch sensor panel. 
         FIG. 5  illustrates a touch sense circuitry configuration that can be used to mitigate the effects of parasitic capacitance according to one example. 
         FIG. 6  illustrates another exemplary circuit to mitigate the effects of parasitic capacitance according to disclosed examples. 
         FIG. 7  illustrates an exemplary computing system including a touch sensor panel utilizing touch sensor common mode noise recovery according to one disclosed example. 
         FIG. 8 a    illustrates an exemplary mobile telephone having a touch sensor panel that includes a touch common mode noise recovery circuit and method according to one disclosed example. 
         FIG. 8 b    illustrates an exemplary digital media player having a touch sensor panel that includes a touch common mode noise recovery circuit and method according to one disclosed example. 
         FIG. 8 c    illustrates an exemplary personal computer having a touch sensor panel that includes a touch common mode noise recovery circuit and method according to one disclosed example. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description of examples, reference is made to the accompanying drawings which form a part hereof, and in which it is shown by way of illustration specific examples that can be practiced. It is to be understood that other examples can be used and structural changes can be made without departing from the scope of the disclosed examples. 
     This relates to a touch sensor panel which can have the ability to not only sense touch events but to also sense proximity events over a wide dynamic range of distances. The touch sensor panel hardware can be switched into various configurations depending on the range of distance that is desired to be sensed. Furthermore, the parasitic capacitance experienced by the touch sensor panel can be mitigated so that it does not act to distort the touch sensor panel&#39;s ability to detect touch and proximity events. Driven shielding can be employed to reduce parasitic capacitance, and the parasitic capacitance&#39;s effect on phase can be calibrated out to reduce its net effect on touch and proximity detection. 
     Although examples disclosed herein may be described and illustrated herein in terms of self-capacitance touch sensor panels, it should be understood that the examples are not so limited, but are additionally applicable to any capacitive touch sensor panel in which a wide dynamic range of detection is required. Additionally, although examples disclosed herein may be described and illustrated in terms of driven shielding being applied to one or more of a border trace area, a display area and electrodes, it should be understood that the examples are not so limited, but may be additionally applicable to any part of a touch input device which contributes parasitic capacitance to sense detection. Furthermore, although examples disclosed herein relate to a method of mitigating parasitic capacitance on a touch sensor panel, it should be understood that the examples are not so limited, but may be additionally applicable to any capacitive touch sensor device such as a capacitive trackpad. 
       FIG. 1 a    illustrates an exemplary touch sensor panel  100  according to some examples of the disclosure. Touch sensor panel  100  can include an array of touch nodes  106  that can be formed by a two-layer electrode structure separated by a dielectric material, although in other examples the electrodes can be formed on the same layer. One layer of electrodes can include a plurality of drive lines  102  positioned perpendicular to another layer of electrodes comprising a plurality of sense lines  104 , with each of the nodes  106  having an associated mutual capacitance  114  (also referred to as coupling capacitance), although in other examples, the drive and sense lines can be positioned in non-orthogonal arrangements. The drive lines  102  and sense lines  104  can cross over each other in different planes separated from one another by a dielectric. Each point in which a drive line  102  intersects a sense line  104  can create a touch node  106 . Thus, for example, a panel which contains for instance 20 drive lines  102  and 15 sense lines  104  will have 300 touch nodes available to detect touch or proximity events. 
     Drive lines  102  (also referred to as rows, row traces, or row electrodes) can be activated by a stimulation signal provided by respective drive circuits  108 . Each of the drive circuits  108  can include an alternating current (AC) or unipolar pulsatile voltage source referred to as a stimulation signal source. To sense touch event(s) on the touch sensor panel  100 , one or more of the drive lines  102  can be stimulated by the drive circuits  108 , and the sense circuitry  110  can detect the resulting change in the charge coupled onto the sense lines  104  in the form of a change in the amplitude of the coupled stimulation signal. The change in voltage amplitude values can be indicative of a finger or conductive object touching or in proximity to the panel. The detected voltage values can be representative of node touch output values, with changes to those output values indicating the node locations  106  where the touch or proximity events occurred and the amount of touch that occurred at those location(s). 
       FIG. 1 b    illustrates an exemplary touch node and the effect that a finger touching the node can have on coupled charge according to one disclosed example. When drive line  102  is stimulated by a signal, electric field lines  118  can form between drive line  102  and sense line  104  due to the mutual capacitance between the drive and sense line, and charge can be coupled from the drive line to the sense line. When a finger or conductive object  116  comes into contact or near proximity to the touch node  106  created by the intersection of drive line  102  and sense line  104 , the object can block some of the electric field lines and the amount of charge coupled between the drive and sense line can decrease, with some of the charge being coupled into the finger or object. This decrease in charge coupled onto sense line  104  from drive line  102  can be detected by sense circuitry  110 . 
       FIG. 1 c    illustrates an exemplary touch node and the effect that the absence of a finger can have on coupled charge according to one disclosed example. When finger  116  is removed from touch node  106 , the charge emanating from drive line  102  is no longer partially coupled into finger  116  and thus the amount of charge coupled into sense line  102  can increase. Finger  116  generally can only couple charge from the drive line  102  if it is touching or in near proximity to touch node  106  and blocking some electric field lines  118 . Once the finger  116  is moved away from touch node  106  and is a certain distance away from the node, then the charge is no longer coupled onto finger  116  and the touch sensor panel can no longer detect the presence of the finger and will not register a touch or proximity event. Thus, capacitive touch sensor panels which employ mutual capacitance to detect touch or proximity events often have a very limited range of distance over which the system can detect proximity events. 
     Touch sensor panels that employ self-capacitance to detect touch or proximity events can be used to detect the presence of a finger or object that is further away from the touch sensor panel than a panel which uses mutual capacitance.  FIG. 2  illustrates an exemplary self-capacitance touch sensor circuit  200  according to one disclosed example. Self-capacitive touch sensor panel circuit  200  contains electrodes  202  which are connected to sense circuitry  204  and have a self-capacitance to ground. When an object touches or is in close proximity with the electrode, an additional capacitance can be formed between the electrode and ground through the object, which can increase the self-capacitance of the electrode. This change in the self-capacitance of an electrode  202  can be detected by sensing circuit  204 . Changes in self-capacitance can be created when objects or fingers are further away from the touch panel, as opposed to mutual capacitance touch panels which require the finger or object to be either touching or in near proximity to panel in order to sense a touch or proximity event. Unlike mutual capacitance touch sensor  100 , each electrode of the circuit acts as a touch node, rather than the intersections of orthogonal electrodes. Thus in 20×15 electrode array, there are only 35 touch nodes. One skilled in the art will recognize that such a self-capacitance architecture can possess a touch resolution that is less than the mutual capacitance touch resolution (e.g., 35 nodes vs. 300 nodes). Since the self-capacitive architecture described above has a reduced spatial resolution when compared to a mutual capacitive touch sensor, a self-capacitive touch sensor panel may not be able to detect touch or proximity event location with as much accuracy or unambiguity as a mutual capacitive touch sensor panel. 
       FIG. 3 a    illustrates an exemplary electrical circuit corresponding to a self-capacitance touch sensor electrode and sensing circuit according to one disclosed example. Electrode  202  can have a self-capacitance  304  to ground associated with it. Touch electrode  202  can be coupled to sensing circuit  314  and stimulation source  302 . Stimulation source  302  can for example, be a current and voltage source and can act to provide a charge on electrode  202  for the purpose of measuring the self capacitance of the electrode. Sensing circuit  314  can include operational amplifiers  320  and  306 . Operational amplifier  320  can receive Vin at its non-inverting input and can output a voltage V 1 . Vin can be correlated with both Istim and self-capacitance  304 . Operational amplifier  306  can receive Vstim  308  at it&#39;s non-inverting input and can output a voltage V 2 . Both V 1  and V 2  can be supplied to subtractor  316  that can subtract V 2  from V 1  to produce Vout. Assuming a substantially stable Istim and Vstim, Vout can be correlated to the value of self-capacitance  304 . Thus Vout, after passing through band pass filter  312 , can be indicative of changes in self-capacitance  304 . The frequency response of band pass filter  312  can depend on the frequency composition of Vstim and Istim. In some examples band pass filter  312  can be coupled to an analog to digital converter, an integrator and a demodulator. When a finger or object approaches electrode  202 , the self-capacitance of the electrode  304  can change in response to the object or finger&#39;s presence. This change in self-capacitance can create a subsequent change in Vout. This change is Vout can be used to detect the change in self-capacitance caused by a touch or proximity event. 
       FIG. 3 b    illustrates an exemplary electrical circuit corresponding to a self-capacitance touch sensor electrode and sensing circuit when a parasitic capacitance is present on the touch electrode according to one disclosed example. Parasitic capacitance  318  can represent capacitances found on touch electrode  202  that can be derived from various sources on a device which employs a touch sensor panel. As an example, parasitic capacitance can be created by the interaction between the touch electrodes  202  and other circuitry of the device such as a display or other conductive plates that can exist within a device which employs a touch sensor panel. One of ordinary skill in the art will recognize that in a self-capacitance touch sensing system, parasitic capacitance  318  (Cb) will be in parallel to the self-capacitance  304  (Cf) as shown in  FIG. 3 b   . When two capacitors are in parallel they add together, thus the change in capacitance being measured by sense circuit  314  can be Cf+Cb, where Cself represents the signal of interest which is the self-capacitance of electrode  202 . Since sense circuit  314  detects a combination of self-capacitance  304  and parasitic capacitance  314 , the relationship between self-capacitance  304  and parasitic capacitance  318  can be important. 
     For instance, the magnitude of parasitic capacitance  314  in relation to the magnitude of self-capacitance  304  can have an effect on how accurately sense circuit  314  is able to detect changes in self-capacitance created by a finger or object in proximity to touch electrode  202 .  FIG. 4 a    illustrates an exemplary relationship between Cf and parasitic capacitance when a hand or object is near the self-capacitance touch sensor panel. As illustrated, when hand  402  is a short distance  410  from touch panel  404 , the magnitude of Cf (self-capacitance)  406  is larger than the magnitude of Cb (parasitic capacitance)  408 . Note that the magnitudes are illustrative and shown for purposes of relative comparison only, and are not intended to represent actual magnitudes. If Cf  406  is considered the signal of interest and Cb  408  is considered a noise source, then the touch sensor panel  404  can be said to have a good signal to noise ratio (SNR), thus making touch and proximity detection more reliable. 
       FIG. 4 b    illustrates an exemplary relationship between Cf and parasitic capacitance when a hand or object is far from the self-capacitance touch sensor panel. When hand  402  increases its distance  410  from touch panel  404 , the magnitude of Cf  406  can become much smaller since Cf is inversely proportional to the distance that an object is located from the touch sensor panel. While fluctuations in distance  410  can cause fluctuations in the magnitude of Cf  406 , Cb  408  can remain roughly constant. This means that as the hand  402  moves farther away from touch sensor panel  404 , the SNR of the sensor system can decrease. Eventually, when hand  402  is a certain distance  410  from touch sensor panel  404 , Cb  408  can be said to “drown out” Cf  406 . In other words, the magnitude of Cb  408  can be so great as compared to the magnitude of Cf  406  that touch sense circuitry  204  may no longer be able to detect changes in Cf. For instance in the circuit of  FIG. 3 b   , if Cb is substantially larger than Cf, any changes to Vout may be undetectable. 
     These relationships between Cf and Cb can mean that a self-capacitance touch sensor panel&#39;s performance over distance can be constrained by at least two factors: the distance  410  that a hand or object  402  is away from touch panel  404 , and the amount of parasitic capacitance  408  present on the touch sensor panel  404 . In order to achieve an acceptable SNR across a wide dynamic range of distance, parasitic capacitance&#39;s  408  effect on SNR can be reduced or eliminated so that its magnitude relative to Cf is small. 
     In some examples the sense circuitry used to detect changes in self-capacitance can only measure a constant total amount of charge. In such a scenario the Cb signal rather than acting as a noise source, can act as an offset signal to Cf. If the sense circuitry can only measure a constant total amount of charge, a larger Cb can mean that a smaller amount of the constant measureable charge will be allocated to Cf. Therefore a large Cb indirectly translates to a smaller signal, and hence a lower SNR. If Cb can be made smaller or appear smaller to the sense circuitry, a larger amount of the constant measureable charge can be allocated to Cf which translates to a larger signal and thus a higher SNR. 
     Thus, in order to increase the dynamic range within which proximity events can be detected, the effect that parasitic capacitance has on touch and proximity event detection can be mitigated in order to allow for errors in proximity event detection to be minimized.  FIG. 5  illustrates a touch sense circuitry configuration that can be used to mitigate the effects of parasitic capacitance according to one example. In order to mitigate the effect that parasitic capacitance has on a touch sensor panel, the value of Istim can be calibrated so that the value of Vout equals zero when no touch or proximity event is occurring on the panel. During a calibration procedure, the value of Vout can be detected by touch processor  520 . Touch processor  520  can include an analog to digital converter that can convert the detected Vout into a digital signal to be used by the touch processor. Touch processor  520  can detect the value of Vout during a calibration procedure, and send a signal to signal generator  522  to adjust the value of Istim. In order to mitigate the effect of parasitic capacitance on the panel, Vout can be made to equal zero or close to zero such that in the presence of a touch or proximity event, touch processor  520  can readily detect the change in Vout. In other words, by adjusting Istim such that Vout is zero, the parasitic capacitance will no longer “drown out” the self-capacitance signal, and thus a change in self-capacitance can be detected. In order to make Vout equal zero, the inputs to subtractor  316  should be equal. This means that the outputs of operational amplifiers  320  and  306  should be equal. In order to make the outputs of operational amplifiers  320  and  306  equal, the input to operational output  320 , Vin, should be equal to the input of operational amplifier  306 , Vstim. This relationship can be expressed mathematically as follows:
 
 V   out   =V   in   −V   stim   (1)
 
Vstim, as an example, can be expressed as:
 
 V   stim   =A   stim  sin(ω c   t+φ   1 )  (2)
 
In order for Vout to be equal to zero, Vin=Vstim. Vin can be expressed as:
 
                     V   in     =       ∫       I   stim     ⁢     ⅆ   t           C   b               (   3   )               
When Vin=Vstim, then Istim can be expressed as:
 
                     I   stim     =       C   b     ⁢       ⅆ     V   stim         ⅆ   t                 (   4   )               
When Istim is set to equal equation 4, then Vout becomes zero. If a finger or object is touching or in close proximity to electrode  202 , Vout can become:
 
     
       
         
           
             
               V 
               out 
             
             = 
             
               
                 - 
                 
                   C 
                   f 
                 
               
               ⁢ 
               
                 
                   V 
                   stim 
                 
                 
                   ( 
                   
                     
                       C 
                       f 
                     
                     + 
                     
                       C 
                       b 
                     
                   
                   ) 
                 
               
             
           
         
       
     
     Thus, as shown in equation 4, when Cf is zero, Vout can be zero. When Cf is non-zero (in other words a touch or proximity event in occurring on the panel), the value of Vout can be non-zero and proportional to the value of Cf. This is due to the fact that Cb is greater than Cf and therefore the denominator of the equation can be considered constant. During a calibration procedure, touch processor  520  can monitor Vout, and if Vout is not zero or substantially zero, touch processor  520  can command signal generator  522  to adjust the value of Istim. Once the value of Istim is set such that the value of Vout is zero when no touch or proximity event is occurring, the value of Istim can remain constant during a touch or proximity event detection mode. 
     The calibration procedure described above can be done at the time of manufacturing of the panel, or it can be done periodically during use of the touch sensitive device to account for changes in parasitic capacitance caused by thermal variation, for example. 
       FIG. 6  illustrates another exemplary circuit to mitigate the effects of parasitic capacitance according to disclosed examples. In this example, in order to obtain a Vout of zero when no touch or proximity event is detected, Vstim and Istim can be held constant by signal generator  606 , while a resistor  602  and a capacitor  604 , which are coupled to the non-inverting input of operation amplifier  306 , are adjusted in order to drive the outputs of operation amplifier  320  and  306  to be equal, thus driving Vout to be zero. One skilled in the art will recognize that current generator Istim can be replaced by a voltage generator and use the same calibration procedure in order to drive Vout to be zero. Resistor  602  and capacitor  604  can form an RC filter on the input of operation amplifier  306 . During the calibration procedure, the RC filter can be adjusted by adjusting resistor  602  and capacitor  604  so that the sensing frequency we of equation 2 is slightly higher than the pass band of the RC filter. By tuning the RC filter in this manner, changes in self-capacitance caused by a touch or proximity event can cause amplitude modulation on the excitation signal. 
       FIG. 7  illustrates exemplary computing system  700  that can include one or more of the examples described above. Computing system  700  can include one or more panel processors  702  and peripherals  704 , and panel subsystem  706 . Peripherals  704  can include, but are not limited to, random access memory (RAM) or other types of memory or storage, watchdog timers and the like. Panel subsystem  706  can include, but is not limited to, one or more sense channels  708  which can utilize operational amplifiers that can be configured to minimize saturation time, channel scan logic  710  and driver logic  714 . Channel scan logic  710  can access RAM  712 , autonomously read data from the sense channels and provide control for the sense channels including calibrating the sense channels for changes in phase correlated with a parasitic capacitance. In addition, channel scan logic  710  can control driver logic  714  to generate stimulation signals  716  at various frequencies and phases that can be selectively applied to drive lines of touch sensor panel  724 . In some examples, panel subsystem  706 , panel processor  702  and peripherals  704  can be integrated into a single application specific integrated circuit (ASIC). 
     Touch sensor panel  724  can include a capacitive sensing medium having a plurality of drive lines and a plurality of sense lines, although other sensing media can also be used. Each intersection of drive and sense lines can represent a capacitive sensing node and can be viewed as picture element (node)  726 , which can be particularly useful when touch sensor panel  724  is viewed as capturing an “image” of touch. Each sense line of touch sensor panel  724  can drive sense channel  708  (also referred to herein as an event detection and demodulation circuit) in panel subsystem  706 . The drive and sense lines can also be configured to act as individual electrodes in a self-capacitance touch sensing configuration. 
     Computing system  700  can also include host processor  728  for receiving outputs from panel processor  702  and performing actions based on the outputs that can include, but are not limited to, moving an object such as a cursor or pointer, scrolling or panning, adjusting control settings, opening a file or document, viewing a menu, making a selection, executing instructions, operating a peripheral device coupled to the host device, answering a telephone call, placing a telephone call, terminating a telephone call, changing the volume or audio settings, storing information related to telephone communications such as addresses, frequently dialed numbers, received calls, missed calls, logging onto a computer or a computer network, permitting authorized individuals access to restricted areas of the computer or computer network, loading a user profile associated with a user&#39;s preferred arrangement of the computer desktop, permitting access to web content, launching a particular program, encrypting or decoding a message, and/or the like. Host processor  728  can also perform additional functions that may not be related to panel processing, and can be coupled to program storage  732  and display device  404  such as an LCD display for providing a UI to a user of the device. Display device  404  together with touch sensor panel  724 , when located partially or entirely under the touch sensor panel, can form touch screen  718 . 
     Note that one or more of the functions described above can be performed by firmware stored in memory (e.g. one of the peripherals  704  in  FIG. 7 ) and executed by panel processor  702 , or stored in program storage  732  and executed by host processor  728 . The firmware can also be stored and/or transported within any non-transitory computer-readable storage medium for use by or in connection with an instruction execution system, apparatus, or device, such as a computer-based system, processor-containing system, or other system that can fetch the instructions from the instruction execution system, apparatus, or device and execute the instructions. In the context of this document, a “non-transitory computer-readable storage medium” can be any medium that can contain or store the program for use by or in connection with the instruction execution system, apparatus, or device. The computer readable storage medium can include, but is not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus or device, a portable computer diskette (magnetic), a random access memory (RAM) (magnetic), a read-only memory (ROM) (magnetic), an erasable programmable read-only memory (EPROM) (magnetic), a portable optical disc such a CD, CD-R, CD-RW, DVD, DVD-R, or DVD-RW, or flash memory such as compact flash cards, secured digital cards, USB memory devices, memory sticks, and the like. 
     The firmware can also be propagated within any transport medium for use by or in connection with an instruction execution system, apparatus, or device, such as a computer-based system, processor-containing system, or other system that can fetch the instructions from the instruction execution system, apparatus, or device and execute the instructions. In the context of this document, a “transport medium” can be any medium that can communicate, propagate or transport the program for use by or in connection with the instruction execution system, apparatus, or device. The transport readable medium can include, but is not limited to, an electronic, magnetic, optical, electromagnetic or infrared wired or wireless propagation medium. 
       FIG. 8 a    illustrates exemplary mobile telephone  836  that can include touch sensor panel  824  and display device  830 , the touch sensor panel including circuitry to mitigate the effects of parasitic capacitance on a self-capacitance touch detection device. 
       FIG. 8 b    illustrates exemplary digital media player  840  that can include touch sensor panel  824  and display device  830 , the touch sensor panel including circuitry to mitigate the effects of parasitic capacitance on a self-capacitance touch detection device. 
       FIG. 8 c    illustrates exemplary personal computer  844  that can include touch sensor panel (trackpad)  824  and display  830 , the touch sensor panel and/or display of the personal computer (in examples where the display is part of a touch screen) including circuitry to mitigate the effects of parasitic capacitance on a self-capacitance touch detection device. The mobile telephone, media player and personal computer of  FIGS. 8 a , 8 b  and 8 c    can achieve a wider dynamic range of sensing capabilities by switching its configuration to detect near field and far field events, and mitigating parasitic capacitance. 
     Although  FIGS. 8 a - c    discuss a mobile telephone, a media player and a personal computer respectively, the disclosure is not so restricted and the touch sensor panel can be included on a tablet computer, a television, or any other device which utilizes the touch sensor panel including circuitry to switch between near field far field sensing configurations and mitigate the effects of parasitic capacitance on the touch sensor panel. 
     Although the disclosed examples have been fully described with reference to the accompanying drawings, it is to be noted that various changes and modifications will become apparent to those skilled in the art. Such changes and modifications are to be understood as being included within the scope of the disclosed examples as defined by the appended claims.

Metadata:
Filing Date: 20120921
Publication Date: 20160628
Grant Date: 20160628
Priority Date: 20120921
Inventors: SHAHPARNIA SHAHROOZ
Assignee: APPLE INC
CPC Classifications: [{"code": "G06F3/044", "inventive": true, "first": true, "tree": "[]"}, {"code": "G06F3/0418", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/0445", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/0418", "inventive": true, "first": true, "tree": "[]"}, {"code": "G06F3/0443", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/0445", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/0418", "inventive": true, "first": true, "tree": "[]"}, {"code": "G06F3/0443", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 50338369