Patent Publication Number: US-10310659-B2

Title: Pressure-sensitive touch panel

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     The present application claims priority to and the benefit of U.S. Provisional Patent Application No. 62/095,853 filed Dec. 23, 2014, the entire contents of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a touch panel for combined capacitive and pressure sensing. 
     BACKGROUND 
     Resistive and capacitive touch panels are used as input devices for computers and mobile devices. One type of capacitive touch panel, projected capacitance touch panels, is often used for mobile devices because an exterior layer may be made of glass, providing a hard surface which is resistant to scratching. An example of a projected capacitance touch panel is described in US 2010/0079384 A1. 
     Projected capacitance touch panels operate by detecting changes in electric fields caused by the proximity of a conductive object. The location at which a projected capacitance touch panel is touched is often determined using an array or grid of capacitive sensors. Although projected capacitance touch panels can usually differentiate between single-touch events and multi-touch events, they suffer the drawback of not being able to sense pressure. Thus, projected capacitance touch panels tend to be unable to distinguish between a relatively tight tap and a relatively heavy press. A touch panel which can sense pressure can allow a user to interact with a device in new ways by providing additional information to simply position of a touch. 
     Different approaches have, been proposed to allow a touch panel to sense pressure. One approach is to provide capacitive sensors which include a gap whose size can be reduced by applied pressure, so as to produce a measurable difference in the mutual capacitance. For example, US 2014/043289 A describes a pressure sensitive capacitive sensor for a digitizer system which includes an interaction surface, at least one sensing layer operable to sense interaction by mutual capacitive sensing, and an additional layer comprising resilient properties and operable to be locally compressed responsive to pressure locally applied during user interaction with the capacitive sensor. However, the need for a measurable displacement can make it more difficult to use a glass touch surface and can cause problems with material fatigue after repeated straining. 
     Other pressure sensitive touch panels have proposed using one or more discrete force sensors supporting a capacitive touch panel, such that pressure applied to the capacitive touch panel is transferred to one or more sensors located behind the panel or disposed around the periphery. For example, US 2013/0076646 A1 describes using strain gauges with a force sensor interface which can couple to touch circuitry. WO 2012/031564 A1 describes a touch panel including a first panel, a second panel, and a displacement sensor sandwiched between the first panel and the second panel. The displacement sensors, such as capacitive or piezoresistive sensors, are placed around the edge of the second panel. However, it can be difficult to distinguish the pressure of multiple touches using sensors located behind a touch panel or disposed around the periphery. 
     Other pressure sensitive touch panels have been proposed which attempt to combine capacitive touch sensing with force sensitive piezoelectric layers. For example, WO 2009/150498 A2 describes a device including a first layer, a second layer, a third layer, a capacitive sensing component coupled to the first layer, and a force sensing component coupled to the first layer and the third layer and configured to detect the amount of force applied to the second layer. WO 2015/046289 A1 describes a touch panel formed by stacking a piezoelectric sensor and an electrostatic sensor. The piezoelectric sensor is connected to a pressing force detection signal generation unit, and the electrostatic sensor is connected to a contact detection signal generation unit. However, systems which use separate electronics to sense changes in capacitance and pressures can make a touch panel more bulky and expensive. Systems in which electrodes are directly applied or patterned onto a piezoelectric film can be more complex and expensive to produce. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to provide an improved capacitive touch panel. 
     According to a first aspect of the invention there is provided apparatus including a multiplexer having a plurality of inputs and an output. The apparatus also includes a touch panel including a layer structure including one or more layers, each layer extending perpendicularly to a thickness direction. The one or more layers include, a layer of piezoelectric material. The layer structure has first and second opposite faces. The one or more layer(s) are arranged between the first and second faces such that the thickness direction of each layer is perpendicular to the first and second faces. The layer structure includes a plurality of first electrodes disposed on the first face, each first electrode connected to a respective input of the multiplexer. The layer structure includes at least one second electrode disposed on the second face. The apparatus includes a front end module configured to receive an input signal from the multiplexer output. The front end module includes a first stage configured to provide an amplified signal based on the input signal. The front end module includes a second stage including first and second frequency-dependent filters configured to receive the amplified signal and to provide respective first and second filtered signals. The first filtered signal has a first frequency bandwidth, and the second filtered signal has a second frequency bandwidth which has a relatively higher start-frequency than the first frequency bandwidth. 
     Thus, pressure and capacitance measurements may be performed using a touch panel without the need for separate pressure and capacitance electrodes. A single input signal is received from an electrode including pressure and capacitance information and the input signal may be amplified and processed using a single front end. This can allow the apparatus to be more readily integrated into existing projected capacitance touch panels anchor to be easily used in conjunction with existing devices such as touch controller ICs. 
     The apparatus may further include a signal source configured to provide a periodic signal. The front end module may be configured to receive the periodic signal and the first stage may be configured to provide the amplified signal based on the input signal and the periodic signal. The second filtered signal may be based on the periodic signal and the first filtered signal may be not based on the periodic signal. 
     Providing a periodic signal to the front end module instead of through the touch panel electrodes directly can allow the gain for amplifying signals from the layer of piezoelectric material to be increased without causing saturation of the first stage output. It can allow an analogue-to-digital converter (ADC) to be used in a subsequent stage having a lower dynamic range. 
     The signal source may provide a periodic signal having a basic frequency of at least 0.5 kH, optionally at least 1 kHz, optionally at least 10 kHz. The signal source may provide a periodic signal having a basic frequency of at least 20 kHz. The signal source may provide a periodic signal having a basic frequency of at least 50 kHz. The signal source may provide a periodic signal having a basic frequency of at least 100 kHz. The signal source may provide a periodic signal having a sinusoidal, square, triangular or saw-toothed waveform. The signal source may provide a periodic signal comprising a superposition of two or more sinusoidal waveforms having different frequencies. The signal source may be a digital-to-analogue converter (DAC). 
     The apparatus may include a controller configured to cause the multiplexer to connect each one of the plurality of first electrodes to the front end module according to a sequence determined by the controller. The sequence may be pre-determined. The sequence may be dynamically determined. 
     The first and second stages may be configured such that the amplitude of the first filtered signal is dependent upon a pressure applied to the layer of piezoelectric material proximate to a given first electrode connected to the front end module by the multiplexer. 
     The first and second stages may be configured such that the amplitude of the second filtered signal is dependent upon a capacitance of a given first electrode connected to the front end module by the multiplexer. The amplitude of the second filtered signal may depend upon a self capacitance associated with a given first electrode. The amplitude of the second filtered signal may depend upon a mutual capacitance between the given first electrode and the second electrode(s). 
     The first frequency-dependent filter may comprise a low-pass filter and the second frequency-dependent filter may comprise at least one band-pass filter. The first frequency-dependent filter may comprise at least one band-stop filter and the second frequency-dependent filter may comprise at least one band-pass filter. The first frequency-dependent filter may comprise a low-pass filter and the second frequency-dependent filter may comprise a high-pass filter. Each band-pass filter may be a notch filter. Each band-stop filter may be a notch filter. Filters may comprise active filter circuits. Filters may comprise passive filter circuits. Filters may comprise a single stage. Filters may comprise multiple stages. Filters may comprise filter circuits selected from the group consisting of Butterworth filters, Chebyshev filters, Gaussian filters and Bessel filters. 
     The first stage may have a low-frequency cut-off configured to reject a pyroelectric response of the layer of piezoelectric material. The first stage may have a low-frequency cut-off configured to reject a mains power distribution frequency. The low frequency cut-off may be at least 50 Hz. The low frequency cut-off may be 60 Hz. The low frequency cut-off may be at least 100 Hz. The low frequency cut-off may be at least 200 Hz. 
     The first stage may include one or more integrating amplifiers configured to integrate the input signal. 
     The first stage may include one or more differential amplifier(s) configured to receive the input signal. 
     The plurality of first electrodes may include a plurality of conductive pads disposed on the first face in a two dimensional array. 
     The touch panel may further include a plurality of third electrodes disposed overlying the first face of the layer structure and arranged such that the layer structure is between the plurality of third electrodes and the second electrode(s). Each of the plurality of third electrodes may be connected to a respective input of the multiplexer. 
     The apparatus may include a controller configured to cause the multiplexer to connect each one of the plurality of first electrodes and each one of the plurality of third electrodes to the front end module according to a sequence determined by the controller. 
     The apparatus may further include a second multiplexer having a plurality of inputs and an output. The apparatus may further include a second front end module configured to receive an input signal from the output of the second multiplexer. The second front end module may have the same electronic configuration as the front end module. The touch panel may further include a plurality of third electrodes disposed overlying the first face of the layer structure and arranged such that the layer structure lies between the plurality of third electrodes and the second electrode(s). Each third electrode may be connected to a respective input of the second multiplexer. 
     The apparatus may include a controller configured to cause the second multiplexor to connect each one of the plurality of third electrodes to the second front end module according to a sequence determined by the controller. 
     Each first electrode may extend in a first direction and the plurality of first electrodes may be arrayed spaced apart perpendicular to the first direction. Each third electrode may extend in a second direction and the plurality of third electrodes may be arrayed spaced apart perpendicular to the second direction. The first and second directions may be different. The first and second directions may be substantially perpendicular. The first and second directions may meet at an angle of more than 30 and less than 90 degrees. 
     The touch panel may further include a second layer structure including one or more dielectric layers. Each dielectric layer may extend perpendicularly to a thickness direction. The second layer structure may have third and fourth opposite faces. The dielectric layers may be arranged between the third and fourth faces such that the thickness direction of each dielectric layer is perpendicular to the third and fourth faces. The plurality of third electrodes may be disposed on the third face of the second layer structure and the fourth face of the second layer structure may contact the first plurality of electrodes. 
     The plurality of third electrodes may be disposed on the first face of the layer structure. Each first electrode may comprise a continuous conductive region and each third electrode may comprise a plurality of conductive regions electrically connected to one another by jumpers. Each jumper may span a conductive region forming a portion of one of the first electrodes. 
     The apparatus may further include a second multiplexer having a plurality of inputs and an output. The apparatus may further include a second front end module configured to receive an input signal from the output of the second multiplexer. The second front end module may have the same electronic configuration as the front end module. The touch panel may include a plurality of second electrodes. The touch panel may further include a plurality of third electrodes disposed on the second face of the layer structure. Each third electrode may be connected to a respective input of the second multiplexer. Each first electrode may extend in a first direction and the plurality of first electrodes may be arrayed spaced apart perpendicular to the first direction. Each second electrode may extend in a second direction and the plurality of second electrodes may be arrayed spaced apart perpendicular to the second direction. Each third electrode may extend in a second direction and the plurality of third electrodes may be arrayed spaced apart perpendicular to the second direction. The third electrodes may be arranged parallel to and be interleaved with the plurality of second electrodes. The first and second directions may be different. The first and second directions may be substantially perpendicular. The first and second directions may meet at an angle of more than 30 and less than 90 degrees. 
     The layer structure may include one or more dielectric layers stacked between the layer of piezoelectric material and the first face of the layer structure. 
     The layer structure may include one or more dielectric layers stacked between the second face of the layer structure and the layer of piezoelectric material. 
     Thus, none of the electrodes need to be disposed directly on the layer of piezoelectric material. This allows a bare layer of piezoelectric material to be included in the layer structure. This can reduce the costs and complexity of producing the layer structure. 
     The second electrode may be a region of conductive material which is substantially coextensive with the second face. 
     The at least one second electrode may be a region of conductive material arranged in a grid. 
     The signal source may include a voltage controlled source, and the apparatus may further include a bias source coupled to the second electrode(s). The bias source may provide a constant bias. The constant bias may be ground potential. The bias source may be provided by the signal source. 
     The signal source may include one or more synchronized current controlled sources, and each current controlled source may provide the periodic signal to a respective front end module. 
     The first stage of each front end module may include an operational amplifier having at least an inverting input coupled to a first rail, a non-inverting input coupled to the voltage controlled source via a path comprising a first resistor, and an output coupled to a second rail. The first stage of each front end module may include a second resistor coupling the first rail to the output of the multiplexer corresponding to the front end module, a third resistor coupling the first rail to the second rail, and a first capacitor coupling the first rail to the second rail. The second rail may provide the amplified signal. 
     A second capacitor may be connected in parallel with the first resistor. The capacitance of the first capacitor may be substantially equal to a mutual capacitance between a given first electrode and at least one second electrode. 
     The first stage of each front end module may include a first operational amplifier having at least an inverting input coupled to a first rail, a non-inverting input coupled to the voltage source via a path comprising a first resistor and a second rail, and an output coupled to a third rail. The first stage of each front end module may include a second operational amplifier having at least an inverting input coupled to a fourth rail, a non-inverting input coupled to the second rail via a path comprising a second resistor, and an output coupled to a fifth rail. The first stage of each front end module may include a comparator having at least an inverting input coupled to the third rail, a non-inverting input coupled to the fifth rail, and an output providing the amplified signal. The first stage of each front end module may include a third resistor coupling the first rail output of the multiplexer corresponding to the front end module, a fourth resistor coupling the first rail to the third rail, a first capacitor coupling the first rail to the third rail, a fifth resistor coupling the fourth rail to the fifth rail, a second capacitor coupling the fourth rail to the fifth rail, and a sixth resistor coupling the fourth rail to ground via a path comprising a third capacitor. 
     The first resistor may have a resistance substantially equal to the second resistor. The third resistor may have a resistance substantially equal to the sixth resistor. The fourth resistor may have a resistance substantially equal to the fifth resistor. The first capacitor may have a capacitance substantially equal to the second capacitor. The third capacitor may have a capacitance substantially equal to a mutual capacitance between the given first electrode and at least one second electrode. A fourth capacitor may be connected in parallel with the first resistor. A fifth capacitor may be connected in parallel with the second resistor. 
     The first stage of each front end module may include an operational amplifier having at least an inverting input coupled to a first current controlled source via a first rail, a non-inverting input coupled to ground via a path comprising a first resistor, and an output coupled to a second rail. The first stage of each front end module may include a second resistor coupling the first rail to the second rail, and a third resistor coupling the first rail to ground via a path comprising a first capacitor. The first rail may be coupled to the output of the multiplexer corresponding to the front end module and the second rail may be coupled to the second electrode(s). The first rail may be coupled to the second electrode(s) and the second rail may be coupled to the output of the multiplexer corresponding to the front end module. The second rail may provide the amplified signal. 
     A second capacitor may be connected in parallel with the first resistor. The first capacitor may have a capacitance substantially equal to a mutual capacitance between the given first electrode and at least one second electrode. 
     The first stage of each front end module may include a first operational amplifier having at least an inverting input coupled to a first current controlled source via a first rail, a non-inverting input coupled to ground via a path comprising a first resistor, and an output coupled to a second rail. The first stage of each front end module may include a second operational amplifier having at least an inverting input coupled to a second current source by a third rail, a non-inverting input coupled to ground via a path comprising a second resistor, and an output coupled to a fourth rail. The first stage of each front end module may include a comparator having at least an inverting input coupled to the second rail, a non-inverting input coupled to the fourth rail, and an output providing the amplified signal. The first stage of each front end module may include a third resistor coupling the first rail and the second rail, a fourth resistor coupling the first rail to ground via a path comprising a first capacitor, a fifth resistor coupling the third rail to ground via a path comprising a second capacitor, a sixth resistor coupling the third rail to the fourth rail, and a third capacitor coupling the third rail to the fourth rail. The first rail may be coupled to the output of the multiplexer corresponding to the front end module and the second rail may be coupled to the second electrode(s). The first rail may be coupled to the second electrode(s) and the second rail may be coupled to the output of the multiplexer corresponding to the front end module. The first current controlled source may be synchronised with the second current controlled source. 
     The third resistor may have a resistance substantially equal to the sixth resistor. The first capacitor may have a capacitance substantially equal to the second capacitor. The third capacitor may have a capacitance substantially equal to a mutual capacitance between the given first electrode and at least one second electrode. A fourth capacitor may be connected in parallel with the first resistor. A fifth capacitor may be connected in parallel with the second resistor. 
     The apparatus may further comprise a signal processor arranged to receive the first and second filtered signals and to calculate pressure values and/or capacitance values in dependence upon the first and second filtered signals. 
     The signal processor may be configured to employ correlated double sampling methods so as to improve signal to noise ratio of the pressure values and/or capacitance values. The signal processor may be configured to treat the pressure values and/or the capacitance values as image data. 
     According to a second aspect of the invention there is provided a portable electronic device comprising the apparatus. 
     According to a third aspect of the invention there is provided a method in a touch panel including a layer structure comprising one or more layers, each layer extending perpendicularly to a thickness direction, the one or more layers including a layer of piezoelectric material, the layer structure having first and second opposite faces, and the layer(s) arranged between the first and second faces such that the thickness direction of each layer is perpendicular to the first and second faces, a plurality of first electrodes disposed on the first face, and at least one second electrode disposed on the second face. The method includes selecting each given first electrode of the plurality of first electrodes according to a predetermined sequence. The method includes, for each given first electrode, generating an amplified signal based on an input signal received from the given first electrode, filtering the amplified signal using a first frequency-dependent filter to provide a first filtered signal having a first frequency bandwidth, and filtering the amplified signal using a second frequency-dependent filter to provide a second filtered signal having a second frequency bandwidth which has a relatively higher start-frequency than the first frequency bandwidth. 
     The method may further include providing a periodic signal. The amplified signal may be generated based on the input signal and the periodic signal. The second filtered signal may be based on the periodic signal and the first filtered signal may be based on the periodic signal. 
     According to a fourth aspect of the invention there is provides a portable electronic device carrying out the method. 
     According to a fifth aspect of the invention there is provided a method of fabricating a layer structure for a touch panel. The method includes providing a transparent substrate having first and second opposite faces. The method includes providing a dielectric layer having first and second opposite faces. The method includes providing a layer of piezoelectric material having first and second opposite faces. The method includes providing a plurality of first conductive regions extending in a first direction and spaced apart perpendicular to the first direction. The method includes providing a plurality of second conductive regions extending in a second direction and spaced apart perpendicular to the second direction, the second direction different to the first. The method includes providing a third conductive material region extending such that, when assembled, the third conductive material region at least partially overlaps each first conductive region and each third conductive region. The method includes assembling the layer structure such that the first face of the transparent substrate is opposed to the second face of the dielectric layer, and the first face of the dielectric layer is opposed to the second face of the layer of piezoelectric material. The method includes assembling the layer structure such that the plurality of first conductive regions are disposed between the transparent substrate and the dielectric layer, the plurality of second conductive regions are disposed between the transparent substrate and the layer of piezoelectric material, and the third conductive material region is disposed over the first face of the layer of piezoelectric material. 
     Thus, the layer structure for a touch panel can be fabricated without the need for complex and/or expensive deposition of patterned electrode on the layer of piezoelectric material. Additionally, the layer of piezoelectric material may be provided as a single sheet without the need to deposit or pattern piezoelectric material regions to provide discrete devices. 
     The plurality of first conductive regions may be disposed on the second face of the dielectric layer. Assembling the layer structure may include bonding the second face of the dielectric layer to the first face of the transparent substrate. 
     The plurality of first conductive regions may be disposed on the first face of the transparent substrate. Assembling the layer structure may include bonding the second face of the dielectric layer to the first face of the transparent substrate. 
     The plurality of first conductive regions may be disposed on the first face of the dielectric layer. Assembling the layer structure may include bonding the second face of the dielectric layer to the first face of the transparent substrate. 
     The plurality of second conductive regions may be disposed on the same face of the dielectric layer as the plurality of first electrodes. Each first conductive region may be a continuous conductive region and each second conductive region may be a plurality of separate conductive regions connected by jumpers. Each jumper may span a portion of a first conductive region. Assembling the layer structure may include bonding the second face of the layer of piezoelectric material to the first face of the dielectric layer. 
     The plurality of second conductive regions may be disposed on the first face of the dielectric layer. Assembling the layer structure may include bonding the second face of the layer of piezoelectric material to the first face of the dielectric layer. 
     The method may further include providing a second dielectric layer having first and second opposite faces. The plurality of second conductive regions may be disposed on the first face or the second face of the second dielectric layer. Assembling the layer structure may include bonding the second face of the second dielectric layer to the first face of the dielectric layer, and bonding the second face of the layer of piezoelectric material to the first face of the second dielectric layer. 
     The plurality of second conductive regions may be disposed on the second face of the layer of piezoelectric material. The method may further include bonding the second face of the layer of piezoelectric material to the first face of the dielectric layer. The method may further include providing a third dielectric layer having first and second opposite faces. The third conductive material region may be disposed on the first face or the second face of the third dielectric layer. Assembling the layer structure may include bonding the second face of the third dielectric layer to the first face of the layer of piezoelectric material. 
     The third conductive material region may be disposed on the first face of the layer of piezoelectric material. 
     Bonding a second face of one layer to a first face of another layer may include providing a layer of pressure sensitive adhesive material between the opposed first and second faces, and applying pressure between the first and second faces. “Pressure sensitive adhesive” (PSA) as used herein includes optically clear adhesives (OCA), optically clear resins (OCR) and liquid optically clear adhesives (LOCA). 
     The method may include providing a polarizer on the second conductive regions. Any one of the dielectric layers may comprise a polarizer. Any one of the dielectric layers may comprise a layer of colour filter material. 
     According to a sixth aspect of the invention there is provided a portable electronic device comprising a layer structure for a touch panel fabricated according to the method. 
     According to a seventh aspect of the invention there is provided a method of fabricating a layer structure for a touch panel. The method includes providing a transparent substrate having first and second opposite faces, providing a first dielectric layer having first and second opposite faces, wherein a plurality of first conductive regions extending in a first direction and spaced apart perpendicular to the first direction are disposed on the second face of the first dielectric layer, and bonding the second face of the first dielectric layer to the first face of the transparent substrate. The method includes providing a second dielectric layer having first and second opposite faces, wherein a plurality of second conductive regions extending in a second direction and spaced apart perpendicular to the second direction are disposed on the second face of the second dielectric layer, and wherein the second direction is different to the first direction, and bonding the second face of the second dielectric layer to the first face of the first dielectric layer. The method includes providing a layer of piezoelectric material having first and second opposite faces, wherein a third conductive material region is disposed on the first face of the layer of piezoelectric material such that, when assembled, the third conductive region at least partially overlaps each first conductive region and each second conductive region, and bonding the second face of the layer of piezoelectric material to the first face of the second dielectric layer. 
     According to an eighth aspect of the invention there is provided a method of fabricating a layer structure for a touch panel. The method includes providing a transparent substrate having first and second opposite faces, providing a dielectric layer having first and second opposite faces, wherein a plurality of first conductive regions extending in a first direction and spaced apart perpendicular to the first direction are disposed on the second face of the dielectric layer, and bonding the second face of the dielectric layer to the first face of the transparent substrate. The method includes providing a layer of piezoelectric material having first and second opposite faces, wherein a plurality of second conductive regions extending in a second direction are and spaced apart perpendicular to the second direction are disposed on the second face of the layer of piezoelectric material, wherein the second direction is different to the first direction, and wherein a third conductive material region is disposed on the first face of the layer of piezoelectric material such that, when assembled, the third conductive region at least partially overlaps each first conductive region and each second conductive region, and bonding the second face of the layer of piezoelectric material to the first face of the dielectric layer. 
     According to a ninth aspect of the invention there is provided a method of fabricating a layer structure for a touch panel. The method includes providing a transparent substrate having first and second opposite faces, providing a first dielectric layer having first and second opposite faces, wherein a plurality of first conductive regions extending in a first direction and spaced apart perpendicular to the first direction are disposed on the second face of the first dielectric layer, and bonding the second face of the first dielectric layer to the first face of the transparent substrate. 
     The method includes providing a second dielectric layer having first and second opposite faces, wherein a plurality of second conductive regions extending in a second direction and spaced apart perpendicular to the second direction are disposed on the second face of the second dielectric layer, and wherein the second direction is different to the first direction, and bonding the second face of the second dielectric layer to the first face of the first dielectric layer. The method includes providing a layer of piezoelectric material having first and second opposite faces, and bonding the second face of the layer of piezoelectric material to the first face of the second dielectric layer. The method includes providing a third dielectric layer having first and second faces, wherein a third conductive material region is disposed on the second surface of the third dielectric layer such that, when assembled, the third conductive region at least partially overlaps each first conductive region and each second conductive region, and bonding the second face of the third dielectric layer to the first face of the layer of piezoelectric material. 
     According to a tenth aspect of the invention there is provided a method of fabricating a layer structure for a touch panel. The method includes providing a transparent substrate having first and second opposite faces, providing a dielectric layer having first and second opposite faces, wherein a plurality of first conductive regions extending in a first direction and spaced apart perpendicular to the first direction are disposed on the second face of the dielectric layer, and wherein a plurality of second conductive regions extending in a second direction and spaced apart perpendicular to the second direction are disposed on the first face of the dielectric layer, and wherein the second direction is different to the first direction, and bonding the second face of the dielectric layer to the first face of the transparent substrate. 
     The method includes providing a layer of piezoelectric material having first and second opposite faces, wherein a third conductive material region is disposed on the first face of the layer of piezoelectric material such that, when assembled, the third conductive region at least partially overlaps each first conductive region and each second conductive region, and bonding the second face of the layer of piezoelectric material to the first face of the dielectric layer. 
     According to an eleventh aspect of the invention there is provided a method of fabricating a layer structure for a touch panel. The method including providing a transparent substrate having first and second opposite face, providing a first dielectric layer having first and second opposite faces, wherein a plurality of first conductive regions extending in a first direction and spaced apart perpendicular to the first direction are disposed on the second face of the first dielectric layer, and wherein a plurality of second conductive regions extending in a second direction and spaced apart perpendicular to the second direction are disposed on the second surface of the first dielectric layer, and wherein the second direction is different to the first direction, and bonding the second face of the first dielectric layer to the first face of the transparent substrate. 
     The method includes providing a layer of piezoelectric material having first and second faces, and bonding the second face of the layer of piezoelectric material to the first face of the first dielectric layer. The method includes providing a second dielectric layer having first and second opposite faces, wherein a third conductive material region is disposed on the second face of the second dielectric layer such that, when assembled, the third conductive region at least partially overlaps each first conductive region and each second conductive region, and bonding the second face of the second dielectric layer to the first face of the layer of piezoelectric material. Each first conductive region comprises, a continuous conductive region and each second conductive region comprise a plurality of separate conductive regions connected by jumpers, each jumper spanning a portion of a first conductive region. 
     According to a twelfth aspect of the invention there is provided a method of fabricating a layer structure for a touch panel. The method includes providing a transparent substrate having first and second opposite faces, wherein a plurality of first conductive regions extending in a first direction and spaced apart perpendicular to the first direction are disposed on the first face of the glass sheet, providing a first dielectric layer having first and second opposite faces, wherein a plurality of second conductive regions extending in a second direction and spaced apart perpendicular to the second direction are disposed on the second surface of the first dielectric layer, and wherein the second direction is different to the first direction, and bonding the second face of the first dielectric layer to the first face of the transparent substrate. The method includes providing a layer of piezoelectric material having first and second opposite faces, and bonding the second face of the layer of piezoelectric material to the first face of the first dielectric layer. The method includes providing a second dielectric layer having first and second opposite faces, wherein a third conductive material region is disposed on the second face of the second dielectric layer such that, when assembled, the third conductive region at least partially overlaps each first conductive region and each second conductive region, and bonding the second face of the second dielectric layer to the first face of the layer of piezoelectric material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Certain embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings in which: 
         FIG. 1  schematically illustrates a first apparatus including a first touch sensor and a front end module for combined capacitive and pressure sensing; 
         FIG. 2  schematically illustrates a second apparatus including a second touch sensor; 
         FIG. 3  is a block diagram of an electronic device; 
         FIG. 4  illustrates the operation of the front end module shown in  FIG. 1 ; 
         FIG. 5  illustrates the operation of the front end module shown in  FIG. 1  using a single ended amplifier; 
         FIG. 6  illustrates the operation of the front end module shown in  FIG. 1  using a differential amplifier; 
         FIG. 7  schematically illustrates an example of a filter configuration for the front end module shown in  FIG. 1 ; 
         FIG. 8  schematically illustrates an example of a filter configuration for the front end module shown in  FIG. 1 ; 
         FIG. 9  schematically illustrates an example of a filter configuration for the front end module shown in  FIG. 1 ; 
         FIG. 10  is a schematic circuit diagram of a first amplifier included in the apparatus shown in  FIG. 1 ; 
         FIG. 11  is a schematic circuit diagram of the first amplifier shown in  FIG. 10  included in the apparatus shown in  FIG. 2 ; 
         FIG. 12  is a schematic circuit diagram of a second amplifier included in the apparatus shown in  FIG. 1 ; 
         FIG. 13  is a schematic circuit diagram of a third amplifier included in the apparatus shown in  FIG. 1 ; 
         FIG. 14  is a schematic circuit diagram of a fourth amplifier included in the apparatus shown in  FIG. 1 ; 
         FIG. 15  is a cross-sectional view of a first touch panel for combined capacitive and pressure sensing; 
         FIG. 16  schematically illustrates a third apparatus including the touch panel shown in  FIG. 15 ; 
         FIG. 17  is a schematic circuit diagram of the first amplifier shown in  FIG. 10  included in the apparatus shown in  FIG. 16 ; 
         FIG. 18  is a schematic circuit diagram of the second amplifier shown in  FIG. 12  included in the apparatus shown in  FIG. 16 ; 
         FIG. 19  is a plan view of a patterned electrode for the touch panel shown  FIG. 15 ; 
         FIG. 20  illustrates using interpolation based on measured pressure values to estimate a location and a pressure of a user interaction with a touch panel; 
         FIG. 21  is a cross-sectional view of a second touch panel for combined capacitive and pressure sensing; 
         FIG. 22  schematically illustrates a fourth apparatus including the touch panel shown in  FIG. 21 ; 
         FIG. 23  is a plan view of an arrangement of electrodes for the touch panel for combined capacitive and pressure sensing shown in  FIG. 21 ; 
         FIG. 24  is a plan view of an arrangement of electrodes for a third touch panel for combined capacitive and pressure sensing; 
         FIG. 25  schematically illustrates a fifth apparatus-including the touch panel shown in  FIG. 21 ; 
         FIG. 26  is a schematic circuit diagram of the first amplifier shown in  FIG. 10  included in the apparatus shown in  FIG. 25 ; 
         FIG. 27  is a cross-sectional view of a fourth touch panel for combined capacitive and pressure sensing; 
         FIG. 28  schematically illustrates a sixth apparatus including the touch panel shown in  FIG. 27 ; 
         FIG. 29  is a schematic circuit diagram of the third amplifier shown in  FIG. 13  included in the apparatus shown in  FIG. 28 ; 
         FIGS. 30A to 30C  illustrate a first display stack-up at different stages during fabrication; 
         FIGS. 31A to 31C  illustrate a second display stack-up at different stages during fabrication; 
         FIGS. 32A to 32C  illustrate a third display stack-up at different stages during fabrication; 
         FIGS. 33A to 33D  illustrate a fourth display stack-up at different stages during fabrication; 
         FIGS. 34A and 34B  illustrate a fifth display stack-up at different stages during fabrication; 
         FIGS. 35A and 35B  illustrate a sixth display stack-up at different stages during fabrication; 
         FIGS. 36A to 36C  illustrate a seventh display stack-up at different stages during fabrication; 
         FIGS. 37A to 37D  illustrate an eighth display stack-up at different stages during fabrication; 
         FIG. 38  illustrates a first embedded display stack-up; 
         FIG. 39  illustrates a second embedded display stack-up; 
         FIG. 40  illustrates a third embedded display stack-up; 
         FIG. 41  illustrates a fourth embedded display stack-up; 
         FIG. 42  illustrates a fifth embedded display stack-up; 
         FIG. 43  illustrates a sixth embedded display stack-up; 
         FIG. 44  illustrates a seventh embedded display stack-up; 
         FIG. 45  illustrates an eighth embedded display stack-up; 
         FIG. 46  is a plan view of an arrangement of electrodes for a fifth touch panel for combined capacitive and pressure sensing; and 
         FIG. 47  is a cross-sectional view of the touch panel shown in  FIG. 46 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, like parts are denoted by like reference numerals. 
     First combined capacitance and pressure sensing apparatus and first touch sensor:  FIG. 1  schematically illustrates a first apparatus  1  for combined capacitive and pressure sensing which includes a first touch sensor  2 , a front end module  3 , and a first signal processing module  4 . 
     The first touch sensor  2  includes a layer structure  5  having a first face  6  and a second, opposite, face  7 , a first electrode  8  and a second electrode  9 . The layer structure  5  includes one or more layers, including at least a layer of piezoelectric material  10 . Each layer included in the layer structure  5  is generally planar and extends in first x and second y directions which are perpendicular to a thickness direction z. The one or more layers of the layer structure  5  are arranged between the first and second faces  6 ,  7  such that the thickness direction z of each layer of the layer structure  5  is perpendicular to the first and second faces  6 ,  7 . The first electrode  8  is disposed on the first face  6  of the layer structure  5 , and the second electrode  9  is disposed on the second face  7  of the layer structure  5 . The first electrode  8  is electrical coupled to a terminal A and the second electrode  9  is coupled to a terminal B. 
     Preferably, the piezoelectric material is a piezoelectric polymer such as polyvinylidene fluoride (PVDF). However, the piezoelectric material may alternatively be a layer of a piezoelectric ceramic such as lead zirconate titanate (PZT). Preferably, the first and second electrodes are indium tin oxide (ITO) or indium zinc oxide (IZO). 
     However, the first and second electrodes  8 ,  9  may be metal films, such as aluminium, copper, silver or other metals suitable for deposition and patterning as a thin film. The first and second electrodes  8 ,  9  may be conductive polymers such as polyaniline, polythiphene, polypyrrole or poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT/PSS). The first and second electrodes may be formed from a metal mesh; nanowires, optionally silver nanowires; graphene; and carbon nanotubes. 
     The front end module  3  is coupled to the first touch sensor  2  via terminal A in order to receive an input signal  11  from the first electrode  8 . The front end module includes a first stage  12  in the form of an amplification stage, and a second stage in the form of a first frequency-dependent filter  13  and a second frequency-dependent filter  14 . The first stage  12  receives the input signal  11  from the first electrode  8 , and provides an amplified signal  15  based on the input signal  11 . The first frequency-dependent filter  13  receives and filters the amplified signal  15  to provide a first filtered signal  16  having a first frequency bandwidth. The second frequency-dependent filter  14  receives and filters the amplified signal  15  to provide a second filtered signal  17  having a second frequency bandwidth. The frequency bandwidth has a relatively higher start-frequency than the first frequency bandwidth. 
     The input signal  11  is produced in response to a user interaction with the touch sensor  2  or with a layer of material overlying the touch sensor  2 . In the following description, reference to a “user interaction” shall be taken to include a user touching or pressing a touch sensor, a touch panel or a layer of material overlying either. The term “user interaction” shall be taken to include interactions involving a user&#39;s digit or a stylus (whether conductive or not). The term “user interaction” shall also be taken to include a user&#39;s digit or conductive stylus being proximate to a touch sensor or touch panel without direct physical contact. 
     The terminal B may couple the second electrode  9  to ground, to a voltage bias source  52  ( FIG. 10 ) providing a constant potential, to a signal source  44  providing a periodic signal  43  or to the front end module  3  such that the front end module  3  is connected across the terminals A and B. 
     The terminals A, B, and other terminals denoted herein by capitalised Latin letters are used as reference points for describing electrical coupling between electrodes and other elements of an apparatus. Although the terminals A, B may actually be physical terminals, the description that an element, for example a front end module, is coupled to a terminal, for example, the terminal A shall be taken to mean that the front end module is directly coupled to the first electrode  8 . Similarly for other elements and other terminals denoted by capitalised Latin letters. 
     The first signal processing module  4  receives the first and second filtered signals  16 ,  17 . The first signal processing module  4  calculates pressure values  18  based on the first filtered signal  16  and capacitance values  19  based on the second filtered signal  17 . The pressure values  18  depend upon a deformation, which may be a strain, applied to the layer of piezoelectric material  10  and corresponding to a user interaction. The capacitance values  19  depend upon the self-capacitance of the first electrode  8  and/or a mutual capacitance between the first and second electrodes  8 ,  9 . The capacitance values  19  vary in response to a user interaction involving a digit or a conductive stylus. 
     In this way, pressure and capacitance measurements may be performed using the touch sensor  2  without the need for separate pressure and capacitance electrodes. A single input signal  11  is received from the first electrode  8  which includes pressure and capacitance information. Additionally, the input signal  11  may be amplified and processed using a single front end module  3 . This can allow the apparatus  1  to be more readily integrated into existing projected capacitance touch panels. 
     The layer structure  5  may include only the layer of piezoelectric material  10 , such that the first and second opposite faces  6 ,  7  are faces of the piezoelectric material layer  10  ( FIGS. 15, 21, 27, 32 and 38 ). Alternatively, the layer structure  5  may include one or more dielectric layers which are stacked between the layer of piezoelectric material  10  and the first face  6  of the layer structure  5  ( FIGS. 31, 33, 35 and 36 ). The layer structure  5  may include one or more dielectric layers stacked between the second face  7  of the layer structure  5  and the layer of piezoelectric material  10  ( FIG. 33 ). Preferably, one or more dielectric layer(s) include layers of a polymer dielectric material such as polyethylene terephthalate (PET), or layers of pressure sensitive adhesive (PSA) material. However, one or more dielectric layer(s) may include layers of a ceramic insulating material such as aluminium oxide. 
     In  FIG. 1 , the first and second faces  6 ,  7  and the layers of the layer structure  5  are shown extending along orthogonal axes labelled x and y, and the thickness direction of each layer of the layer structure  5  is aligned with an axis labelled z which is orthogonal to the x and y axes. However, the first, second and thickness directions need not form a right handed orthogonal set as shown. For example, the first and second directions x, y may intersect at an angle of 30 degrees or 45 degrees or any other angle greater than 0 degrees and less than 90 degrees. 
     Second combined capacitance and pressure sensing apparatus and second touch sensor; 
     Referring also to  FIG. 2 , a second apparatus  20  is shown which includes a second touch sensor  21 , a first front end module  3   a , a second front end module  3   b  and a second signal processing module  22 . 
     The second touch sensor  21  is similar to the first touch sensor  2 , except that the second touch sensor  21  also includes a Second layer structure  23  having a third face  24  and a fourth, opposite, face  25 , and a third electrode  26 . The second layer structure  23  includes one or more dielectric layers  27 . Each dielectric layer  27  is generally planar and extends in first x and second y directions Which are perpendicular to a thickness direction z. The one or more dielectric layers  27  of the second layer structure  23  are arranged between the third and fourth faces  24 ,  25  such that the thickness direction z of each dielectric layer  27  of the second layer structure  23  is perpendicular to the third and fourth faces  24 ,  25 . The third electrode  26  is disposed on the third face  24  of the second layer structure  23 , and the fourth face  25  of the second layer structure  23  contacts the first electrode  8 . 
     Preferably, the dielectric layer(s)  27  include layers of a polymer dielectric material such as PET or layers of PSA materials. However, the dielectric layer(s)  27  may include layers of a ceramic insulating material such as aluminium oxide. Preferably, the third electrode  26  is made of indium tin oxide (ITO) or indium zinc oxide (IZO). However, the third electrode  26  may be a metal mesh film such as aluminium, copper, silver other metals suitable for deposition and patterning as a thin film. The third electrode  26  may be made of a conductive polymer such as polyaniline, polythiphene, polypyrrole or poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT/PSS). 
     The first and second front-end modules  3   a ,  3   b  are the same as the front end module  3 . The first front end module  3   a  is coupled to the second touch sensor  21  via a terminal D in order to receive a first input signal  11   a  from the first electrode  8 . The second front end module  3   b  is coupled to the second touch sensor  21  via a terminal C in order to receive a second input signal  11   b  from the third electrode  26 . A terminal  6  may couple the second electrode  9  to ground, to a voltage bias source  52  ( FIG. 10 ) providing a constant potential, or to a signal source  44  providing a periodic signal  43 . Alternatively, the terminal E may be coupled to the first front end module  3   a  such that the first front end module  3   a  is connected across the terminals D and E, and the terminal E may also be coupled to the second front end module  3   b  such that the second front end module  3   b  is connected across the terminals C and E. 
     The second signal processing module  22  receives first and second filtered signals  16   a ,  17   a  from the first front end module  3   a  and first and second filtered signals  16   b ,  17   b  from the second front end module  3   b . The second signal processing module  22  calculates first pressure values  18   a  and capacitance values  19   a  based on the filtered signals  16   a ,  17   a  from the first front end module  3   a  and the second filtered signal  17   b  from the second front end module  3   b . The second signal processing module  22  calculates second pressure values  18   b  and capacitance values  19   b  based on the filtered signals  16   b ,  17   b  from the second front end module  3   b  and the second filtered signal  17   a  from the first front end module  3   a . The pressure values  18   a ,  18   b  depend upon a deformation applied to the layer of piezoelectric material  10  by a user interaction. The first capacitance values  19   a  depend upon the self-capacitance of the first electrode  8  and/or a mutual capacitance between the first and second electrodes  8 ,  9  and/or upon a mutual capacitance between the first and third electrodes  8 ,  23 . The second capacitance values  19   b  depend upon the self-capacitance of the third electrode  26  and/or a mutual capacitance between the third and second electrodes  23 ,  9 , and/or upon a mutual capacitance between the first and third electrodes  8 ,  23 . The capacitance values  19  vary in response to a user interaction involving a digit or a conductive stylus. 
     The second layer structure  23  may include only a single dielectric layer  27 , such that the third and fourth opposite faces  24 ,  25  are faces of a single dielectric layer  27  ( FIGS. 21, 23, 30, 34, 36 ). Alternatively, a second layer structure need not be used, and the third electrode  26  could be disposed on the first face  6  along with the first electrode  8  ( FIGS. 24, 35, 37 ). In  FIG. 2 , the third and fourth faces  24 ,  25  and the dielectric layers  27  of the second layer structure  23  are shown extending along orthogonal axes labelled x and y, and the thickness direction of each dielectric layer  23  of the second layer structure  23  is aligned with an axis labelled z which is orthogonal to the x and y axes. However, the first, second and thickness directions need not form a right handed orthogonal set as shown. 
     Electronic device: Referring also to  FIG. 3 , an electronic device  28  may include a touch panel  29  and a touch controller  30  for providing combined capacitive and pressure sensing. 
     The electronic device  28  may be a relatively immobile electronic device such as, for example a desktop computer, an automated teller machine (ATM), a vending machine, a point of sale device, or a public access information terminal. Alternatively, an electronic device  28  may be a portable electronic device such as a laptop, notebook or tablet computer, a mobile phone, a smart phone, a personal data assistant or a music playing device. The electronic device  28  includes a touch panel  29  including one or more touch sensors  2 ,  21 . The touch panel  29  is coupled to a touch controller  30  including one or more front end modules  3  by a link  31 . In a case where the link  31  is a multiplexed link, one front end module  3  may receive input signals  11  from multiple touch sensors  2 ,  21 . For example, using a multiplexed link  31  the touch controller  30  may include one front end module and the touch panel  29  may include two, four, eight, sixteen, thirty two, sixty four, one hundred and twenty eight, two hundred and fifty six or more touch sensors  2 ,  21 . The number of touch sensors  2 ,  21  coupled to a front end module  3  by a multiplexed link  31  need not be a power of two. 
     The electronic device  28  may include a processor  32  for executing programs and processing information. The electronic device  28  may include a memory  33  such as a volatile random access memory for temporarily storing programs and information, and/or storage  31  such as non-volatile random access memory (NVRAM) or a hard disc drive (HDD) for long term storage of programs and information. The electronic device  28  may include a network interface  35  for transmitting and/or receiving information from wired or wireless communication networks. The electronic device  28  may include a removable storage interface  36  which can interface with removable storage media to read and/or write programs and information. The electronic device  28  may include output means such as a display  37  and/or speaker(s)  38 . The display  37  may be any type of display such as, for example, an liquid crystal display (LCD), a light emitting diode display (LED), an organic LED display, an electrophoretic display or other type of electronic-ink display. 
     The touch controller  30  provides input information to the electronic device  28  which corresponds to user interactions with the touch panel  29 . For example, input information may be the locations and/or pressures of one or more user interactions. The electronic device may include other input means such as a microphone  39 , or other input devices  40  such as, for example, a keyboard, keypad, mouse or trackball. When the touch panel  29  includes a plurality of touch sensors  2 ,  21 , the touch controller  30  may provide positional information in the form of coordinates and/or pressures corresponding to one or more simultaneous user interactions with the touch panel  29 . 
     The touch panel  29  may be provided overlying the display  37 , such that the touch panel  29  and display  37  provide a touch screen. Alternatively, the touch sensors  2 ,  21  of the touch panel  29  may be integrated into or embedded within the display  37 . When the touch panel  29  is used overlying or integrated into the display  37 , the layer structure(s)  5 ,  23  and electrodes  8 ,  9 ,  26  may be transparent or substantially transparent. For example, the layer structure(s)  5 ,  23  and electrodes  8 ,  9 ,  26  may transmit 50% or more, preferably at least 75% preferably at feast 90% of light in visible wavelengths. For example, the piezoelectric material may be PVDF, dielectric layers included in the layers structures  5 ,  23  may be PET or an optically clear PSA, and the electrodes  8 ,  9 ,  26  may be ITO. Alternatively, the electrodes  8 ,  9 ,  26 , and any connections thereto, may be opaque and sufficiently thin in a direction perpendicular to the thickness direction z that they are not immediately noticeable to the human eye, for example, electrodes, and any connections thereto, may be less than 100 micrometers (1×10-4 m) wide, less than 10 micrometers (1×10-5 m) wide or thinner. 
     Operation of the first and second apparatuses: Referring also to  FIG. 4 , operation of the first end module  3  will be explained. 
     The layer of piezoelectric material  10  is poled such that a polarisation P of the layer of piezoelectric material  10  having a component Pz in the thickness direction z will be generated by the application of a pressure (or stress or force) in the thickness direction z which results from a user interaction with the touch sensor  2 ,  21 . The polarisation P of the layer of piezoelectric material results in an induced electric field Ep, which has a component Ez in the thickness direction. Preferably, the layer of piezoelectric material  10  is poled such that the induced electric field Ep is orientated substantially in the thickness direction z, such that the component of the induced electric field Ep in the thickness direction Ez is substantially larger than any components perpendicular to the thickness direction Ex, Ey. Preferably, the induced electric field Ep is orientated at an angle within 10 degrees of the thickness direction z. However, the induced electric field Ep may be orientated at an angle within 30 degrees, within 45 degrees or within 60 degrees of the thickness direction z. The deformation which produces the polarisation P may result from a compression or a tension. The deformation which produces the polarisation P may be an in-plane stretching of the piezoelectric material layer  10 . 
     The induced electric field Ep produces a potential difference between the first and second electrodes  8 ,  9  of the first or second touch sensors  2 ,  21 . The induced electric field Ep produces a potential difference between the third and second electrodes  26 ,  9  of the second touch sensor  21 . If a conductive path is provided between the first or third electrodes  8 ,  26  and the second electrode  9 , charges will flow between them until the induced electric field Ep is cancelled by an electric field Eq produced by the charging of the electrodes  8 ,  9 ,  26 . Intimate contact between the layer of piezoelectric material  10  and the electrodes  8 ,  9 ,  26  is not required, provided that intervening layers of the layer structures  5 ,  23  are not so thick that the induced electric field Ep is negligible at the location of an electrode  8 ,  9 ,  26 . A potential difference may be produced between the third and second electrodes  23 ,  9  of the second touch sensor  21  provided that the first electrode  8  is arranged such that the third electrode  23  is not entirely screened from the induced electric field Ep. 
     The input signal  11  received from the first electrode  8  or the third electrode  23  includes a current signal Ipiezo(t) which depends upon the induced electric field Ep (because there exists a finite resistance between the first or third electrodes  8 ,  26  and the second electrode  9 ). Generally, a greater deformation applied to the layer of piezoelectric material  10  will result in a greater induced electric field Ep and a correspondingly larger magnitude of Ipiezo(t). The first stage  12  includes a circuit providing an integrating amplifier which integrates the current signal Ipiezo(t) and multiplies by a gain G in order to provide an integrated output voltage signal Vpiezo(t). The gain G need not be fixed, and in general may be by a function of time, frequency and/or the electrical parameters of a feedback network included in the first stage  12 . The current signal Ipiezo(t) and leakage currents result in the magnitude of the induced electric field Ep decaying slowly over time (also referred to herein as “rolling off”) in response to a static pressure applied by a user. For example, when a user presses the touch sensor  2 ,  21  the integrated output voltage signal Vpiezo(t) will display a rapidly rising period  41 , followed by a relatively slowly decaying period  42 . 
     The amplified signal  15  is a superposition of the integrated output voltage signal Vpiezo(t) and a capacitance measurement voltage signal Vcap(t). The capacitance voltage signal Vcap(t) is a periodic signal having a basic frequency of fd. The capacitance voltage signal Vcap(t) is based on the capacitance of the touch sensor  2 ,  21  and a periodic signal  43  provided by a signal source  44 . Relative to the periodic signal  43 , one or more of the amplitude, phase or frequency of the capacitance voltage signal Vcap(t) depends directly upon the capacitance of the touch sensor  2 ,  21 . 
     For the first touch sensor  2 , a signal source  44  may be coupled to the front end module  3  or to the second electrode  9  via terminal B. For the second touch sensor  21 , signal source(s)  44  may be coupled to the first and second front end modules  3   a ,  3   b  or to the second electrode  9  via terminal E. The signal source  44  may be a voltage controlled source Vd(fd) or a current controlled source Id(fd). When the signal source  44  is a current controlled source Id(fd) and the periodic signal  43  is an input to the front end modules  3   a ,  3   b  of the second apparatus  20 , a pair of synchronised current controlled sources Id(fd) are used so that current drawn by one front end module  3   a ,  3   b  does not disturb the periodic signal  43  supplied to the other. 
     The signal source  44  may provide a periodic signal  43  having a sinusoidal, square, triangular or saw-toothed waveform. The signal source may provide a periodic signal comprising a superposition of two or more sinusoidal waveforms having different frequencies. 
     Preferably, the front end module  3  receives the periodic signal  43  and the first stage  12  provides the amplified signal  15  based on the input signal  11  and the periodic signal  43 . The amplified signal  15  is a superposition of the integrated output voltage signal Vpiezo(t) and the capacitance measurement voltage signal Vcap(t). However, the integrated output voltage signal Vpiezo(t) and the capacitance measurement voltage signal Vcap(t) generally have distinctly different frequency contents, which facilitates separation using the first and second frequency-dependent filters  13 ,  14 . Where a user interaction does not apply a pressure to the layer of piezoelectric material the contribution of the integrated output voltage signal Vpiezo(t) to the amplified signal  15  may be zero or negligible. 
     Self capacitances of the first or third electrodes  8 ,  26 , or mutual capacitances between any pair of the first, second or third electrodes  8 ,  9 ,  26  may typically fall within the range of 0.1 to 3000 pF or more, and preferably 100-2500 pF. In order to effectively couple to capacitances in this range, the periodic signal  43  may typically have a base frequency of greater than or equal to 10 kHz, greater than or equal to 20 kHz, greater than or equal to 50 kHz or greater than or equal to 100 kHz. The periodic signal  43  may be provided with a narrow frequency band or may be provided by a single frequency signal, such as a sinusoidal signal. 
     By contrast, the integrated output voltage signal Vpiezo(t) typically includes a broadband frequency content spanning a range from several Hz to several hundreds or thousands of Hz. This is partly because the integrated output voltage signal Vpiezo(t) arises from user interactions by a human user and partly because of the slowly decaying roll off period  42 . 
     Preferably, the first frequency-dependent filter  13  attenuates the capacitance measurement voltage signal Vcap(t) such that the first filtered signal  16  is not based on the periodic signal  43 . Preferably the first filtered signal  16  is substantially equal to the integrated output voltage signal Vpiezo(t). 
     Preferably, the second frequency-dependent filter  14  selects the capacitance measurement voltage signal Vcap(t) such that the second filtered signal  17  is based on the periodic signal  43  and the capacitance of the touch sensor  2 ,  21 . Preferably, the second filtered signal  17  is substantially equal to the capacitance measurement voltage signal Vcap(t). Preferably, the first stage  12  provides the amplified signal  15  such that the amplitude of the capacitance measurement voltage signal Vcap(t) depends upon the capacitance of the touch sensor  2 ,  21 . 
     In this way, the amplitude of the first filtered signal  16  is dependent upon a pressure applied to the layer of piezoelectric material  10  by a user interaction, and the amplitude of the second filtered signal  17  is dependent upon a capacitance of a the touch sensor  2 ,  21  as modified by a user interaction. 
     Referring also to  FIG. 5 , a change in amplitude of the second filtered signal  17  in response to a user interaction in a case where the first stage  12  uses a single ended amplifier circuit is shown. 
     When there is no user interaction with a touch sensor  2 ,  21 , the second filtered signal  17  has a baseline amplitude V 0 . In response to a user interaction with the touch sensor  2 ,  21 , the amplitude of the second filtered signal changes to Vcap, which may be greater than, less than or equal to V 0 , depending on the configuration of the first stage  12 . The user interaction is detected by the change in the amplitude V 0 −Vcap of the second filtered signal  17 . 
     Referring also to  FIG. 5 , a change in amplitude of the second filtered signal  17  in response to a user interaction in a case where the first stage  12  uses a differential amplifier circuit is shown. 
     When there is no user interaction with a touch sensor  2 ,  21 , the second filtered signal  17  has a baseline amplitude V 0  which is zero, negligible or as small as possible. In response to a user interaction, the amplitude of the second filtered signal changes to Vcap which is greater than V 0 . In the same way as the case using a single ended amplifier, the user interaction is detected by the change in amplitude V 0 −Vcap of the second filtered signal  17 . However, in a case where the first stage  12  uses a differential amplifier circuit, it may be possible to increase the sensitivity of the first stage  12  amplification without requiring an analog to digital converter with a very high dynamic range in order to digitise and further process the second filtered signal  17 . 
     Referring also to  FIGS. 7 to 9 , the frequency-attenuation behaviour of the first stage  12 , and the second stage including the first and second frequency-dependent filters  13 ,  14  are shown. 
     The first stage  12  has a frequency response  45  having a low frequency cut-off fl and a high frequency cut-off fu. Below the low frequency cut-off fl and above the high frequency cut-off fu the gain G of the first stags drops rapidly so that frequencies outside the range fl to fu are blocked. The high frequency cut-off fu is greater than the base-frequency fd of the periodic signal  43 . The low-frequency cut-off is preferably at least 1 hertz, or at least sufficiently high to substantially block voltage signals resulting from a pyroelectric effect in the layer of piezoelectric material  10  which result from the body temperature of a user&#39;s digit. For application in an industrial or domestic environment, the low frequency cut-off fl may be at least 50 Hz, at least 60 Hz or at least sufficiently high to reject noise pick-up at a frequency of a domestic of industrial power distribution network and resulting from ambient electric fields. The low frequency cut-off fl may be at least 100 Hz. The low frequency cut-off fl may be at least 200 Hz. For application in aircraft, the low frequency cut-off fl may be at least 400 Hz. 
     Referring in particular to  FIG. 7 , the first frequency-dependent filter  13  may be a low-pass filter  46  having a cut-off frequency foff which is lower than the base frequency fd of the periodic signal  43 , and the second frequency-dependent filter  14  may be a band-pass filter  47  having a pass-band including the base frequency fd. 
     Referring in particular to  FIG. 8 , the first frequency-dependent filter  13  may be a band-reject filter  48  having a stop-band including the base frequency fd, and the second frequency-dependent filter  14  may be a band-pass filter  47  having a pass-band including the base frequency fd. 
     Referring in particular to  FIG. 9 , the first frequency-dependent filter  13  may be a low-pass filter  46  having a cut-off frequency foff which is lower than the base frequency fd of the periodic signal  43 , and the second frequency-dependent filter  14  may be a high-pass filter  49  having a cut-off frequency fon which is lower than the base frequency fd of the periodic signal  43  and higher than the cut-off frequency foff of the first frequency-dependent filter  13 . 
     The band-pass filter  47  and/or the band-reject filter  48  may be notch filters or comb filters. If the periodic signal  43  has a sinusoidal waveform the band-pass filter  47  and/or the band-reject filter  48  are preferably notch filters centered at the base frequency fd. If the periodic signal  43  has a non-sinusoidal waveform, then the band-pass filter  47  and/or the band-reject filter  48  are preferably wide band-filters, or comb-filters having pass/reject bands centered at the base frequency fd and harmonics thereof. 
     The first and second frequency-dependent filters  13 ,  14  may be provided by active filter circuits. The first and second frequency-dependent filters  13 ,  14  may be provided by passive filter circuits. The first and second frequency-dependent filters  13 ,  14  may be provided by single stage filters or multiple stage filters. The first and second frequency-dependent filters  13 ,  14  may be Butterworth filters, Chebyshev filters, Gaussian filters and Bessel filters. The first frequency-dependent filter  13  may be of different type to the second frequency-dependent filter. 
     Alternatively, the second stage of the front end module  3  and the first and second frequency-dependent filters  13 ,  14  may be provided by an suitably programmed information processing device such as a microprocessor or a microcontroller. 
     Examples of circuits which may provide the first stage  12  in cases where the periodic signal  43  is received by the front end module  3  shall now be described. 
     First amplifier: Referring also to  FIG. 10 , a first amplifier  50  for the first stage  12  of the front end module  3  will be explained in a case where the signal source  44  is a voltage controlled source Vd(fd) which supplies the periodic signal  43  to the front end module  3 . 
     The first touch sensor  2  is represented in the circuit diagram by an equivalent circuit  51  in which Cself represents the self capacitance of the first electrode  8 , ΔCself represents the change in the self capacitance of the first electrode resulting from the touch or proximity of a user&#39;s digit or a conductive stylus. Csensor represents the mutual capacitance between the first and second electrodes  8 ,  9  and Psensor represents the piezoelectric response of the layer of piezoelectric material  10 . The first electrode  8  is coupled to the terminal A and the second electrode  9  is coupled to a voltage bias source  52  which provides a constant bias voltage Vbias to the second electrode  9 . The voltage bias Vbias may be a positive, negative or ground potential. 
     The first amplifier  50  provides the first stage  12  of the front end module  3 . The first amplifier  50  includes an operational amplifier OP 1  having at least an inverting input coupled to a first rail  53 , a non-inverting input coupled to the voltage controlled source Vd(fd) via a path  54  including a first resistor Rd, and an output coupled to a second rail  55 . The first amplifier  50  also includes a second resistor Ri coupling the first rail  53  to the terminal A. In this way, the first amplifier  50  is coupled to the first electrode  8 . The first amplifier  50  also includes a third resistor Rf coupling the first rail  53  to the second rail  55 , and a first capacitor Cf coupling the first rail  53  to the second rail  55 . Optionally, a second capacitor Cd may be connected in parallel with the first resistor Rd. Other terminals of the operational amplifier OP 1 , such as power supply terminals, may be present but are not shown in this or other schematic circuit diagrams described herein. 
     The gain and frequency dependence of the first amplifier  50  are controlled by the third resistor Rf and the first capacitor Cf which provide a negative feedback network to the operational amplifier OP 1 . In the first amplifier  50 , the second rail  55  provides the amplified signal  15  via an output terminal Vout. Alternatively, the second rail  55  may be directly coupled to the second stage of the front end module  3 . 
     Because the non-inverting input of the operational amplifier OP 1  is coupled to the voltage controlled source Vd(fd), the amplifier is effectively provided with a periodically varying virtual earth. In this way, the amplified signal  15  output by the first amplifier  50  is modulated by the periodic signal  43 , and includes a superposition of the integrated output voltage signal Vpiezo(t) and the capacitance measurement voltage signal Vcap(t). One simple way to view the interaction with the periodic signal  43  is using one of the “Golden rules” of operational amplifiers, namely that when an ideal operational amplifier is provided with a negative feedback network, the inverting and non-inverting inputs will be at the same potential. Thus, the potential at the non-inverting input of the operational amplifier OP 1  varies with the periodic signal  43 , and couples to the capacitances of the equivalent circuit  51 . Coupling the periodic signal  43  to the capacitances of the equivalent circuit  51  using the described virtual earth configuration may have the advantage of allowing higher gains to be used to amplify the current signal Ipiezo(t) without saturating the output of the operational amplifier OP 1 . 
     The capacitance of the first capacitor Cf may be selected to be approximately equal to the mutual capacitance Csensor between the first electrode  8  and the second electrode  9 . The low frequency cut-off fl of the first amplifier  50  may be approximated as fl=1/(2π×Rf×Cf). The high frequency cut-off fu may be approximated as fu=1/(2π×Ri×Cf). In this way, the second and third resistors Ri, Rf and the first capacitor Cf may be selected such that the high frequency cut-off fu is greater than the base frequency fd of the periodic signal  43  and the low-frequency cut-off is at least 1 hertz or at least sufficiently high to substantially block voltage signals resulting from a pyroelectric effect in the layer of piezoelectric material  10 . Optionally, the low frequency cut-off fl may be at least 50 Hz or at least 60 Hz or at least sufficiently high to reject noise pick-up at a frequency of a domestic or industrial power distribution network. 
     The voltage bias source  52  need not provide a constant bias voltage Vbias, and the voltage bias source  52  may instead provide a time varying voltage. In some cases, the voltage bias source  52  may alternatively be provided by the signal source  44  such that the periodic signal  43  is provided to the second electrode  9 . 
     The equivalent circuit  51  has been shown as including a variable capacitance ΔCself arising from self capacitance of the first electrode  8 . However, the equivalent circuit  51  of the touch sensor  2  may additionally or alternatively include a variable capacitance ΔCsensor arising from a mutual capacitance between the first and second electrodes  8 ,  9 . In general, the equivalent circuit  51  may be different depending on the exact geometries of the first and second electrodes  8 ,  9  and the touch sensor  2 . 
     Referring also to  FIG. 11 , the first amplifier  50  may provide the first stages  12  of the first and second front end modules  3   a ,  3   b  for the second apparatus  20 . 
     A pair of first amplifiers  50   a ,  50   b  may provide the first stage of the respective front end modules  3   a ,  3   b  of the second apparatus  20  in a similar way to the first apparatus  1 . The input to a first amplifier  50   a  which is included in the first front end module  3   a  is coupled via terminal D to the first electrode  8  of the second touch sensor  21 . The input to a first amplifier  50   b  which is included in the second front end module  3   b  is coupled via terminal C to the third electrode  26  of the second touch sensor  21 . The second electrode  9  of the second touch sensor  21  is coupled via terminal E to a voltage bias source  52 . The same voltage controlled source  44 , Vd(fd) may be coupled to both front end modules  3   a ,  3   b  in parallel. 
     Thus, each first amplifier  50   a ,  50   b  provides a corresponding amplified signal  15   a ,  15   b  depending upon input signals  11   a ,  11   b  from the first and third electrodes  8 ,  26  respectively. One difference to the first touch sensor  2  is that the equivalent circuit  56  of the second touch sensor  21  also includes a mutual capacitance Cmut between the first electrode  8  and the third electrode  26 . 
     Second amplifier: Referring also to  FIG. 12 , a second amplifier  57  for the first stage  12  of the front end module  3  will be explained in a case where the signal source  44  is a voltage controlled source Vd(fd) which supplies the periodic signal  43  to the front end module  3 . 
     The first touch sensor  2  is represented in the circuit diagram by an equivalent circuit  51 . The first electrode  8  is coupled to the terminal A and the second electrode  9  is coupled to a voltage bias source  52  which provides a constant bias voltage Vbias to the second electrode  9 . The voltage bias Vbias may be a positive, negative or ground potential. 
     The second amplifier  57  includes a first operational amplifier OP 1  having at least an inverting input coupled to a first rail  58 , a non-inverting input coupled to the voltage controlled source Vd(fd) via a path including a first resistor Rd 1  and a second rail  59 , and an output coupled to a third rail  60 . The second amplifier  57  includes a second operational amplifier OP 2  having at least an inverting input coupled to a fourth rail  61 , a non-inverting input coupled to the second rail  59  via a path including a second resistor Rd 2 , and an output coupled to a fifth rail  62 . The second amplifier includes a comparator CM 1  having at least an inverting input coupled to the third rail  60 , a non-inverting input coupled to the fifth rail  62 , and an output providing the amplified signal  15 . The second amplifier  57  also includes a third resistor Ri 1  coupling the first rail  53  to the terminal A, a fourth resistor Rf 1  coupling the first rail  58  to the third rail  60 , a first capacitor Cf 1  coupling the first rail  58  to the third rail  60 , a fifth resistor Rf 2  coupling the fourth rail  61  to the fifth rail  62 , a second capacitor Cf 2  coupling the fourth rail  61  to the fifth rail  62 , and a sixth resistor Ri 2  coupling the fourth rail  61  to ground via a path  63  including a third capacitor Cref. 
     The first resistor Rd 1  may have a resistance substantially equal to the second resistor Rd 2 . The fourth resistor Rf 1  may have a resistance substantially equal to the fifth resistor Rf 2 . The first capacitor Cf 1  may have a capacitance substantially equal to the second capacitor Cf 2 , and also approximately equal to a mutual capacitance Csensor between the first electrode  8  and the second electrode  9 . The third capacitor Cref may have a capacitance approximately equal to a mutual capacitance Csensor between the first electrode  8  and the second electrode  9 . A fourth capacitor Cd 1  may be connected in parallel with the first resistor Rd 1 . A fifth capacitor Cd 2  may be connected in parallel with the second resistor Rd 2 . 
     The second amplifier  57  operates in substantially the same way as the first amplifier  50 , except that the second amplifier  57  is a differential amplifier which ideally provides an amplified signal  15  which has zero or negligible amplitude when there is no user interaction with the touch sensor  2 . 
     Referring also to  FIGS. 2 and 11 , the second amplifier  57  may be used in the second apparatus in the same way, as the first amplifier  50  by connecting the respective inputs of a pair of second amplifiers  57  to the first and third electrodes  8 ,  26  of the second touch sensor  21  via the terminals D and C respectively. 
     Third amplifier: Referring also to  FIG. 13 , a third amplifier  63  for the first stage  12  of the front end module  3  will be explained in a case where the signal source  44  is a current controlled source Id(fd) which supplies the periodic signal  43  to the front end module  3 . 
     The first touch sensor  2  is represented in the circuit diagram by the equivalent circuit  51 . The first electrode  8  is coupled to the terminal A and the second electrode  9  is coupled to the terminal B. Both the terminal A and terminal B are coupled to the third amplifier  63 . 
     The third amplifier  63  includes an operational amplifier OP 1  having at least an inverting input coupled to the current controlled source Id(fd) via in first rail  63 , a non-inverting input coupled to ground via a path  65  including a first resistor Rd, and an output coupled to a second rail  66 . The third amplifier  63  also includes a second resistor (Rf) coupling the first, rail  64  to the second rail  66 , and a third resistor Ri coupling the first rail  64  to ground via a path including a first capacitor Cf. In the third amplifier  63 , the second rail  66  provides the amplified signal  15 . 
     The first rail  64  is coupled to the first electrode  8  via terminal A and the second rail  66  is coupled to the second electrode  9  via terminal B. Alternatively, the first rail  64  may be coupled to the second electrode  9  via terminal B and the second rail  66  may be coupled to the first electrode  8  via terminal A. 
     A second capacitor Cd may be connected in parallel with the first resistor Rd. The first capacitor Cf may have a capacitance substantially equal to a mutual capacitance Csensor between the first electrode  8  and the second electrode  9 . 
     In many respects, for example the high and low frequency cut-offs fl, fu, the third amplifier  63  is configured similarly to the first and second amplifiers  50 ,  57 . However, the feedback network of the third amplifier  63  differs from the first or second amplifiers  50 ,  57  because it includes the first touch sensor  2 . 
     The second apparatus  20  may use front end modules  3   a ,  3   b  which have first stages  12  provided by the third amplifier  63 . A third amplifier  63  included in the first front end module  3   a  may be connected across the terminals D and E instead of A and B, and a third amplifier  63  included in the second front end module  3   b  may be connected across the terminals C and E. Optionally, a third front end module (not shown) may be used which includes a third amplifier  63  connected across the terminals C and D. When more than one third amplifier  63  is used, each third amplifier  63  should be coupled to the output of a separate, synchronised current controlled source Id(fd). 
     Fourth amplifier: Referring also to  FIG. 14 , a fourth amplifier  67  for the first stage  12  of the front end module  3  will be explained in a case where the signal source  44  is a pair of synchronised current controlled sources Id 1 ( fd ), Id 2 ( fd ) which supply the periodic signal  43  to the front end module  3 . 
     The first touch sensor  2  is represented in the circuit diagram by the equivalent circuit  51 . The first electrode  8  is coupled to the terminal A and the second electrode  9  is coupled to the terminal B. Both terminal A and terminal B are coupled to the third amplifier  63 . 
     The fourth amplifier  67  includes a first operational amplifier OP 1  having at least an inverting input coupled to a first current controlled source Id 1 ( fd ) via a first rail  68 , a non-inverting input coupled to ground via a path  69  including a first resistor Rd 1 , and an output coupled to a second rail  70 . The fourth amplifier  67  also includes a second operational amplifier OP 2  having at least an inverting input coupled to a second current Id 2 ( fd ) source by a third rail  71 , a non-inverting input coupled to ground visa path  72  including a second resistor Rd 2 , and an output coupled to a fourth rail  73 . The fourth amplifier  67  also includes a comparator CM 1  having at least an inverting input coupled to the second rail  70 , a non-inverting input coupled to the fourth rail  73 , and an output providing the amplified signal  15 . The fourth amplifier  67  also includes a third resistor Rf 1  coupling the first rail  68  and the second rail  70 , a fourth resistor Ri 1  coupling the first rail  68  to ground via a path including a first capacitor Cf 1 , a fifth resistor Ri 2  coupling the third rail  71  to ground via a path including a second capacitor Cf 2 , a sixth resistor Rf 2  coupling the third rail  71  to the fourth rail  73 , and a third capacitor Cref coupling the third rail  71  to the fourth rail  73 . The first current controlled source Id 1 ( fd ) is synchronised with the second current controlled source Id 2 ( fd ). 
     The first rail  68  is coupled to the first electrode  8  via terminal A and the second rail  70  is coupled to the second electrode  9  via terminal B. Alternatively, the first rail  68  may be coupled to the second electrode  9  via terminal B and the second rail  70  may be coupled to the first electrode  8  via terminal A. 
     The first resistor Rd 1  may have a resistance substantially equal to the second resistor Rd 2 . The third resistor Rf 1  may have a resistance substantially equal to the sixth resistor Rf 1 . The first capacitor Cf 1  may have a capacitance substantially equal to the second capacitor Cf 2  and approximately equal to a mutual capacitance Csensor between the first electrode  8  and the second electrode  9 . The third capacitor Cref may have a capacitance approximately equal to a mutual capacitance Csensor between the first electrode  8  and the second electrode  9 . A fourth capacitor Cd 1  may be connected in parallel with the first resistor Rd 1 . A fifth capacitor Cd 2  may be connected in parallel with the second resistor Rd 2 . 
     The fourth amplifier  67  operates in substantially the same way as the third amplifier  63 , except that the fourth amplifier  67  is a differential amplifier which ideally provides an amplified signal  15  which has zero or negligible amplitude when user does not touch and/or press the touch sensor  2 . 
     The second apparatus  20  may use front end modules  3   a ,  3   b  which each have a first stage  12  provided by the fourth amplifier  67 . A fourth amplifier  67  included in the first front end module  3   a  may be connected across the terminals D and G instead of A and B as shown in  FIG. 14 , and a fourth amplifier  67  included in the second front end module  3   b  may be connected the terminals C and E. Optionally, a third front end module (not shown) may be used which includes a fourth amplifier  67  connected across the terminals C and D. 
     Third combined capacitance and pressure sensing apparatus and first touch panel: Referring also to  FIGS. 15 and 16 , a third apparatus  74  including a first touch panel  29 , a first touch controller  30  and a multiplexer  75  will be explained. 
     The multiplexer  75  has a plurality of inputs and one output, the output is coupled to a terminal F. 
     The first touch panel  29  includes a layer structure  5  which is generally the same as the layer structure of the first touch sensor  2 , except that the layer structure  5  of the first touch panel  29  is shared between multiple first electrodes  8  disposed on the first face  6  in the form of pads of conductive material. The first electrodes  8  are disposed on the first face  6  in an array extending in the first and second directions x, y. Each first electrode  8  is coupled to a corresponding input of the multiplexer  75  by a respective conductive trace  76 . The conductive traces  76  may be made of the same material as the first electrodes  8 . Alternatively, the conductive traces  76  may be made of a material having a higher conductivity than the material used for the first electrodes  8 . The conductive traces  76  are generally much thinner in the plane defined by the first and second directions x, y than the corresponding first electrodes  8 . The second electrode  9  is disposed on the second face  9  and is extensive such that the second electrode at least partial underlies each first electrode  8 . The second electrode  9  may be substantially coextensive with the second face  7 . The second electrode is coupled to a terminal G. 
     In this way, each first electrode  8  effectively provides a first touch sensor  2  which may be individually addressed using the multiplexer  75  and the conductive traces  76 . 
     The first touch panel  29  may be bonded overlying the display  37  of an electronic device  28 . In this case, the materials of the first touch panel  29  should be substantially transparent as described hereinbefore. A cover lens  77  may be bonded overlying the first touch panel  29 . The cover lens  77  is preferably glass but may be any transparent material. The coyer lens  77  may be bonded to the touch panel  29  using a layer of PSA material  78 . The layer of PSA material  78  may be substantially transparent. The array of first electrodes  8  mid the corresponding conductive traces  76  may be fabricated using index matching techniques to minimise visibility to a user. 
     The first touch controller  30  includes a front end module  3 , a signal processing module  4  and a controller  79 . The controller  79  may communicate with the processor  32  of the electronic device  28  using a link  80 . The touch controller  30  may include a signal source  44  providing the periodic signal  43  to the front end module  3 . 
     The front end module  3  is coupled to the output of the multiplexor  75  by the terminal F. In this way, the front end module  3  may receive an input signal  11  from any one of the first electrodes  8  which is addressed by the multiplexer  75 . The front end module  3  may include a first stage  12  provided by any one of the first, second, third or fourth amplifiers  50 ,  57 ,  63 ,  67 . Alternatively, the front end module  3  may include a first stage  12  using any circuit which provides an amplified signal  15  based on the input signal  11  and including a superposition of the integrated output voltage signal Vpiezo(t) and the capacitance measurement voltage signal Vcap(t) as hereinbefore described. 
     In a case where the front end module  3  includes the first or second amplifiers  50 ,  57 , the second electrode  9  may be coupled to a bias voltage source  52  via the terminal G. Alternatively, in a case when the front end module  3  includes the third or fourth amplifiers  63 ,  67 , the second electrode  9  may be coupled to the front end module  3  via the terminal G. 
     The controller  79  may provide a control signal  81  to the multiplexer  75 . The control signal  81  may cause the multiplexer  75  to address each input according to a sequence determined by the controller  79 . In this way, the from end module  3  may receive an input signal  11  from each first electrode  8  according to a sequence determined by the controller  80 . The sequence may be pre-defined, for example, the sequence may select each first electrode  8  once before repeating. The sequence may be dynamically determined, for example, when one or more user interactions are detected, the controller  79  may scan the subset of first electrodes  8  adjacent to each detected user interaction in order to provide faster and/or more accurate tracking of user touches. The sequence may be arranged so that the multiplexor  75  addresses each first electrode  8  during a quiet period or blanking period of the display  37 . The sequence may be provided to the controller  79  by the processor  32  via the link  80 . Alternatively, the processor may directly control the sequence via the link  80 . 
     In this way, the front end module  3  may receive input signals  11  from each of the first electrodes  8  which are disposed in an array extending in the first and second directions. The signal processing module  4  provides respective pressure values  18  and capacitance values  19  to the controller. The controller  79  uses the received pressure values  18  and capacitance values  19  to calculate a position and an applied pressure for one or more user interactions with the touch panel  29 . The controller  79  provides the positions and/or pressures of one or more user interactions to the processor  32  as input information via the link  80 . Alternatively, the pressure values  18  and capacitance values  19  may be provided to the processor  32  via the link  80  and the processor  32  may calculate the positions and/or pressures of one or more user interactions. The controller  79  or the processor  32  are calibrated by applying known pressures to known locations so that the accuracy of calculated positions and/or pressures of one or more user interactions may be optimised and/or verified. 
     Referring also to  FIG. 17 , a configuration of the first amplifier  50  included in the third apparatus  74  is shown. 
     The configuration of the first amplifier  50  included in the third apparatus  74  is substantially the same as the configuration of the first amplifier  50  included in the first apparatus  1 , except that the first rail  53  is coupled to the output of the multiplexor  75  via the terminal F instead of being coupled to the terminal A, the voltage bias source  52  is coupled to the second electrode  9  via the terminal G instead of the terminal B, and that the first amplifier  50  further includes a switch SW 1 . 
     The switch SW 1  is coupled between the first rail  53  and the second rail  55  of the first amplifier  50 . When the switch SW 1  is closed, the first capacitor C-f of the first amplifier  50  is discharged. The opening and closing of the switch SW 1  may be governed by a control signal  82  provided by the controller  79 . In this way, after an input signal  11  has been received from one of the first electrodes  8 , the first capacitor Cf may be discharged in order to reset the feedback network of the first amplifier  50  before the multiplexer  75  addresses a different first electrode  8 . 
     Referring also to  FIG. 18 , a configuration of the second amplifier  57  included in the third apparatus  74  is shown. 
     The configuration of the second amplifier  57  included in the third apparatus  74  is substantially the same as the configuration of the first amplifier  50  included in the third apparatus  74 , except that the second amplifier  57  is used instead of the first amplifier  50  and the second amplifier  57  further includes a first switch SW 1  and a second switch SW 2 . 
     The first switch SW 1  couples the first rail  58  to the third rail  60  of the second amplifier  57  and the second switch SW 2  couples the fourth rail  61  to the fifth rail  62 . When the switch SW 1  is closed, the first capacitor C-f 1  of the second amplifier  57  is discharged. When the switch SW 2  is closed, the second capacitor C-f 2  of the second amplifier  57  is discharged. The opening and closing of the switches SW 1 , SW 2  may be governed by control signals  82  provided by the controller  79 . In this way, after an input signal  11  has been received from one of the first electrodes  8 , the capacitors Cf 1 , Cf 2  may be discharged so as to reset the feedback network of the second amplifier  57  before the multiplexer  75  addresses a different first electrode  8 . 
     Alternatively, the third apparatus  74  may include the third amplifier  63 . In such a case, the first rail  64  of the third amplifier  63  may be coupled to the output of the multiplexor  75  via terminal F and the second rail  66  of the third amplifier  63  may be coupled to the second electrode  9  via terminal G. Alternatively, the first rail  64  of the third amplifier  63  may be coupled to the second electrode  9  via terminal G and the second rail  66  of the third amplifier  63  may be coupled to output of the multiplexor  75  via terminal F. 
     Alternatively, the third apparatus  74  may include the fourth amplifier  67 . In such a case, the first rail  68  of the fourth amplifier  67  may be coupled to the output of the multiplexor  75  via terminal F and the second rail  70  of the fourth amplifier  67  may be coupled to the second electrode  9  via terminal G. Alternatively, the first rail  68  of the fourth amplifier  67  may be coupled to the second electrode  9  via terminal G and the second rail  70  of the fourth amplifier  67  may be coupled to the multiplexor  75  via terminal F. 
     When a user interacts with the first touch panel  29  proximate to a given first electrodes  8 , the resulting change in capacitance of the respective touch sensor  2  formed by the given first electrode  8 , the local region of the layer structure  5  and the local region of the second electrode  9 , is mostly due to a change in the self-capacitance of the first electrode  8 . This is because the magnitude of the mutual capacitance Csensor between the first electrode  8  and the second electrode  9  may be large such that a change in the mutual capacitance is relatively small. The value of the mutual capacitance Csensor between the first electrode B and the second electrode  9  may be reduced if required by using a patterned second electrode  9 . Using a patterned second electrode  9  may allow the second electrode  9  to be disposed between a user&#39;s digit/stylus and the first and/or third electrodes  8 ,  26  without screening electrostatic interactions between the user&#39;s digit/stylus and the first and/or third electrodes  8 ,  26 . 
     Referring also to  FIG. 19 , a patterned second electrode  83  is in the from of a Cartesian grid. The conductive region of the patterned second electrode  83  includes struts  84  extending in the first direction x and having a width W in the second direction y, and struts  85  extending in the second direction y and having a width W in the first direction x. The struts  84  extending in the first direction x are evenly spaced in the second direction y with a spacing S, and the struts  85  extending in the second direction y are evenly spaced in the first direction x with the same spacing S. The struts  84 ,  85  are joined where they intersect such that the patterned second electrode  83  is formed of a single region of conductive material. 
     The patterned second electrode  83  may be arranged such that the magnitude of the mutual capacitance Csensor between the first electrode  8  and the second electrode  9  is reduced. This may increase the relative size of changes in the mutual capacitance ΔCsensor between the first electrode  8  and the second electrode  9  resulting from a user&#39;s touch, making such changes ΔCsensor easier to detect. 
     Referring also to  FIG. 20 , using pressure values to infer a location and/or pressure of a user interaction occurring between two first electrodes  8  will be explained. 
     The separation between adjacent electrodes in a projected capacitance touch panel, sometimes referred to as the pitch, may be relatively coarse, for example, 1 to 5 mm or larger than 5 mm. If the positions of user interactions were determined only at the pitch length, projected capacitance touchscreens would not be able to provide accurate positions of user interactions or to smoothly follow a path traced by a user. To provide more accurate locations, projected capacitance touchscreens employ interpolation, for example, using an electrode having a peak signal and also the adjacent electrode signals, in order to infer a touch location using linear interpolation, quadratic interpolation or interpolation using higher order polynomials or other suitable functions. Such interpolation is possible because a user interaction may alter the capacitances of several adjacent electrodes simultaneously. 
     Similarly, when a user presses on the cover lens  77 , the layer of piezoelectric material  10  underlying the cover lens  77  will experience strain across a broader area because of the rigidity of the cover lens  77 . For example, a user interaction at a precise location  86  may result in pressure values  87   a  and  87   b  being calculated for first electrodes  8  at locations  88   a ,  88   b  which bracket the precise location  86 . A user interaction al a precise location  86  may also result in pressure values  89   a  and  89   b  being calculated for first electrodes  8  at locations  90   a ,  90   b  adjacent to the pair of bracketing locations  88   a ,  88   b.    
     The controller  79  or the processor  32  may calculate an estimate of the precise location  86  and/or a precise pressure value  91  using the largest value  87   a  and the corresponding location  88   a  in combination with the two next nearest values  87   b ,  89   a  and the corresponding locations  88   b ,  90   a . Alternatively, the controller  79  or the processor  32  may calculate an estimate of the precise location  86  and/or a precise pressure value  91  using the pair of bracketing values  87   a ,  87   b  and locations  88   a ,  88   b . The controller  79  or the processor  32  may calculate an estimate of the precise location  86  and/or a precise pressure value  91  using the pair of bracketing values  87   a ,  87   b  and locations  88   a ,  88   b  and the adjacent values and locations  89   a ,  89   b ,  90   a ,  90   b . The controller  79  or the processor  32  may calculate an estimate of the precise location  86  and/or a precise pressure value  91  using linear interpolation, quadratic interpolation or interpolation using higher order polynomials or other suitable functions. 
     The third apparatus  73  may operate using interpolation of capacitance values  19  and/or pressure values  18  to determine locations and pressures of one or more user interactions. 
     Fourth combined capacitance and pressure sensing apparatus and second touch panel: Referring also to  FIGS. 21 and 22 , a fourth apparatus  93  including a second touch panel  92 , a first touch controller  30  and a multiplexer  75  will be explained. 
     The fourth apparatus  93  is substantially the same as the third apparatus  74 , except that the fourth apparatus  93  includes a second touch panel  92  instead of the touch panel  29 . 
     The second touch panel  92  includes a layer structure  5  which is generally the same as the layer structure  5  of the second touch sensor  21 , except that in the touch panel  92 , the layer structure  5  is shared by multiple first electrodes  8  disposed on the first face  6  of the layer structure  5 , and the second layer structure  23  is shared by multiple third electrodes  26  disposed on the third face  24  of the second layer structure  23 . The first electrodes  8  each extend in the first direction x and the first electrodes  8  are disposed in an array evenly spaced in the second direction y. The third electrodes  26  each extend in the second direction y and the third electrodes  26  are disposed in an array evenly spaced in the first direction x. Each first electrode  8  and each third electrode  26  is coupled to a corresponding input of the multiplexer  75  by a respective conductive trace  76 . The second electrode  9  is disposed on the second face  9  and is extensive such that the second electrode at least partially underlies each first electrode  8  and each third electrode  26 . The second electrode  9  may be substantially coextensive with the second face  7 . The second electrode is coupled to a terminal G. 
     In this way, the area around each intersection of a first electrode  8  with a third electrode  26  effectively provides a second touch sensor  21  and each of the first and third  8 ,  26  electrodes may be individually addressed using the multiplexer  75  and the conductive traces  76 . 
     The second touch panel  92  may be bonded overlying the display  37  of an electronic device  28  and a cover lens  77  may be bonded overlying the second touch panel  92  in the same way as for the touch panel  29 . 
     The controller  79  may provide a control signal  81  to the multiplexer  75  in the same way as for the third apparatus  74 . However, in the fourth apparatus  93 , the control signal  81  may cause the multiplexer  75  to address each input according to a different sequence to the third apparatus  74 . For example, the control signal  81  may cause the multiplexer  75  to address a given first electrode  8 , and to subsequently address each third electrode  26  intersecting the given first electrode  8  before addressing a further first electrode  8  and repeating the scan through the third electrodes  26 . The control signal  81  may cause the multiplexer to dynamically address first and third electrodes  8 ,  26  proximate to first and third electrodes  8 ,  26  at which a user touch was previously detected. 
     In this way, a raster scan of the first and third electrodes  8 ,  26  may be performed which allows the first touch controller  30  to determine the positions and/or pressures of one or more user interactions. 
     In the similar way to the third apparatus  74 , the changes in capacitance values  19  generated in the fourth apparatus in response to user interactions may be predominantly due to changes in self-capacitance of the addressed first or second electrode  8 ,  26 . However, if a given pair of first and third electrodes  8 ,  26  are addressed sequentially and without excessive delay, a change in the mutual capacitance between the given pair of first and third electrodes  8 ,  26  may additionally be determined. 
     Although the first electrode  8  and the third electrode  26  have been shown as being substantially rectangular, other shapes can be used. 
     Referring also to  FIG. 23 , an alternative arrangement of the first and third electrodes  8 ,  26  is shown. Instead of being rectangular, each first electrode  8  may include several pad segments  94  evenly spaced in the first direction x and connected to one another in the first direction x by relatively narrow bridging segments  95 . Similarly each third electrode  26  may comprise several pad segments  96  evenly spaced in the second direction y and connected to one another in the second direction y by relatively narrow bridging segments  97 . The pad segments  94  of the first electrodes  8  are diamonds having a first width W 1  in the second direction  7  and the bridging segments  95  of the first electrodes  8  have a second width W 2  in the second direction y. The pad segments  96  and bridging segments  97  of the third electrodes  26  have the same respective shapes and widths W 1 , W 2  as the first electrodes  8 . 
     The first electrodes  8  and the third electrodes  26  are arranged such that the bridging segments  97  of the third electrodes  26  overlie the bridging segments  95  of the first electrodes  8 . Alternatively, the first electrodes  8  and the third electrodes  26  may be arranged such that the pad segments  96  of the third electrodes  26  overlie the pad segments  94  of the first electrodes  8 . The pad segments  94 ,  96  need not be diamond shaped, and may instead be circular. The pad segments  94 ,  96  may be a regular polygon such as a triangle, square, pentagon or hexagon. The pad segments  94 ,  96  may be I shaped or Z shaped. 
     Third touch panel: Referring also  FIG. 24 , a third touch panel  98  may be included in the fourth apparatus  93  instead of the second touch panel  92 . 
     The third touch panel  98  is substantially the same as the second touch panel  92  except that the third touch panel  98  does not include the second layer structure  23  and the third electrodes  26  are disposed on the first face  6  of the layer structure  5  in addition to the first electrodes  8 . Each first electrode  8  is a continuous conductive region extending in the first direction x in the same way as the second touch panel  92 , for example, each first electrode  8  may include several pad segments  94  evenly spaced in the first direction x and connected to one another in the first direction x by relatively narrow bridging segments  95 . Each third electrode  26  may comprise several pad segments  99  evenly spaced in the second direction y in the same way as the second touch panel  92 . However, unlike the second touch panel  92 , the pad segments  99  of the third touch panel are disposed on the first face  6  of the layer structure  5  and are interspersed with, and separated by, the first electrodes  8 . The pad segments  99  corresponding to each third electrode  26  are connected together by conductive jumpers  100 . The jumpers  100  each span a part of a first electrode  8  and the jumpers  100  are insulated from the first electrodes  8  by a thin layer of dielectric material (not shown) which may be localised to the area around the intersection of the juniper  100  and the first electrode  8 . 
     Alternatively, a dielectric layer (not shown) may overlie the first face  6  of the layer structure  5  and the first and third electrode  8 ,  26 . Conductive traces (not shown) extending in the second direction y may be disposed over the dielectric layer (not shown), each conductive trace (not shown) overlying the pad segments  99  making up one third electrode  26 . The overlying conductive traces (not shown) may connect the pad segments  99  making up each third electrode  26  using vias (not shown) formed through the dielectric layer (not shown). 
     Fifth combined capacitance and pressure sensing apparatus: Referring also to  FIG. 25 , a fifth apparatus  93  including a second touch panel  92 , a second touch controller  102  and first and second multiplexers  75   a ,  75   b  will be explained. 
     The first multiplexer  75   a  has a plurality of inputs and an output coupled to the terminal H. The second multiplexer  75   b  has a plurality of inputs and an output coupled to the terminal I. 
     The second touch panel  92  of the fifth apparatus  101  is the same as the second touch panel  92  of the fourth apparatus  93 , except that each of the first electrodes  8  is coupled to a corresponding input of the first multiplexor  75   a  by a respective first conductive trace  76   a , each of the third electrodes  26  is coupled to a corresponding input of the second multiplexer  75   b  by a respective second conductive trace  76   b , and the second electrode  9  is coupled to a terminal J. 
     The second touch controller  102  includes the first and second front end modules  3   a ,  3   b , the second signal processing module  22  and the controller  79 . Each of the first and second front end modules  3   a ,  3   b  is substantially the same as the front end module  3  of the third and fourth apparatuses  74 ,  93 . The second signal processing module  22  is substantially the same as the second signal processing module  22  of the second apparatus  20 . The controller  79  is substantially the same as the controller  79  of the third of fourth apparatuses  74 ,  93 , except that the control signals  81  may cause the first and second multiplexers  75   a ,  75   b  to address each given pair of first and third electrodes  8 ,  26  according to a sequence determined by the controller  79  or communicated to the controller  79  from the processor  32  by the link  80 . The sequence may be a predetermined sequence or a dynamically determined sequence. 
     In this way, each intersection of the first and third electrodes  8 ,  26  effectively provides a second touch sensor  21  which may be individually addressed by the first and second multiplexers  75   a ,  75   b . When a particular intersection is addressed by the first and second multiplexers  75   a ,  75   b , a first input signal  11   a  from the respective first electrode  8  is received by the first front end module  3   a  and a second input signal  11   b  from the respective third electrode  26  is concurrently received by the second front end module. 
     Alternatively, the fifth apparatus  101  may use the third touch panel  98  instead of the second touch panel  92 . 
     In addition to changes in the self-capacitances of given first and third electrode  8 ,  26  addressed by the multiplexers  75   a ,  75   b , the respective capacitance values  19   a ,  19   b  also include a change in the mutual capacitance between the addressed pair of first and third electrodes  8 ,  26 . 
     Referring also to  FIG. 26 , a configuration using first amplifier  50  to provide the first stages  12   a ,  12   b  of the first and second front end modules  3   a ,  3   b  will be explained. 
     A configuration of the fifth apparatus  101  including the first amplifier  50  is substantially the same as the configuration of the third or fourth apparatuses  74 ,  93  including the first amplifier  50 , except that a pair of multiplexers  75   a ,  75   b , a pair of front end modules  3   a ,  3   b  and a pair of first amplifiers  50   a ,  50   b  are used, and in that the first amplifiers  50   a ,  50   b  further include a first switch SW 1  and a second switch SW 2 . 
     The pair of first amplifiers  50   a ,  50   b  provide the first stages  12   a ,  12   b  of the first and second front end modules  3   a ,  3   b  respectively. The first rail  53  of the first amplifier  50   a  which is included in the first front end module  3   a  is coupled via the terminal H to the output of the first multiplexer  75   a . The first rail  53  of the first amplifier  50   b  which is included in the second front end module  3   b  is coupled via the terminal I to the output of the second multiplexer  75   b . The second electrode  9  of the second or third touch panels  92 ,  98  is coupled via terminal J to the voltage bias source  52 . The same voltage controlled source Vd(fd) may be coupled to both front end modules  3   a ,  3   b  in parallel. 
     The pair first amplifiers  50   a ,  50   b  include a first switch SW 1  and a second switch SW 2 . The first switch SW 1  couples the first rail  53  to the second rail  55  of the first amplifier  50   s  included in the first front end module  3   a . The second switch SW 2  couples the first rail  53  to the second rail  55  of the first amplifier  50   b  included in the second front end module  3   b . When the switch SW 1  is closed, the first capacitor C-f of the first amplifier  50   a  is discharged. When the switch SW 2  is closed, the first capacitor C-f of the first amplifier  50   b  is discharged. The opening and closing of the switches SW 1 , SW 2  may be governed by control signals  82  provided by the controller  79 . In this way, after the first front end module  3   a  has received, an input signal  11  from one of the first electrodes  8  and the second front end module  3   b  has received an input signal  11  from one of the third electrodes  26 , the capacitors Cf of the first amplifiers  50   a ,  50   b  may be discharged so as to reset the respective feedback networks before the multiplexers  75   a ,  75   b  connect a different pairing of first and third electrodes  8 ,  26 . 
     Thus, each first amplifier  50   a ,  50   b  provides a corresponding amplified signal  15   a ,  15   b  depending upon first and second input signals  11   a ,  11   b  received from the first and third electrodes  8 ,  26  addressed by the respective first and second multiplexers  75   a ,  75   b . One difference to the third or fourth apparatuses  74 ,  93  is that the equivalent circuit  56  of each second touch sensor  21  provided by an addressed pair of first and second electrodes  8 ,  26  additionally includes a mutual capacitance Cmut between the selected first electrode  8  and the selected third electrode  26 . 
     Alternatively, the fifth apparatus  102  may be configured using first and second front end modules  3   a ,  3   b  including respective second amplifiers  57 . In such a case, the first rail  58  of the second amplifier  57  included in the first front end module  3   a  may be coupled to the output of the first multiplexer  75   a  via the terminal H, the first rail  58  of the second amplifier  57  included in the second frontend module  3   b  may, be coupled to the output of the second multiplexer  75   b  via the terminal I, and the second electrode  9  of the second or third touch panels  92 ,  98  may be coupled via terminal J to a voltage bias source  52 . 
     Alternatively, the fifth apparatus  102  may be configured using first and second front end modules  3   a ,  3   b  including respective third amplifiers  63 . In such a case, the first rail  64  of the third amplifier  63  included in the first front end module  3   a  may be coupled to the terminal H and the corresponding second rail  66  may be coupled to the terminal J, and the first rail  64  of the third amplifier  63  included in the second front end module  3   b  may be coupled to the terminal I and the corresponding second rail  66  may be coupled to the terminal J. Alternatively, the first rail  64  of the third amplifier  63  included in the first front end module  3   a  may be coupled to the terminal J and the corresponding second rail  66  may be coupled to the terminal H, and the first rail  64  of the third amplifier  63  included in the second front end module  3   b  may be coupled to the terminal J and the corresponding second rail  66  may be coupled to the terminal I. 
     Alternatively, the fifth apparatus  102  may be configured using first and second front end modules  3   a ,  3   b  including respective fourth amplifiers  67 . In such a case, the first rail  68  of the fourth amplifier  67  included in the first front end module  3   a  may be coupled to the terminal H and the corresponding second rail  70  may be coupled to the terminal J, and the first rail  68  of the fourth amplifier  67  included in the second front end module  3   b  may be coupled to the terminal I and the corresponding second rail  70  may be coupled to the terminal J. Alternatively, the first rail  68  of the fourth amplifier  67  included in the first front end module  3   a  may be coupled to the terminal J and the corresponding second rail  70  may be coupled to the terminal H, and the first rail  68  of the fourth amplifier  67  included in the second front end module  3   b  may be coupled to the terminal J and the corresponding second rail  70  may be coupled to the terminal I. 
     Sixth combined capacitance and pressure sensing apparatus and fourth touch panel: Referring also to  FIGS. 27 and 28 , a sixth apparatus  104  including a fourth touch panel  103 , a third touch controller  105  and first and second multiplexers  75   a ,  75   b  will be explained. 
     The sixth apparatus  104  includes first and second multiplexers  75   b , a fourth touch panel  103  and a third touch controller  105 . In many respects the sixth apparatus  104  is substantially similar or analogous to the third to fifth apparatuses  74 ,  93 ,  101 , and only the differences of the sixth apparatus  104  shall be explained. 
     The first and second multiplexers  75   a ,  75   b  each include a plurality of inputs and an output. The output of the first multiplexer  75   a  is coupled to a terminal K and the output of the second multiplexer  75   b  is coupled to a terminal L. 
     The fourth touch panel  103  includes a layer structure  5  which is generally the same as the layer structure  5  of the first touch sensor  1 , except that in the fourth touch panel  103 , the layer structure  5  is common to many first electrodes  8  disposed on the first face  6  of the layer structure  5  and many second electrodes  9  disposed on the second face  7  of the layer structure  5 . The first electrodes  8  each extend in the first direction x and the first electrodes  8  are disposed in an array evenly spaced in the second direction y. The second electrodes  9  each extend in the second direction y and the second electrodes  9  are disposed in an array evenly spaced in the first direction x. Each second electrode  9  is coupled to a corresponding input of the first multiplexer  75   a  by a respective conductive trace  76   a , and each first electrode  8  is coupled to a corresponding input of the second multiplexer  75   b  by a respective conductive trace  76   b . In this way, the area around each intersection of a first electrode  8  with a second electrode  9  effectively provides a first touch sensor  2  and each of the first and second  8 ,  9  electrodes may be individually addressed using the first and second multiplexers  75   a ,  75   b  and the conductive traces  76   a ,  76   b.    
     The fourth touch panel  103  may be bonded overlying the display  37  of an electronic device  28  and a cover lens  77  may be bonded overlying the fourth touch panel  103  in the same way as for the touch panel  29 , the second touch panel  92  or the third touch panel  98 . 
     The third touch controller  105  includes a front end module  3 , a signal source  44 , a signal processing module  4  and a controller  79  which are the same as hereinbefore described, except that the first stage  12  of the front end module  3  uses the third amplifier  63  of the fourth amplifier  67  and the signal source  44  is one or more synchronised current controlled sources Id(fd). In this case, the front end module  3  is connected across the terminals K and L. 
     The controller  79  may provide a control signal  81  to the first and second multiplexers  75   a ,  75   b  in the same way as for the fifth apparatus  101 . In this way, each intersection of the first and third electrodes  8 ,  26  effectively provides a first touch sensor  2  which may be individually addressed by the first and second multiplexers  75   a ,  75   b.    
     In addition to changes in the self-capacitances of given first and second electrodes  8 ,  9  addressed by the multiplexers  75   a ,  75   b , the capacitance values  19  also include a change in the mutual capacitance between the addressed pair of first and second electrodes  8 ,  9 . 
     Although the first electrode  8  and the third electrode  26  have been shown as being substantially rectangular, other shapes can be used. For example, the first and second electrodes  8 ,  9  of the fourth touch panel  103  may have shapes and configurations substantially similar to the first and third electrodes  8 ,  26  of the second touch panel  92 . 
     Referring also to  FIG. 29 , a configuration of the third amplifier  63  included in the sixth apparatus  104  will be explained. 
     The first rail  64  of the third amplifier  63  may be coupled to the terminal K and the second rail  66  may be coupled to the terminal L. Alternatively, first rail  64  of the third amplifier  63  may be coupled to the terminal L and the second rail  66  may be coupled to the terminal K. 
     Alternatively, the fourth amplifier  67  may be included in the sixth apparatus  104 . In such a case, the first rail  68  of the fourth amplifier  67  may be coupled to the terminal K and the second rail  70  may be coupled to the terminal L. Alternatively, first rail  68  of the fourth amplifier  67  may be coupled to the terminal L, and the second rail  70  may be coupled to the terminal K. 
     First display stack up: Referring also to  FIGS. 30A to 30C , a first display stack-up  106  and a method of fabricating the first display stack  106  up will be explained. 
     Referring in particular to  FIG. 30A , the cover tens  77  is a transparent substrate extending in the first x and second y directions and having first  77   a  and second  77   b  opposite faces. A first dielectric layer  107  extends in the first x and second y directions and has first  107   a  and second  107   b  opposite faces. Third electrodes  26  in the form of a set of conductive regions extending in the first direction x and spaced apart in the second direction y are disposed on the second face  107   b  of the first dielectric layer  107 . The second face  107   b  of the first dielectric layer  107  is bonded to the first face  77   a  of the cover lens  77 . 
     Referring in particular to  FIG. 30B , a second dielectric layer  108  extends in the first x and second y directions and has first  108   a  and second  108   b  opposite faces. First electrodes  8  in the form of a set of conductive regions extending in the second direction y and spaced apart in the first direction x are disposed on the second face  108   b  of the second dielectric layer  108 . The second face  108   b  of the second dielectric layer  108  is bonded to the first face  107   a  of the first dielectric layer  107 . 
     Referring in particular to  FIG. 30C , a layer of piezoelectric material  10  extends in the first x and second y directions and has first  10   a  and second  10   b  opposite faces. A second electrode  9  in the form of a conductive material region is disposed on the first face  10   a  of the layer of piezoelectric material  10  such that, when assembled, the second electrode  9  at least partially overlaps each first electrode  8  and each third electrode  26  region. The second face  10   b  of the layer of piezoelectric material  10  is bonded to the first face  108   a  of the second dielectric layer  108 . 
     The first display stack-up  106  may be bonded overlying the display  37  of an electronic device  28 . The elements of the first display stack-up  106  are stacked in the thickness direction z from the display  37  to the cover lens  77 . The layer structure  5  includes the second dielectric layer  108  and the layer of piezoelectric material  10  and the second layer structure  23  includes the first dielectric layer  108 . 
     The cover lens  77  is made of glass, or PET or any oilier substantially transparent material. The cover lens  77  may be up to about 20 mm thick and may be at least 0.05 mm thick. Preferably, the cover lens  77  is up to about 2 mm thick and may be at least 0.05 mm thick. The layer of piezoelectric material  10  is made of PVDF or any other substantially transparent piezoelectric material. The layer of piezoelectric material  10  may be poled before assembling the first stack-up  106 . Alternatively, the layer of piezoelectric material may be poled after assembling the first stack-up  106 . The layer of piezoelectric material may be up to about 110 μm thick, and may be at least 0.5 μm or at least 1 μm thick. The second electrode  9  and the first electrodes  8  and/or third electrodes  26  may be used to produce a poling field. The dielectric layers  107 ,  108  may be PET or any other substantially transparent polymer. The dielectric layers  107 ,  108  may be between 10 μm and 100 μm thick, for example, around 20 to 25 μm thick. Preferably the dielectric layers  107 ,  108  are in the range of about 10-100 μm thick. The conductive regions providing the electrodes  8 ,  9 ,  26  may be ITO, IZO or any other substantially transparent conductive material. The conductive regions providing the electrodes  8 ,  9 ,  26  may be applied to the dielectric layers  107 ,  108  and/or the layer of piezoelectric material  10  using lithography, printing or other suitable methods. The shapes of the conductive regions providing the first, second and third electrodes  8 ,  9   26  may be any suitable electrode shape described in relation to one of the third to sixth apparatuses  74 ,  93 ,  101 ,  104 . The sheet resistance of conductive regions providing electrodes may be between 1 and 200 Ω/sq. The sheet resistance may be below 10 Ω/sq. Preferably the sheet resistance is as low as is practical. 
     The assembly of the first display stack-up  106  has been described in a certain sequence. However, the elements of the first display stack-up may be bonded together in any other sequence resulting in the same ordering of layers  107 ,  108 ,  10 . In particular, the first and second dielectric layers  107 ,  108  and the layer of piezoelectric material  10  may be bonded together using continuous roll-to-roll production methods before being bonded to the cover lens  77 . When the cover lens  77  is a flexible material, the first display stack-up  106  may be fabricated entirely using continuous roll-to-roll processes. 
     The first display stack-up  106  does not require complex patterning of the layer of piezoelectric material  10  or of electrodes disposed on the layer of piezoelectric material  10 . This allows fabrication of the first display stack-up to avoid complex multi-stage and/or duplex patterning of electrodes. As a result, fabrication of the first display stack-up  106  may be fast, efficient and cheap. 
     Second display stack up: Referring also to  FIGS. 31A to 31C , a second display stack-up  109  and a method of fabricating the second display stack-up  109  will be explained. 
     The second display stack-up  109  is the same as the first display stack-up  106 , except that elements of the second display stack-up  109  are bonded to one another using layers of pressure sensitive adhesive (PSA) material  110  extending in the first x and second y directions. For example, the cover tens  77  and the first dielectric layer  107  are arranged so that the first face  77   a  of the cover lens  77  is opposite to and separated from the second face  107   b  of the first dielectric layer  107 . Pressure applied in the thickness direction z to bond the cover lens  77  and the first dielectric layer  107  together. Layers of PSA material  110  are used in the same way to bond the first and second dielectric layers  107 ,  108 , to bond the second dielectric layer  108  to the layer of piezoelectric material  10  and to bond the second stack-up  109  overlying the display  37 . Layers of PSA material  100  may be between 10 and 50 μm thick. Preferably, the layers of PSA material  110  are 25 μm thick. 
     The elements of the second display stack-up  109  are stacked in the thickness direction z from the display  37  to the cover lens  77 . The layer structure  5  includes the second dielectric layer  108 , the layer of piezoelectric material  10  and a layer of PSA material  110 . The second layer structure  23  includes the first dielectric layer  107  and a layer of PSA material  110 . 
     Third display stack-up: Referring also to  FIGS. 32A to 32C , a third display stack-up  111  and a method of fabricating the third display stack-up  111  will be explained. 
     Referring in particular to  FIG. 32A , the cover lens  77  is a transparent substrate extending in the first x and second y directions and having first  77   a  and second  77   b  opposite faces. A first dielectric layer  107  extends in the first x and second y directions and has first  107   a  and second  107   b  opposite faces. Third electrodes  26  in the form of a set of conductive regions extending in the first direction x and spaced apart in the second direction y are disposed on the second face  107   b  of the first dielectric layer  107 . The second face  107   b  of the first dielectric layer  107  is bonded to the first face  77   a  of the cover lens  77  using a layer of PSA material  110 . 
     Referring in particular to  FIG. 32B , a layer of piezoelectric material  10  extends in the first x and second y directions and has first  10   a  and second  10   b  opposite faces. First electrodes  8  in the form of conductive regions extending in a second direction y and spaced apart in the first direction x are disposed on the second face  10   b  of the layer of piezoelectric material  10 . A second electrode  9  in the form of a conductive material region is disposed on the first face  10   a  of the layer of piezoelectric material  10  such that, when assembled, the second electrode  9  at least partially overlaps each first electrode  8  and each third electrode  26 . The second face  10   b  of the layer of piezoelectric material  10  may be bonded to the first face  107   a  of the first dielectric layer  107  using a layer of PSA material  110 . 
     Referring in particular to  FIG. 32C , the third display stack up  111  may be bonded overlying the display  37  using a layer of PSA material  110 . 
     The elements of the third display stack-up  111  are stacked in the thickness direction z from the display  37  to the cover lens  77 . The layer structure  5  includes the layer of piezoelectric material  10 . The second layer structure  23  includes the first dielectric layer  107  and a layer of PSA material  110 . 
     Fourth display stack-up: Referring also to  FIGS. 33A to 33D , a fourth display stack-up  112  and a method of fabricating the fourth display stack-up  112  will be explained. 
     Referring in particular to  FIG. 33A , the cover lens  77  is a transparent substrate extending in the first x and second y directions and having first  77   a  and second  77   b  opposite faces. A first dielectric layer  107  extends in the first x and second y directions and has first  107   a  and second  107   b  opposite faces. Third electrodes  26  in the form of a set of conductive regions extending in the first direction x and spaced apart in the second direction y are disposed on the second face  107   b  of the first dielectric layer  107 . The second face  107   b  of the first dielectric layer  107  is bonded to the first face  77   a  of the cover lens  77  using a layer of PSA material  110 . 
     Referring in particular to  FIG. 33B , a second dielectric layer  108  extends in the first x and second y directions and has first  108   a  and second  108   b  opposite faces. First electrodes  8  in the form of a set of conductive regions extending in the second direction y and spaced apart in the first direction x are disposed on the second face  108   b  of the second dielectric layer  108 . The second face  108   b  of the second dielectric layer  108  is bonded to the first face  107   a  of the first dielectric layer  107  using a layer of PSA material  110 . 
     Referring in particular to  FIG. 33C , a layer of piezoelectric material  10  extends in the first x and second y directions and has first  10   a  and second  10   b  opposite faces. The second face  10   b  of the layer of piezoelectric material  10  is bonded to the first face  108   a  of the second dielectric layer  108  using a layer of PSA material  110 . 
     Referring in particular to  FIG. 33D , a third dielectric layer  113  extends in the first x and second y directions and has first  113   a  and second  113   b  opposite faces. A second electrode  9  in the form of a conductive material region is disposed on the second face  113   b  of the layer third dielectric layer  113  such that, when assembled, the second electrode  9  at least partially overlaps each first electrode  8  and each third electrode  26  region. The third dielectric layer  113  is substantially the same as the first or second dielectric layers  107 ,  108 . The second face  113   b  of third dielectric layer  113  is bonded to the first face  10   a  of the layer of piezoelectric material  10  using a layer of PSA material  110 . 
     The fourth display stack-up  112  may be bonded overlying the display  37  of an electronic device  28 . The elements of the fourth display stack-up  112  are stacked in the thickness direction z from the display  37  to the cover lens  77 . The layer structure  5  includes the second dielectric layer  108 , the layer of piezoelectric material  10  and two layers of PSA material  110 . The second layer structure  23  includes the first dielectric layer  107  and a layer of piezoelectric material  110 . 
     Thus, in the fourth display stack-up, the layer of piezoelectric material  10  does not have any electrodes disposed thereon. This simplifies the fabrication of the fourth stack-up substantially because processing steps to deposit electrodes on the layer of piezoelectric material  10  are not required. In a case when the layer of piezoelectric material  10  is PVDF, the fourth stack-up  112  can be fabricated by sandwiching a PVDF film providing the layer of piezoelectric material  10  between PET layers bearing patterned and unpatterned ITO electrodes. In this way, methods for manufacturing a regular projected capacitance touch panel may be quickly and easily adapted to allow production of combined pressure and capacitance touch panels. 
     Fifth display stack-up: Referring also to  FIGS. 34A and 34B , a fifth display stack-up  114  and a method of fabricating the fifth display stack-up  114  will be explained. 
     Referring in particular to  FIG. 34A , the cover lens  77  is a transparent substrate extending in the first x and second y directions and having first  77   a  and second  77   b  opposite faces. A fourth dielectric layer  115  extends in the first x and second y directions and has first  115   a  and second  115   b  opposite faces. Third electrodes  26  in the form of a set of conductive regions extending in the first direction x and spaced apart in the second direction y are disposed on the second face  115   b  of the fourth dielectric layer  115 . First electrodes  8  in the form of a set of conductive regions extending in the second direction y and spaced apart in the first direction x are disposed on the first face  115   a  of the fourth dielectric layer  115 . The second face  115   b  of the fourth dielectric layer  115  is bonded to the first face  77   a  of the cover lens  77  using a layer of PSA material  110 . The fourth dielectric layer  115  is substantially the same as the first, second or third dielectric layers  107 ,  108 ,  113 . 
     Referring in particular to  FIG. 34B , a layer of piezoelectric material  10  extends in the first x and second y directions and has first  10   a  and second  10   b  opposite faces. A second electrode  9  in the form of a conductive material region is disposed on the first face  10   a  of the layer of piezoelectric material  10  such that, when assembled, the second electrode  9  at least partially overlaps each first electrode  8  and each third electrode  26  region. The second face  10   b  of the layer of piezoelectric material  10  is bonded to the first face  115   a  of the fourth dielectric layer  115  using a layer of PSA material  110 . 
     The fifth display stack-up  114  may be bonded overlying the display  37  of an electronic device  28  using a layer of PSA material  110 . The elements of the fifth display stack-up  114  are stacked in the thickness direction z from the display  37  to the cover lens  77 . The layer structure  5  includes the layer of piezoelectric material  10  and a layer of PSA material  110 . The second layer structure  23  includes the fourth dielectric layer  115 . 
     The second electrode  9  need not be disposed on the layer of piezoelectric, material  10 . Alternatively, the fifth display stack-up  114  may include the third dielectric layer  113  with the second face  113   b  of the third dielectric layer  113  bonded to the first face  10   a  of the layer of piezoelectric material  10  using a layer of PSA material  110 . 
     Sixth display stack-up: Referring also to  FIGS. 35A and 35B , a sixth display stack-up  116  and a method of fabricating the sixth display stack-up  116  will be explained. 
     Referring in particular to  FIG. 35A , the cover lens  77  is a transparent substrate extending in the first x and second y directions and having first  77   a  and second  77   b  opposite faces. A fifth dielectric layer  117  extends in the first x and second y directions and has first  117   a  and second  117   b  opposite faces. Third electrodes  26  in the form of a set of conductive regions extending in the first direction x and spaced apart in the second direction y are disposed on the second face  117   b  of the fifth dielectric layer  117 . First electrodes  8  in the form of a set of conductive regions extending in the second direction y and spaced apart in the first direction x are disposed on the second face  117   a  of the fifth dielectric layer  117 . The second face  117   b  of the fifth dielectric layer  117  is bonded to the first face  77   a  of the cover lens  77  using a layer of PSA material  110 . The fifth dielectric layer  115  is substantially the same as the first, second, third of fourth dielectric layers  107 ,  108 ,  113 ,  115 . Each first electrode  8  is a continuous conductive region and each third electrode is made up of a number of separate conductive regions connected by jumpers  100 . Each jumper spans a portion of a conductive region belonging to a first electrode  8 . The first and third electrodes  8 ,  26  may be substantially the same as the first and third electrodes  8 ,  26  of the third touch panel  98 . 
     Referring in particular to  FIG. 34B , a layer of piezoelectric material  10  extends in the first x and second y directions and has first  10   a  and second  10   b  opposite faces. A second electrode  9  in the form of a conductive material region is disposed on the first face  10   a  of the layer of piezoelectric material  10  such that, when assembled, the second electrode  9  at least partially overlaps each first electrode  8  and each third electrode  26  region. The second face  10   b  of the layer of piezoelectric material  10  is bonded to the first face  117   a  of the fifth dielectric layer  117  using a layer of PSA material  110 . 
     The sixth display stack-up  116  may be bonded overlying the display  37  of an electronic device  28  using a layer of PSA material  110 . The elements of the sixth display stack-up  116  are stacked in the thickness direction z from the display  37  to the cover lens  77 . The layer structure  5  includes the layer of piezoelectric material  10 , a layer of PSA material  110  and the fifth dielectric layer  117 . 
     The second electrode  9  need not be disposed on the layer of piezoelectric material  10 . Alternatively, the sixth display stack-up  116  may include the third dielectric layer  113  with the second face  113   b  of the third dielectric layer  113  bonded to the first face  10   a  of the layer of piezoelectric material  10  using a layer of PSA material  110 . 
     Seventh display stack-up: Referring also to  FIGS. 36A to 36C , a seventh display stack-up  118  and a method of fabricating the seventh display stack up  118  will be explained. 
     Referring in particular to  FIG. 36A , the coyer lens  77  is a transparent substrate extending in the firsthand second y directions and having first  77   a  and second  77   b  opposite faces. Third electrodes  26  in the form of a set of conductive regions extending in the first direction x and spaced apart in the second direction y are disposed on the first face  77   a  of the cover lens  77 . 
     Referring in particular to  FIG. 36B , a second dielectric layer  108  extends in the first x and second y directions and has first  108   a  and second  108   b  opposite faces. First electrodes  8  in the form of a set of conductive regions extending in the second direction y and spaced apart in the first direction x are disposed on the second face  108   b  of the second dielectric layer  108 . The second face  108   b  of the second dielectric layer  108  is bonded to the first face  77   a  of the cover lens  77  using a layer of PSA material  110 . 
     Referring in particular to  FIG. 36C , a layer of piezoelectric material  10  extends in the first x and second y directions and has first  10   a  and second  10   b  opposite faces. A second electrode  9  in the form of a conductive material region is disposed on the first face  10   a  of the layer of piezoelectric material  10  such that, when assembled, the second electrode  9  at least partially overlaps each first electrode  8  and each third electrode  26  region. The second face  10   b  of the layer of piezoelectric material  10  is bonded to the first face  108   a  of the second dielectric layer  108  using a layer of PSA material  110 . 
     The seventh display stack-up  118  may be bonded overlying the display  37  of an electronic device  28  using a layer of PSA material  110 . The elements of the seventh display stack-up  118  are stacked in the thickness direction z from the display  37  to the cover lens  77 . The layer structure  5  includes the layer of piezoelectric material  10 , a layer of PSA material  110  and the second dielectric layer  108 . The second layer structure  23  includes a layer of PSA material  110 . 
     The second electrode  9  need not be disposed on the layer of piezoelectric material  10 . Alternatively, the seventh display stack-up  118  may include the third dielectric layer  113  with the second face  113   b  of the third dielectric layer  113  bonded to the first face  10   a  of the layer of piezoelectric material  10  using a layer of PSA material  110 . 
     Eighth display stack-up: Referring also to  FIGS. 37A to 37D , an eighth display stack-up  125  and a method of fabricating the eighth display stack-up  125  will be explained. 
     Referring in-particular to  FIG. 37A , the cover lens  77  is a transparent substrate extending in the first x and second y directions and having first  77   a  and second  77   b  opposite faces. First electrodes  8  in the form of a set of conductive regions extending in the second direction y and spaced apart in the first direction x are disposed on the first face  77   a  of the cover lens  77 . Third electrodes  26  in the form of a set of conductive regions extending in the first direction x and spaced apart in the second direction y are also disposed on the first face  77   a  of the cover lens  77 . Each first electrode  8  is a continuous conductive region and each third electrode is made up of a number of separate conductive regions connected by jumpers  100 . Each jumper spans a portion of a conductive region belonging to a first electrode  8 . The first and third electrodes  8 ,  26  may be substantially similar to the first and third electrodes  8 ,  26  of the third touch panel  98 . 
     Referring in particular to  FIG. 37B , a layer of piezoelectric material  10  extends in the first x and second y direction&#39;s and has first  10   a  and second  10   b  opposite faces. The second face  10   b  of the layer of piezoelectric material  10  is bonded to the first face  77   a  of the cover lens  77  using a layer of PSA material  110 . 
     Referring in particular to  FIG. 37C , a third dielectric layer  113  extends in the first x and second y directions and has first  113   a  and second  113   b  opposite faces. A second electrode  9  in the form of a conductive material region is disposed on the second face  113   b  of the third dielectric layer  113  such that, when assembled, the second electrode  9  at least partially overlaps each first electrode  8  and each third electrode  26  region. The second face  113   b  of third dielectric layer  113  is bonded to the first face  10   a  of the layer of piezoelectric material  10  using a layer of PSA material  110 . 
     Referring/in particular to  FIG. 37D , the eighth display stack-up  125  may be bonded overlying the display  37  of an electronic device  28  using a layer of PSA material  110  to bond the first surface  113   a  of the third dielectric layer  113  to the display  37 . The elements of the eighth display stack-up  125  are stacked in the thickness direction z from the display  37  to the cover lens  77 . The layer structure  5  includes the layer of piezoelectric material  10  and two layers of PSA material  110 . 
     The second electrode  9  need not be disposed on the third dielectric layer  113 . Alternatively, the eighth display stack-up  125  may include a layer of piezoelectric material  10  having the second electrode  9  disposed onto the first face  10   a  of the layer of piezoelectric material  10 . 
     Modifications: It will be appreciated that many modifications may be made to the embodiments hereinbefore described. Such modifications may involve equivalent and other features which are already known in the design, manufacture and use of projected capacitance touch panels and which may be used instead of or in addition to features already described herein. Features of one embodiment may be replaced or supplemented by features of another embodiment. 
     For example, touch panels  29 ,  92 ,  98 ,  103  and stack-ups  106 ,  109 ,  111 ,  112 ,  114 ,  116 ,  118 ,  125  have been described which overlie the display  37  of an electronic device  28 . However, the apparatuses  1 ,  20 ,  74 ,  93 ,  101 ,  104  described herein may equally be used with other touch panels which are integrated into a display  37  such as, for example, an LCD display, an OLED display, a plasma display or an electrophoretic display. 
     Referring also to  FIG. 38 , a first embedded stack-up  119  includes a pixel array  120  of a display  37 , a colour filter glass  121 , first and third electrodes  8 ,  26 , a layer structure  5 , a patterned second electrode  83 , a polariser  122  and a cover lens  77  stacked in the thickness direction from the pixel array  120  to the cover lens  77 . The first and third electrodes  8 ,  26  are disposed on the same face of the layer structure  5  in substantially the same way as the third touch panel  98 . 
     In this way, the first embedded stack-up  119  can be used in the fourth of fifth apparatuses  93 ,  101  to provide a touch panel with combined capacitive and pressure sensing embedded within an LCD display. This may allow the total thickness of the display  37  and touch panel to be reduced compared to a touch panel overlying the display  37 . 
     Referring also to  FIG. 39 , a second embedded stack-up  123  includes a pixel array  120  of a display  37 , third electrodes  26 , a colour filter glass  121 , first electrodes  8 , a layer structure  5 , a patterned second electrode  83 , a polariser  122  and a cover lens  77  stacked in the thickness direction from the pixel array  120  to the cover lens  77 . The first and third electrodes  8 ,  26  are disposed  5  in substantially the same way as the second touch panel  92 , except that the first and third electrodes  8 ,  26  are disposed on opposite sides of the colour filter glass  121  instead of the second layer structure  23 . 
     Referring also to  FIG. 40 , a third embedded stack-up  124  includes a pixel array  120  of a display  37 , third electrodes  26 , a second layer structure  23 , first electrodes  8 , a colour filler glass  121 , a layer structure  5 , a patterned second electrode  83 , a polariser  122  and a cover lens  77  stacked in the thickness direction from the pixel array  120  to the cover lens  77 . The first and third electrodes  8 ,  26  are disposed in substantially the same way as the second touch panel  92 . 
     In the third embedded stack-up  124 , the first and third electrodes  8 ,  26  are separated by the second layer structure  23 . However, the third embedded stack-up  124  may alternatively omit the second layer structure and include first and third electrodes  8 ,  26  disposed in substantially the same way as the third touch panel  98 . 
     Referring also to  FIG. 41 , a fourth embedded stack-up  126  includes a pixel array  120  of a display  37 , a colour filter glass  121 , first and third electrodes  8 ,  26 , a polariser  122 , a layer structure  5 , a patterned second electrode  83 , and a cover lens  77  stacked in the thickness direction from the pixel array  120  to the coyer lens  77 . The first and third electrodes  8 ,  26  are disposed on the same face of the layer structure  5  in substantially the same way as the third touch panel  98 . 
     Referring also to  FIG. 42 , a fifth embedded stack-up  127  includes a pixel array  120  of a display  37 , third electrodes  26 , a colour filter glass  121 , first electrodes  8 , a polariser  122 , a layer structure  5 , a patterned second electrode  83 , and a cover lens  77  stacked in the thickness direction from the pixel array  120  to the cover lens  77 . The first and third electrodes  8 ,  26  are disposed  5  in substantially the same way as the second touch panel  92 , except that the first and third electrodes  8 ,  26  are disposed on opposite sides of the colour filter glass  121  instead of the second layer structure  23 . 
     Referring also to  FIG. 43 , a sixth embedded stack-up  128  includes a pixel array  120  of a display  37 , third electrodes  26 , a second layer structure  23 , first electrodes  8 , a layer structure  5 , a colour filter glass  121 , a patterned second electrode  83 , a polariser  122  and a cover lens  77  stacked in the thickness direction from the pixel array  120  to the cover lens  77 . The first and third electrodes  8 ,  26  are disposed in substantially the same way as the second touch panel  92 . 
     Referring also to  FIG. 44 , a seventh embedded stack-up  129  includes a pixel array  120  of a display  37 , third electrodes  26 , a second layer structure  23 , first electrodes  8 , a layer structure  5 , a patterned second electrode  83 , a colour filter glass  121 , a polariser  122  and a cover lens  77  stacked in the thickness direction from the pixel array  120  to the cover lens  77 . The first and third electrodes  8 ,  26  are disposed in substantially the same way as the second touch panel  92 . 
     Referring also to  FIG. 45 , an eighth embedded stack-up  130  includes a pixel array  120  of a display  37 , third electrodes  26 , a second layer structure  23 , first electrodes  8 , a colour fiber glass  121 , a polariser  122 , a layer structure  5 , a patterned second electrode  83  and a cover lens  77  stacked in the thickness direction from the pixel array  120  to the cover lens  77 . The first and third electrodes  8 ,  26  are disposed in substantially the same way as the second touch panel  92 . 
     The sixth, seventh and eighth embedded stack-ups  128 ,  129 ,  130  have been described with the first and third electrodes  8 ,  26  separated by the second layer structure  23 . However, the sixth, seventh and eighth embedded stack-ups  128 ,  129 ,  130  may alternatively omit the second layer structure and include first and third electrodes  8 ,  26  disposed in substantially the same way as the third touch panel  98 . 
     The first to eighth embedded stack-ups,  119 ,  123 ,  124 ,  126 ,  127 ,  128 ,  129 ,  130  have been described as including the patterned second electrode  83 . However, the patterned second electrode  83  need not be used and the first to eighth embedded stack-ups,  119 ,  123 ,  124 ,  126 ,  127 ,  128 ,  129 ,  130  may instead include un-patterned second electrodes  9 . 
     Touch panels have been described in which first and third electrodes  8 ,  26  are separated from second, or bias, electrodes  9 ,  83  by the layer structure  5 . However, other arrangements are possible. Referring to  FIGS. 46 and 47 , a fifth touch panel  131  includes a layer structure  5 , a plurality of first electrodes  8  disposed on the first face  6  of the layer structure  5 , a plurality of third electrodes  26  disposed on the second face  7  of the layer structure  5  and a plurality of second electrodes  9  disposed on the second face  7  of the layer structure  5  in the form of a plurality of separated second electrodes  132 . 
     The first electrodes  8  extend in the first direction x and are spaced apart in the second direction y. The third electrodes  26  extend in the second direction y and are spaced apart in the first direction x. The separated second electrodes  132  extend in the second direction y are spaced apart in the first direction x. The separated second electrodes  132  and the third electrodes  26  are interleaved and do not contact one another. The separated second electrodes  132  and third electrodes  26  may be read using conductive traces (not shown) which exit the fifth touch panel  131  on different edges. Each first electrode  8  may take the form of several pad segments  94  evenly spaced in the first direction x and connected to one another in the first direction x by relatively narrow bridging segments  95 . Similarly, each third electrode  26  may include several pad segments  96  evenly spaced in the second direction y and connected to one another in the second direction y by relatively narrow bridging segments  97 . The pad segments  94  by the first electrodes  8  may be diamond shaped. The pad segments  96  and bridging segments  97  of the third electrodes  26  may have the same respective shapes and widths as the first electrodes  8 . Each separated second electrode  9  may include several pad segments  133  evenly spaced in the second direction y and connected to one another in the second direction y by relatively narrow bridging segments  134 . The pad segments  133  and bridging segments  134  of the separated second electrodes  132  may have the same respective shapes and widths as the first and third electrodes  8 ,  26 . Alternatively, the pad segments  94  of the first electrodes  8  may be larger or smaller than the pad segments  133  of the separated second electrodes  132 . 
     The first electrodes  8  and the third electrodes  26  are arranged such that the bridging segments  97  of the third electrodes  26  overlie the bridging segments  95  of the first electrodes  8 . The first electrodes  8  and the third electrodes  26  are arranged such that the respective pad segments  94 ,  96  do not overlap. Instead, the separated second electrodes are arranged such that the pad segments  133  of the separated second electrode  9  overlap the pad segments  94  of the first electrodes  8 . The pad segments  94 ,  96 ,  133  need not be diamond shaped, and may instead be circular. The pad segments  94 ,  96 ,  133  may be a regular polygon such as a triangle, square, pentagon or hexagon. 
     The fifth touch panel may be used in, for example, the fourth of fifth apparatus  93 ,  101  to measure mutual capacitance between a pair of first and third electrodes  8 ,  26 . The separated sensing electrodes  132  may be coupled to each another, for example using external traces (not shown) and addressed collectively to measure pressure values between a first electrode  8  and the separated sensing electrodes  132 . Alternatively, the separated sensing electrodes  132  may be individually addressable to measure, pressure values using a pair of first and separated second electrodes  8 . 
     The first to eighth display stack ups  106 ,  109 ,  111 ,  112 ,  114 ,  116 ,  118 ,  125  or the first to eighth embedded stack-ups,  119 ,  123 ,  124 ,  126 ,  127 ,  128 ,  129 ,  130  may be adapted to incorporate the fifth touch panel  131 , or elements of the fifth touch panel  131  such as, for example, disposing the third electrodes  26  on the same surface as the separated second electrode  132 . The separated second electrodes  132  need not be disposed on the same surface as the third electrode  26 , and may alternatively be disposed on the same surface of the layer structure  5  as the first electrodes  8 . 
     Touch panels and stack-ups have been described which are generally planar. However, touch panels and stack-ups need not be planar or flat and may provide curved or other non-planar surfaces for a user to interact with. Touch panels and stack-ups may be provided overlying or embedded within curved displays. 
     The signal processing module  4 ,  22 , controller  79  or processor  32  may employ correlated double sampling methods to improve the signal to noise ratio of the pressure values  18  and/or the capacitance values  19 . The signal processing module  4 ,  22 , controller  79  or processor  32  may process the pressure values  18  and/or the capacitance values  19  as image data. 
     Touch sensors  2 ,  21  and touch panels  29 ,  92 ,  98 ,  103  have been generally described in relation to first, second and third directions x, y, z forming an orthogonal set. However, the first and second directions need not be perpendicular and may in general intersect at any angle between 1 degree and 90 degrees. Intersection of the first and second directions at 90, 60, 45 or 30 degrees is preferred. 
     Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel features or any novel combination of features disclosed herein either explicitly or implicitly or any generalization thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicant hereby gives notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.