Patent Publication Number: US-8988384-B2

Title: Force sensor interface for touch controller

Description:
FIELD 
     This relates generally to input sensing and more particularly to integrating force sensing with touch sensing. 
     BACKGROUND 
     Many types of input devices are presently 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 sensitive devices, such as touch screens, in particular, are becoming increasingly popular because of their ease and versatility of operation as well as their declining price. A touch sensitive device can include a touch sensor panel, which can be a clear panel with a touch-sensitive surface, and, in some cases, 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. The touch sensitive device can allow a user to perform various functions by touching the touch sensor panel using a finger, stylus or other object at a desired location and, in the case of the display device, at a location often dictated by a user interface (UI) being displayed by the display device. In general, the touch sensitive device can recognize a touch or hover event and the position of the event at the touch sensor panel, and the computing system can then interpret the event and thereafter can perform one or more actions based on the event. 
     In addition to a touch sensor panel, some touch sensitive devices can include a button, which when contacted by a user can cause the device to change a state associated with the button. Pressing or selecting the button can activate or deactivate some state of the device. Not pressing or selecting the button can leave the device in its current state. In general, the touch sensitive device can recognize a press or force event at the button, and the computing system can then interpret the event and thereafter can perform one or more actions based on the event. 
     The use of multiple input mechanisms, such as a touch sensor panel and a button, can provide additional functionality for the user. On the other hand, each mechanism requires circuitry space and operating power, which can undesirably increase device size and decrease battery life. 
     SUMMARY 
     This relates to a force sensor interface in a touch sensitive device that can be coupled with the device&#39;s touch circuitry so as to integrate one or more force sensors with touch sensors of the device. The force sensor interface can include a transmit portion to transmit stimulation signals generated by the touch circuitry to the force sensors to drive the force sensors. The interface can also include a receive portion to receive force signals, indicative of a force applied to the device, from the force sensors for processing by the touch circuitry. This can allow the force sensors and the touch sensors to concurrently operate in an efficient and seamless manner. By the force sensors and the touch sensors sharing touch circuitry, the touch sensitive device can advantageously have multiple input mechanisms, i.e., touch sensors to detect touch or hover events and force sensors to detect force events, without undesirably increasing device size and power consumption. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an exemplary touch sensing circuit according to various embodiments. 
         FIGS. 2A and 2B  illustrate cross-sectional views of exemplary touch sensitive devices according to various embodiments. 
         FIGS. 3A and 3B  illustrate exemplary touch panels that can be used in a touch sensing circuit according to various embodiments. 
         FIGS. 4A and 4B  illustrate an exemplary force sensor that can be used in a touch sensing circuit according to various embodiments. 
         FIG. 4C  illustrates an exemplary noise model for a force sensor that can be used in a touch sensing circuit according to various embodiments. 
         FIG. 4D  is a graph showing noise as a function of frequency in an exemplary force sensor that can be used in a touch sensing circuit according to various embodiments. 
         FIG. 5  illustrates an exemplary transmit section of a touch controller having a force sensor interface according to various embodiments. 
         FIG. 6  illustrates another exemplary transmit section of a touch controller having a force sensor interface according to various embodiments. 
         FIG. 7  illustrates still another exemplary transmit section of a touch controller having a force sensor interface according to various embodiments. 
         FIG. 8  illustrates an exemplary receive section of a touch controller having a force sensor interface according to various embodiments. 
         FIG. 9  illustrates another exemplary receive section of a touch controller having a force sensor interface according to various embodiments. 
         FIG. 10  illustrates still another exemplary receive section of a touch controller having a force sensor interface according to various embodiments. 
         FIG. 11  illustrates yet another exemplary receive section of a touch controller having a force sensor interface according to various embodiments. 
         FIG. 12  illustrates an exemplary receive section of a touch controller having touch sensing circuitry according to various embodiments. 
         FIG. 13  illustrates a method for sensing force and touch at a touch controller having a force sensor interface according to various embodiments. 
         FIG. 14  illustrates an exemplary computing system for sensing force and touch according to various embodiments. 
         FIG. 15  illustrates an exemplary mobile telephone having force and touch sensing capabilities according to various embodiments. 
         FIG. 16  illustrates an exemplary digital media player having force and touch sensing capabilities according to various embodiments. 
         FIG. 17  illustrates an exemplary personal computer having force and touch sensing capabilities according to various embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description of example embodiments, reference is made to the accompanying drawings in which it is shown by way of illustration specific embodiments that can be practiced. It is to be understood that other embodiments can be used and structural changes can be made without departing from the scope of the various embodiments. 
     This relates to a force sensor interface in a touch sensitive device that can be coupled with the device&#39;s touch circuitry so as to integrate one or more force sensors with touch sensors of the device. The force sensor interface can include a transmit portion to transmit stimulation signals generated by the touch circuitry to the force sensors to drive the force sensors. The interface can also include a receive portion to receive force signals, indicative of a force applied to the device, from the force sensors for processing by the touch circuitry. By using touch stimulation signals to drive the force sensors, the modulating nature of the stimulation signals can advantageously reduce noise in the resultant force signals. Additionally, by sharing touch circuitry between the force sensors and the touch sensors, device power consumption and circuitry space can be advantageously reduced over devices having separate circuitry. 
       FIG. 1  illustrates an exemplary touch sensing circuit according to various embodiments. In the example of  FIG. 1 , touch sensing circuit  100  can include touch controller  110  having force sensor interface  112 . The circuit  100  can also include one or more touch sensors  126  and one or more force sensors  136 . The touch controller  110  can be coupled to the touch sensors  126  and can generate and transmit stimulation signals  106  to the touch sensors so as to drive the touch sensors to sense an object touching or hovering over the touch sensors. In some embodiments, the touch controller  110  can transmit multiple simultaneous stimulation signals  106  to the touch sensors to simultaneously drive the touch sensors  126  in a multi-stimulus configuration. In alternate embodiments, the touch controller  110  can transmit one stimulation signal  106  at a time to drive the touch sensors  126  in a single stimulus configuration. The touch controller  110  can also receive and process touch signals  103  from the touch sensors  126  indicative of the touching or hovering object. The touch controller  110  can similarly be coupled to the force sensors  136  via the force sensor interface  112  and can generate and transmit the stimulation signals  106  to the force sensors so as to drive the force sensors to sense a force applied by an object at the force sensors. The touch controller  110  can also receive and process force signals  105  via the interface  112  from the force sensors  136  indicative of the applied force. The interface  112  can be integrated into the touch controller  110  to use existing touch sensing circuitry with the force sensors  136  for seamless operation of all the sensors, which will be described in detail below. 
       FIGS. 2A and 2B  illustrate exemplary touch sensitive devices that can include the sensing circuit of  FIG. 1 . In the example of  FIG. 2A , touch sensitive device  200  can include cover  240  having a touchable surface that an object can hover over, touch, or press on. The device  200  can also include force sensors  136  disposed on a surface of the cover  240  opposite the touchable surface, although in other embodiments the sensors can be supported on another substrate adjacent to the cover. The force sensors  136  can sense the force applied by an object pressing on the touchable surface. The device  200  can further include touch sensors  126  disposed on the surface of the cover  240  opposite the touchable surface, or on another substrate adjacent to the cover. The touch sensors  126  can be adjacent to the force sensors  136 . In other embodiments, the touch sensors  126  can encircle the force sensors  136 . The cover  240  can be glass, plastic, or any suitable material capable of providing a substantially rigid substrate having a touchable surface. In the example of  FIG. 2B , the touch sensors  126  can be disposed on a surface of the cover  240  opposite the touchable surface, although in other embodiments the sensors can be supported on another substrate adjacent to the cover. The force sensors  136  can be disposed on the touch sensors  126 . 
       FIGS. 3A and 3B  illustrate exemplary touch panels that can be used in the sensing circuit of  FIG. 1  to sense a touching or hovering object.  FIG. 3A  illustrates a plan view of a touch panel, which in this case can be self capacitive. In the example of  FIG. 3A , touch panel  124  can have self capacitive electrodes  330 . The self capacitance of the electrodes  330  can be measured relative to some reference, e.g., ground. The electrodes  330  can be spatially separated elements, where each electrode can define a touch sensor (or pixel) of the panel  124 . The electrodes  330  can be coupled to a touch controller driving circuit (not shown) to drive the electrodes with stimulation signals (voltage Vstim, where Vstim can be a positive (+) phase signal Vstim+ or a negative (−) phase signal Vstim−) to sense an object touching at or hovering over the panel  124  and to a touch controller sensing circuit (not shown) to process touch signals (voltage Vo) indicative of the touching or hovering object. 
     When the object is proximate to the touch panel  124 , an electrode  330  can capacitively couple to the object, e.g., a finger, causing a capacitance to be formed between the electrode and the object. This can increase the self capacitance at the electrode. As the object gets closer to the panel  124 , the capacitance to ground can continue to increase and the electrode self capacitance can correspondingly increase. Thus, when the touch controller sensing circuit detects an increase in self capacitance of the electrode  330 , the increase can be interpreted as a touching or hovering object. 
       FIG. 3B  illustrates a plan view of an alternate touch panel, which in this case can be mutually capacitive. In the example of  FIG. 3B , touch panel  124  can have conductive rows  331  and columns  332  forming spatially separated drive and sense lines, respectively. Here, the conductive rows  331  and columns  332  can cross each other to form pixels  334  at the cross locations, where each pixel can define a touch sensor. Other configurations of the drive and sense lines are also possible, such as side by side. The conductive rows  331  can be coupled to a touch controller driving circuit (not shown) to drive the rows and the conductive columns  332  to a touch controller sensing circuit (not shown) to process signals indicative of the object touch or hover. 
     When the object is proximate to the touch panel  124 , the stimulated row associated with a pixel  334  can capacitively couple to the object, e.g., a finger, causing charge to be shunted from the stimulated row to ground through the object. This can reduce the mutual capacitance from the row to the column at the pixel  334 . As the object gets closer to the panel  124 , the amount of shunted charge can continue to increase and the mutual capacitance at the pixel  334  can correspondingly decrease. Thus, when the touch controller sensing circuit detects a drop in mutual capacitance at the pixel  334 , the drop can be interpreted as a touching or hovering object. 
     In an alternate embodiment, the conductive rows  331  and columns  332  in the mutual capacitive touch panel  124  of  FIG. 3B  can be replaced with conductive nodes to form the pixels  334 , where each node includes a pair of electrodes separated by a dielectric, one electrode being the drive electrode and the other electrode being the sense electrode. The conductive nodes can operate in a similar manner as the drive and sense lines to detect a drop in mutual capacitance indicative of a touching or hovering object. 
       FIG. 4A  illustrates a plan view of an exemplary force sensor that can be used in the sensing circuit of  FIG. 1  to sense force applied by an object. In this case, the force sensor can be a strain gauge. In the example of  FIG. 4A , strain gauge  136  can include traces  425  positioned closely together, but not touching, while at rest, i.e., while not strained or otherwise deformed. The strain gauge can have a nominal resistance in the absence of strain or force, such as 1.8KΩ+/−0.1%, and can change as a function of applied strain &amp; as follows.
 
Δ R=R   SG   ·GF·∈,   (1)
 
where ΔR=change in strain gauge resistance, R SG =resistance of undeformed strain gauge, and GF=gauge factor.
 
     As shown in the example of  FIG. 4B , the gauge  136  (illustrated as Rsg) can be included in wheatstone bridge  434  with three other resistors (illustrated as R 1 , R 2 , R 3 ) to sense resistance changes in the gauge (relative to the other resistors) indicative of the applied force. The bridge  434  can be coupled to a force sensor interface (not shown) to receive stimulation signals (voltage Vstim+ and Vstim−) from the touch controller (not shown) to drive the gauge  136  and to transmit force signals (voltage Vb+ and Vb−) indicative of the applied force to the touch controller for processing. Vstim+ and Vstim− can be defined as follows.
 
 V   STIM+ ( t )= V   STM0 ·sin(ω· t )+ V   CM ,  (2)
 
 V   STIM− ( t )=− V   STM0 ·sin(ω STM   ·t )+ V   CM ,  (3)
 
where Vstm 0 =amplitude of the force stimulation signal (mid-2-peak), ω STM =frequency of force stimulation signal in radians, and Vcm=common mode voltage. The force signals Vb+ and Vb− therefore can be as follows.
 
                       V     b   +       =           R   SG         R   1     +     R   SG         ·     V     STM   ⁢           ⁢   0       ·     sin   ⁡     (       ω   STM     ·   t     )         +     V   CM         ,           (   4   )                 V     b   -       =           R   3         R   2     +     R   3         ·     V     STM   ⁢           ⁢   0       ·     sin   ⁡     (       ω   STM     ·   t     )         +       V   CM     .               (   5   )               
With the substitution R 1 =R 2 =R 3 =R, Rsg=R+ΔR, where ΔR=R·GF·∈. The force signal Vb as a function of strain &amp; can therefore be as follows.
 
     
       
         
           
             
               
                 
                   
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     The bridge  434  can include input ports (illustrated as (1) and (2) in  FIG. 4B ), where one input port (1) can be coupled to a stimulation signal Vstim+ and the other input port (2) can be connected to switch  499  to switch between coupling to a stimulation signal Vstim− and coupling to ground. Although the bridge of  FIG. 4B  includes only one strain gauge, it is to be understood that up to four strain gauges can be used at the positions illustrated by R 1 , R 2 , R 3 , Rsg, where the resistance changes of the gauges can be used to sense the applied force. 
     When force is applied to a substrate to which the gauge  136  is attached (such as the cover of  FIGS. 2A ,  2 B), the substrate can bow, causing the gauge traces  425  to stretch and become longer and narrower, thereby increasing the gauge resistance. As the force being applied increases, the gauge resistance can correspondingly increase. Thus, when the touch controller sensing circuit detects a rise in resistance of the gauge  136 , the rise can be interpreted as a force being applied to the gauge. 
     In an alternate embodiment, the bridge  434  can be integrated with the touch controller  110 , where one or more of the resistors R 1 , R 2 , R 3  can be replaced with resistors in the touch controller. For example, resistors R 2 , R 3  can be replaced with resistors in the touch controller, such that the strain gauge  136  (illustrated as Rsg) and resistor R 1  can form the bridge  434  with touch controller resistors. This can reduce the space occupied by the bridge  434 . 
     A temperature change can adversely affect the gauge  136  because a temperature increase can cause the substrate to expand, without applied force, and consequently the gauge attached to the substrate to stretch. As a result, the gauge resistance can increase and be mistakenly interpreted as a force being applied to the gauge. To compensate for the temperature change, one or more of the resistors (R 1 , R 2 , R 3 ) in the bridge  434  can be replaced with a thermistor. The thermistor&#39;s temperature-induced resistance change can counteract the temperature-induced resistance change of the strain gauge due to thermal expansion in the substrate to which the strain gauge is attached, thereby, reducing temperature-induced changes in the force signal Vb. 
     In an alternate embodiment, a separate wheatstone bridge can be used to detect temperature changes while the gauge bridge  434  detects applied force, where the separate bridge can include the thermistor and three other resistors. Alternatively, the bridge can include multiple thermistors. For example, two thermistors can be used, where one thermistor can be in the position of Rsg and the other can be in the position of R 2  in  FIG. 4B . For this configuration, the force signal Vb can be maximum as Vb+ and Vb− change in opposite directions relative to a quiescent operating point, e.g., at ambient temperature. 
     Noise introduced by the bridge  434  can adversely affect the force sensor signals and the touch sensor signals.  FIG. 4C  illustrates an exemplary noise model for the bridge  434  indicating various noise sources (illustrated by voltage noise densities “ENZ”) that can undesirably introduce noise. In the example of  FIG. 4C , voltage noise density ENZ_TX can be a single-ended noise component introduced by the stimulation signals Vstim that drive the bridge  434 . Voltage noise density ENZ_RX can be an input referred noise density introduced by force sense amplifier(s)  443  of the force sensor interface. Voltage noise density ENZ_RB can be a noise density introduced by the bridge resistors (R 1 , R 2 , R 3 , Rsg). Device induced common mode voltage noise density ENZ_TX_CM can be capacitively coupled (illustrated by Cd) to drive lines that transmit the stimulation signals Vstim to the bridge  434 . Similarly, device induced common mode voltage noise density ENZ_RX_CM can be capacitively coupled (illustrated by CO with force sense lines that transmit the force signals Vb from the bridge  434 . 
     Advantageously, the differential configuration of the drive lines and the force sense lines at the bridge  434  can be relied upon to reduce the common mode voltage noise densities ENZ_TX_CM and ENZ_RX_CM. Additionally, in some instances, the contribution of ENZ_TX can be negligible as it translates to a noise component proportional to and below the force signals Vb, provided that the stimulation signal signal-to-noise ratio (SNR) is reasonably high. However, the remaining voltage noise densities ENZ_RX and ENZ_RB can limit the minimum force signal that can be resolved. Because this remaining noise can be a function of frequency, more specifically, can be dominant at lower frequencies (due to 1/f noise), by adjusting the frequency of the stimulation signals Vstim that drive the bridge  434 , this 1/f noise can be reduced, thereby allowing smaller force signals to be resolved and the force signal SNR to be boosted. 
       FIG. 4D  is a graph showing noise density as a function of frequency for the bridge of  FIG. 4C . Below a noise corner frequency, 1/f noise can be the dominant noise. Above the noise corner frequency, thermal noise can dominate. The frequency of the stimulation signals Vstim can be adjusted to be at or above the noise corner frequency so as to substantially reduce or eliminate the 1/f noise and boost the force signal SNR. Because the noise corner frequency is dependent on circuit performance, the noise corner frequency for the bridge  434  can be selected based on the performance of the bridge and associated circuitry. 
     Above the noise corner frequency, the input voltage noise density ENZ_IN at the bridge  434  can be approximately
 
 E   NZ     —     IN =√{square root over (4· K·T·R+ 2 ·E   NZ     —     AMP   2 )},  (7)
 
where K=Boltzman constant, T=temperature in Kelvin, R=bridge resistance (Rsg=R 1 =R 2 =R 3 ), and ENZ_AMP=input referred noise of force sense amplifier. The first term in Equation (7) is the voltage noise density ENZ_RB of each bridge resistor. In order to resolve the minimum force signal, the following condition may be met.
 
 V   B     —     RMS (Min)&gt; E   NZ     —     IN   ·f   BW ,  (8)
 
where Vb_rms=RMS signal amplitude from force sensor bridge at minimum strain &amp;, and f BW =touch/force sensor integration bandwidth.
 
       FIG. 5  illustrates an exemplary transmit section of a touch controller having a force sensor interface according to various embodiments. In the example of  FIG. 5 , transmit section  514  of a touch controller can generate and transmit stimulation signals to touch panel  124  and force sensor bridge  434  to drive them to sense touch and force respectively. The transmit section  514  can include transmit logic  501 , digital-to-analog convertor (DAC)  502 , and one or more drive channels that each includes an analog multiplexer  503  and corresponding touch buffer  505 . The transmit logic  501  can connect to a transmit numerically-controlled oscillator (NCO) (not shown) for phase and frequency digital data signals. The DAC  502  can convert the digital signals from the transmit logic  501  into analog stimulation signals Vstim to supply the multiplexers  503 . Vstim can be a positive (+) phase signal Vstim+ having a waveform at the same frequency as the transmit NCO. Vstim can also be a negative (−) phase signal Vstim− having the same waveform as Vstim+ inverted about a common voltage Vcm. The common voltage Vcm can also supply the multiplexers  503 . The multiplexers  503  can select Vstim+, Vstim−, or Vcm signals to supply the corresponding touch buffers  505  according to the control signals SEL. The touch buffers  505  can gain up the stimulation signals from the transmit DAC  501  and provide the drive capability to drive the touch panel  124  and the force sensor bridge  434 . 
     Force sensor interface  512  can be integrated with the transmit section  514  so that force sensor bridge  434  can be easily coupled via the interface to the touch sensitive device, thereby adding force sensing capability to the device. The force sensor interface  512  can couple one or more of the touch buffers  505  to the force sensor bridge  434 . In this example of  FIG. 5 , the force sensor interface  512  can include a connection from the output of the touch buffer  505  to an input port (illustrated by (1)) of the bridge  434  to supply the stimulation signal Vstim (either Vstim+ or Vstim−) from the buffer to the bridge. The other input port (illustrated by (2)) of the bridge  434  can couple to ground. This configuration can provide a single-ended stimulus input to the bridge  434 . Concurrently, the buffer  505  can supply the stimulation signal Vstim to the touch panel  124 . 
     In response to the stimulation signals, the touch panel  124  and the force sensor bridge  434  can generate touch and force signals respectively, as described previously. The touch and force signals can be transmitted to receive section  507  of a touch controller for further processing, as will be described in detail below. 
       FIG. 6  illustrates another exemplary transmit section of a touch controller having a force sensor interface according to various embodiments. The transmit section of  FIG. 6  is the same as the transmit section of  FIG. 5  except for the force sensor interface. In the example of  FIG. 6 , force sensor interface  612  can include a connection from the output of the touch buffer  505  to an input port (illustrated by (1)) of the bridge  434  to supply the stimulation signal Vstim (either Vstim+ or Vstim−) from the buffer to the bridge. The force sensor interface  612  can also include inverter  613  and force buffer  615  branching off the input to the touch buffer  505  to supply the stimulation signal Vstim to the inverter. The inverter  613  can invert the stimulation signal Vstim and feed the inverted stimulation signal to the force buffer  615 . The force buffer  615  can supply the inverted stimulation signal to the other input port (illustrated by (2)) of the bridge  434 . This configuration can provide a differential stimulus signal to the bridge  434 , where one port provides the stimulation signal Vstim and the other port provides the inverted stimulation signal. Concurrently, the touch buffer  505  can supply the stimulation signal Vstim to the touch panel  124 . 
       FIG. 7  illustrates still another exemplary transmit section of a touch controller having a force sensor interface according to various embodiments. The transmit section of  FIG. 7  is the same as the transmit section of  FIG. 5  except for the force sensor interface. In the example of  FIG. 7 , force sensor interface  712  can couple force buffers  615  (rather than the touch buffers  505 ) to the force sensor bridge  434 . The force sensor interface  712  can include multiplexer  703  to select Vstim+, Vstim−, or Vcm signals to supply the corresponding force buffers  615  according to the control signal SEL. A first of the force buffers  615  can supply the stimulation signal Vstim to an input port (illustrated by (1)) of the bridge  434 . A second of the force buffers  615  and inverter  613  can branch off the input to the first force buffer to supply the stimulation signal Vstim to the inverter. The inverter  613  can invert the stimulation signal Vstim and feed the inverted stimulation signal to the second force buffer  615 . The second force buffer  615  can supply the inverted stimulation signal to the other input port (illustrated by (2)) of the bridge  434 . This configuration can provide a differential stimulus signal to the bridge  434 , where one port provides the stimulation signal Vstim and the other port provides the inverted stimulation signal. Concurrently, one or more of the touch buffers  505  can supply the stimulation signal Vstim to the touch panel  124 . In this example, the touch buffers  505  can be separate and uncoupled from the force sensor interface  712 . 
       FIG. 8  illustrates an exemplary receive section of a touch controller having a force sensor interface according to various embodiments. In the example of  FIG. 8 , receive section  807  of a touch controller can receive and process force signals from force sensor bridge  434 . The receive section  807  can include force sensor interface  812  for receiving and preparing the force signals for processing. The force sensor interface  812  can include differential amplifier  843 , which can connect to the bridge  434  to receive force signals Vb, indicative of the applied force, from the bridge. Vb can be a positive (+) phase signal Vb+ and a negative (−) phase signal Vb−. The amplifier  843  can have common mode noise rejection to reduce common mode noise that can be present on the force signals Vb. In addition to common mode noise rejection, the amplifier  843  can gain up the dynamic force signals Vb to improve the dynamic range of the receive section  807 . The amplifier  843  can have either a static or a programmable gain according to the needs of the touch controller. 
     The equivalent single ended output signal Vo from the differential amplifier  843  as a function of strain ∈ and time t can be as follows. 
                         V   o     ⁡     (     ɛ   ,   t     )       =       (       V     o   +       -     V     o   -         )     =         R   F     R     ·     (       1     (     1   +     GF   ·   ɛ       )       -   1     )     ·     V     STM   ⁢           ⁢   0       ·     sin   ⁡     (       ω   STM     ·   t     )             ,           (   9   )               
where Rf=feedback resistor of differential amplifier, R=bridge resistors (R=Rsg=R 1 =R 2 =R 3 ), GF=gauge factor, V STM0 =force sensor stimulus, and ω STM =stimulus frequency in radians. The overall gain of the amplifier  843  can be a function of the dynamic range of differential analog-to-digital converter (ADC)  849  and the integration bandwidth. The following condition may be met.
 
                         G   AMP     ·     SNDR   ADC       &gt;     20   ·     log   ⁡     (       V   IN_ADC       V   B       )           ,           (   10   )               
where G AMP =gain of amplifier in decibels (dB), SNDR ADC =signal-to-noise and distortion ratio of ADC within the force/touch integration bandwidth in decibels (dB), V IN     —     ADC =dynamic RMS input range of ADC, and V B =RMS force sensor bridge signal. Similarly, the condition of Equation (10) may be met for the receive sections in  FIGS. 9 through 11 , described below.
 
     In some instances, the force sensor(s) and other resistors in the bridge  434  can be mismatched, thereby creating an error signal that can be propagated and amplified at the amplifier  843  and/or differential anti-aliasing filter (AAF)  847  so as to saturate the differential ADC  849 . To compensate for the mismatch, the force sensor interface  812  can include summers  840  and programmable attenuators  841  to perform offset subtraction. The attenuator  841 - a  can receive and attenuate the stimulation signal Vstim− so as to provide a fraction Voff− of the stimulation signal to the summer  840 - a . The summer  840 - a  can then subtract Voff− from the output signal of the amplifier  843 . Similarly, the attenuator  841 - b  can receive and attenuate the stimulation signal Vstim+ so as to provide a fraction Voff+ of the stimulation signal to the summer  840 - b . The summer  840 - b  can then subtract Voff+ from the output signal of the amplifier  843 . Because offset subtraction is performed on the output signals from the amplifier  843 , i.e., the force signals Vb after they have been gained up by the amplifier  843 , the offset subtraction can be performed at higher granularity and the offset mismatch compensated for. 
     The receive section  807  can also include the differential AAF  847 , differential ADC  849 , and touch processor  842 . The AAF  847  can receive the offset-compensated force signals Vout+, Vout− outputted from the summers  840  and can further reject noise in the force signals so as to prevent noise from aliasing back into the operating frequency range of the touch controller and to improve SNR of the force signals. In some embodiments, the AAF  847  can be a bandpass filter to reject noise outside of a particular band, e.g., noise far from the fundamental frequency of the force signals. In some embodiments, the AAF  847  can be a lowpass filter to reject high frequency noise. In some embodiments, the AAF  847  can include multiple filters for rejecting noise based on the needs of the device. The AAF  847  can have either a static or a programmable gain according to the needs of the touch controller. The ADC  849  can receive the noise-reduced force signals from the AAF  847  and can digitize the signals. Touch processor  842  can receive the digitized force signals from the ADC  849  for further processing. 
     In an alternate embodiment, the ADC  849  can be a sigma-delta converter with inherent anti-aliasing filtering, such that the AAF  847  can be omitted. 
     The receive section  807  can be coupled to transmit section  814 , which can be the transmit sections of  FIGS. 5 through 7 , for example, to drive the bridge  434 . 
       FIG. 9  illustrates another exemplary receive section of a touch controller having a force sensor interface according to various embodiments. In the example of  FIG. 9 , receive section  907  of a touch controller can share touch circuitry for processing touch and force signals together. The receive section  907  can include force sensor interface  912  for receiving and preparing the force signals for processing. The force sensor interface  912  can include differential amplifier  943 , which can perform in a similar manner as the differential amplifier  843  of  FIG. 8 . The interface  912  can also include differential-to-single-ended converter (DSC)  973  coupled to the differential amplifier  943  to receive the force signals from the amplifier. The DSC  973  can convert the differential force sensor signals into a single ended signal. Equation (9) can apply here, where Vo in the Equation refers to the output of DSC  973 . To compensate for mismatch between the force sensor(s) and the other resistors in the bridge  434 , the interface  912  can also include summer  940  and programmable attenuator  941  to perform offset subtraction. The attenuator  941  can receive and attenuate the stimulation signal Vstim+ so as to provide a fraction Voff+ of the stimulation signal to summer  940 . The summer  940  can then subtract Voff+ from the output signal of the DSC  973 , thereby compensating for any mismatch in the force sensor bridge  434 . 
     The receive section  907  can also include summer  980 , AAF  947 , ADC  949 , and the touch processor  842 . The summer  980  can combine the touch signals Vo and the offset-compensated force signals Vout. The touch signals Vo from the touch sensors can be conditioned by a sense amplifier (not shown), which will be described in more detail in  FIG. 12 . The composite touch and force signals can be passed to the AAF  947  to reject noise and other undesirable components and to the ADC  949  to digitize the signals. The touch processor  842  can receive the digitized composite touch and force signals for further processing. Because the touch and force signals can share the AAF  947  and ADC  949 , less circuits can be used, thereby saving circuit real estate and power. 
     In an alternate embodiment, the summer  980  can be omitted and the force signals can be processed by the AAF  947  and ADC  949 , while the touch signals are processed concurrently by a different AAF and ADC. 
     The receive section  907  can be coupled to transmit section  914 , which can be the transmit section of  FIG. 7 , for example, to drive the bridge  434 . Touch and force sensor readings can be acquired sequentially using single stimulation signals, i.e., all touch and force sensors are scanned sequentially. Touch and force sensor readings can also be acquired concurrently using multiple stimulation signals, i.e., multiple touch and force sensors are scanned concurrently. In this multiple case, a multi-stimulus matrix for N force sensor channels and M touch sensor channels can include (N+M) 2  entries, each row of N+M entries including a unique phase combination for the force and touch sensor stimulation signals. The composite touch and force signals of receive section  907  can be decoded in the touch processor  842  using the inverse of the multi-stimulus matrix. 
       FIG. 10  illustrates another exemplary receive section of a touch controller having a force sensor interface according to various embodiments. The receive section of  FIG. 10  is the same as the receive section of  FIG. 8 , except that the differential amplifier  843  of  FIG. 8  is replaced with differential instrumentation amplifier  1043  in  FIG. 10 . In the example of  FIG. 10 , force sensor interface  1012  of touch controller receive section  1007  can include the differential instrumentation amplifier  1043  to gain up the force signals Vb from the force sensor bridge  434  and to reduce noise in the signals. The differential instrumentation amplifier  1043  can include first operational amplifier  1043 - a  to receive the force signal Vb+ from the bridge  434 , second operational amplifier  1043 - b  to receive the force signal Vb− from the bridge, and resistors R 1 , Rg. The operational amplifiers  1043 - a ,  1043 - b  can output the amplified differential force signals to the summers  840  and the programmable attenuators  841  to perform offset subtraction, as described previously in  FIG. 8 . The differential DC gain of the instrumentation amplifier  1043  can be Gin=(1+2·R 1 /Rg). In order to maintain the set gain target at the stimulus frequency, the unity gain bandwidth of the instrumentation amplifier  1043  can be above β·Fstm·Gin, where Fstm=the stimulus frequency and β=a derating factor which can be selected based on the maximum tolerable gain error of the instrumentation amplifier at the stimulus frequency. The offset-compensated force signals Vout+, Vout− can be outputted to the differential AAF  847  for further processing as described in  FIG. 8 . The instrumentation amplifier  1043  can have either a static or a programmable gain according to the needs of touch controller. For example, the operational amplifiers  1043 - a ,  1043 - b  can both have a static gain, both a programmable gain, or combinations of the two. 
     The receive section  1007  can be coupled to transmit section  1014 , which can be the transmit sections of  FIGS. 5 through 7 , for example, to drive the bridge  434 . 
       FIG. 11  illustrates still another exemplary receive section of a touch controller having a force sensor interface according to various embodiments. The receive section of  FIG. 11  is the same as the receive section of  FIG. 9 , except the differential amplifier  943  and DSC  973  of  FIG. 9  are replaced with instrumentation amplifier  1143  in  FIG. 11 . In the example of  FIG. 11 , force sensor interface  1112  of touch controller receive section  1107  can include the instrumentation amplifier  1143  to gain up the force signals Vb from the force sensor bridge  434  and to reduce noise in the signals. The instrumentation amplifier  1143  can include first operational amplifier  1143 - a  to receive the force signal Vb+ from the bridge  434 , second operational amplifier  1143 - b  to receive the force signal Vb− from the bridge, and resistors R 1 , R 2 , R 3 , Rg. The instrumentation amplifier  1143  can also include third operational amplifier  1143 - c  to receive the amplified differential force signals outputted from the first and second operational amplifiers  1143 - a ,  1143 - b  and output a single ended force signal to the summer  940  and the programmable attenuator  941  to perform offset subtraction, as described previously in  FIG. 9 . The gain of the instrumentation amplifier  1143  can be Gin=(1+2·R 1 /Rg)·(R 3 /R 2 ). The unity gain bandwidth requirements of the instrumentation amplifier  1143  can be selected according to the targets described previously regarding  FIG. 10 . The offset-compensated force signals Vout and the touch signals can be combined at the summer  980  and outputted to the AAF  947  for further processing as described in  FIG. 9 . The instrumentation amplifier  1143  can have either a static or a programmable gain according to the needs of the touch controller. For example, the operational amplifiers  1143 - a ,  1143 - b ,  1143 - c  can all have a static gain, all have a programmable gain, or combinations of the two. 
     In an alternate embodiment, the summer  980  can be omitted and the force signals processed by the AAF  947  and ADC  949 , while the touch signals are processed concurrently by a different AAF and ADC. 
     The receive section  1107  can be coupled to transmit section  1114 , which can be the transmit section of  FIG. 7 , for example, to drive the bridge  434 . Touch and force sensor readings can be acquired and processed in similar manners as described previously regarding  FIG. 9 . 
     In addition to a force sensor interface as illustrated in  FIGS. 8 through 11 , a touch controller receive section can include touch sensing circuitry for a touch panel.  FIG. 12  illustrates an exemplary receive section of a touch controller having touch sensing circuitry according to various embodiments. In the example of  FIG. 12 , receive section  1207  of a touch controller can include sense amplifier  1283  for receiving and preparing touch signals from the touch panel  124  for processing. The touch panel  124  can have multiple touch sensors. In some embodiments, the touch sensors can be segmented into groups, e.g., columns, and each group coupled to a particular sense amplifier  1283 . In some embodiments, different groups, e.g., columns, of touch sensors can share a sense amplifier  1283 , where the amplifier can be switched between the groups. The sense amplifier  1283  can connect to the touch panel  124  to receive touch signal charge Qo, indicative of a touch or hover at the panel. The sense amplifier  1283  can convert the touch signal charge Qo through feedback resistor Rfb and feedback capacitor Cfb to a touch signal voltage Vo. The sense amplifier  1283  can output the touch signal voltage Vo to the AAF  947  to attenuate noise in the signals. In some embodiments, the sense amplifier  1283  can have either a static or a programmable gain according to the needs of the touch controller. The ADC  949  can digitize the noise-reduced touch signals and output the signals to the touch processor  842  for further processing. The touch circuitry of  FIG. 12  can be operated concurrently with the force and touch circuitry of  FIGS. 8 through 11 . 
     In alternate embodiments, the touch panel  124  of  FIG. 12  can share the AAF  947  and the ADC  949  with the force sensor bridge  434 , as described previously in  FIGS. 9 and 11 , where the summer  980  could be disposed between the sense amplifier  1283  and the AAF  947  to receive the touch signals Vo and the force signals Vout. 
     The receive section  1207  can be coupled to transmit section  1214 , which can be the transmit sections of  FIGS. 5 through 7 , for example, to drive the touch panel  124 . 
     It is to be understood that the circuits of  FIGS. 5 through 12  are not limited to the components described therein, but can include other and/or additional components capable of performing force and touch signal processing according to various embodiments. 
       FIG. 13  illustrates an exemplary method for sensing force and touch at a touch controller having a force sensor interface according to various embodiments. In the example of  FIG. 13 , a touch controller&#39;s transmit section can apply stimulation signals Vstim to a touch panel and a force sensor bridge to drive them to sense a touch or hover at the panel and a force applied at the bridge, respectively ( 1305 ). The stimulation signals Vstim can be applied to the bridge via a force sensor interface, as described previously. The touch controller&#39;s scan logic can control the timing of the stimulation signals Vstim to the panel and bridge. In response to the stimulation signals Vstim, the panel can generate touch signals, indicative of a touch or hover at the panel, and the bridge can generate force signals, indicative of a force applied at the bridge ( 1310 ). 
     The touch controller can then perform a scan of the panel and the bridge outputs to capture the touch signals and the force signals ( 1315 ). The scan logic can control the scanning of the panel and bridge in order to transmit the touch and force signals to the touch controller&#39;s receive section for processing. In some embodiments, during a scan period, the touch controller can perform the panel scan and the bridge scan sequentially. For example, the panel scan can be performed, followed by the bridge scan, or vice versa. In some embodiments, during the scan period, the touch controller can perform the panel scan and the bridge scan concurrently. In some embodiments, during the scan period, the touch controller can perform a hybrid sequential-concurrent scan, in which some portions of the panel and bridge scans can be performed together, while other portions of the scans can be performed in sequence. For example, at the beginning of the scan period, both the panel and bridge scans can be started. After a brief period, the bridge scan can be suspended while the panel scan completes. The bridge scan can then resume and complete. The hybrid scan can be useful in cases where force is applied intermittently, such that an initial bridge scan concurrently with the panel scan can indicate quickly whether an applied force is present, necessitating a full bridge scan. 
     In some embodiments, both scans need not be performed every scan period. For example, the bridge scan can be performed every other scan period or at some other predetermined frequency. In some embodiments, the scans can be triggered, rather than automatically performed. For example, during the scan period, the panel scan can trigger the bridge scan after the panel scan completes. Conversely, the bridge scan can trigger the panel scan upon completion. Alternatively, the panel scan can trigger the bridge scan if the panel scan indicates that the proximity of a touching or hovering object is likely to result in the object applying a force to the bridge. Alternatively, some device condition, e.g., power status, can trigger either or both scans. 
     The touch controller&#39;s receive section can then process the touch and force signals ( 1320 ). The processed touch and force signals can cause some action to be performed by a device having the panel, bridge, and touch controller. 
       FIG. 14  illustrates an exemplary computing system  1400  that can have a touch controller with a force sensor interface according to various embodiments described herein. In the example of  FIG. 14 , computing system  1400  can include touch controller  1406 . The touch controller  1406  can be a single application specific integrated circuit (ASIC) that can include one or more processor subsystems  1402 , which can include one or more main processors, such as ARM968 processors or other processors with similar functionality and capabilities. However, in other embodiments, the processor functionality can be implemented instead by dedicated logic, such as a state machine. The processor subsystems  1402  can also include peripherals (not shown) such as random access memory (RAM) or other types of memory or storage, watchdog timers and the like. 
     The touch controller  1406  can also include receive section  1407  for receiving signals, such as touch signals  1403  of one or more touch sense channels (not shown), force signals  1433  of a force sense channel (not shown), other signals from other sensors such as sensor  1411 , etc. The touch controller  1406  can also include demodulation section  1409  such as a multistage vector demodulation engine, panel scan logic  1410 , and transmit section  1414  for transmitting stimulation signals  1416  to touch sensor panel  1424  to drive the panel and to force sensor bridge  1434  to drive the bridge. The panel scan logic  1410  can access RAM  1412 , autonomously read data from the sense channels, and provide control for the sense channels. In addition, the panel scan logic  1410  can control the transmit section  1414  to generate the stimulation signals  1416  at various frequencies and phases that can be selectively applied to rows of the touch sensor panel  1424  and to the force sensor bridge  1434 . 
     The touch controller  1406  can include force sensor interface  1422 , which can couple to touch circuitry in the transmit section  1414  and the receive section  1407 , to integrate the force sensor bridge  1434  with the touch system. As a result, the touch sensor panel  1424  and the force sensor bridge  1434  can operate concurrently to sense a touch or hover at the panel and a force applied at the bridge. 
     The touch controller  1406  can also include charge pump  1415 , which can be used to generate the supply voltage for the transmit section  1414 . The stimulation signals  1416  can have amplitudes higher than the maximum voltage by cascading two charge store devices, e.g., capacitors, together to form the charge pump  1415 . Therefore, the stimulus voltage can be higher (e.g., 6V) than the voltage level a single capacitor can handle (e.g., 3.6 V). Although  FIG. 14  shows the charge pump  1415  separate from the transmit section  1414 , the charge pump can be part of the transmit section. 
     Computing system  1400  can also include touch sensor panel  1424 , which can be as described above in  FIGS. 3A and 3B , and force sensor bridge  1434 , which can be as described above in  FIGS. 4A and 4B , for example. 
     Computing system  1400  can include host processor  1428  for receiving outputs from the processor subsystems  1402  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. The host processor  1428  can also perform additional functions that may not be related to panel processing, and can be coupled to program storage  1432  and display device  1430  such as an LCD display for providing a UI to a user of the device. In some embodiments, the host processor  1428  can be a separate component from the touch controller  1406 , as shown. In other embodiments, the host processor  1428  can be included as part of the touch controller  1406 . In still other embodiments, the functions of the host processor  1428  can be performed by the processor subsystem  1402  and/or distributed among other components of the touch controller  1406 . The display device  1430  together with the touch sensor panel  1424 , when located partially or entirely under the touch sensor panel or when integrated with the touch sensor panel, can form a touch sensitive device such as a touch screen. 
     Note that one or more of the functions described above, can be performed, for example, by firmware stored in memory (e.g., one of the peripherals) and executed by the processor subsystem  1402 , or stored in the program storage  1432  and executed by the host processor  1428 . 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 non-transitory medium that can contain or store the program for use by or in connection with the instruction execution system, apparatus, or device. The non-transitory 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 medium can include, but is not limited to, an electronic, magnetic, optical, electromagnetic or infrared wired or wireless propagation medium. 
     It is to be understood that the touch sensor panel is not limited to touch, as described in  FIG. 14 , but can be a proximity panel or any other panel according to various embodiments. In addition, the touch sensor panel described herein can be a multi-touch sensor panel. 
     It is further to be understood that the computing system is not limited to the components and configuration of  FIG. 14 , but can include other and/or additional components in various configurations capable of sensing touch and force according to various embodiments. 
       FIG. 15  illustrates an exemplary mobile telephone  1500  that can include a touch controller having a force sensor interface, the touch controller capable of operating on home button  1548  to sense an applied force, touch panel  1524  to sense a touch or hover, display device  1536 , and other computing system blocks according to various embodiments. 
       FIG. 16  illustrates an exemplary digital media player  1600  that can include a touch controller having a force sensor interface, the touch controller capable of operating on clickwheel  1648  to sense an applied force, touch panel  1624  to sense a touch or hover, display device  1636 , and other computing system blocks according to various embodiments. 
       FIG. 17  illustrates an exemplary personal computer  1700  that can include a touch controller having a force sensor interface, the touch controller capable of operating on enter buttons  1748  to sense an applied force, touch pad  1724  to sense a touch or hover, display  1736 , and other computing system blocks according to various embodiments. 
     The mobile telephone, media player, and personal computer of  FIGS. 15 through 17  can provide efficient power consumption, compact device size, and improved force sensing by tightly integrating force sensing circuitry with existing touch sensing circuitry according to various embodiments. 
     Although embodiments 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 various embodiments as defined by the appended claims.