CN103748540B - Active integrator for capacitive sensing array - Google Patents

Abstract

An active integrator for sensing capacitance of a touch-sensing array is disclosed. The active integrator is configured to receive a response signal from the touch-sensing array having a positive portion and a negative portion. The response signal indicates the presence or absence of a conductor on the touch-sensing array. The active integrator is configured to continuously integrate the response signal.

Description

Active Integrators for Capacitive Sense Arrays

related application

This application claims priority to US Provisional Patent Application No. 61/472,161, filed April 5, 2011, the entire disclosure of which is hereby incorporated by reference in its entirety.

technical field

The present invention relates generally to capacitive touch-sensing arrays, and more particularly, to active integrators for receive circuits of touch-sensing arrays.

Background technique

Computing devices, such as notebook computers, personal data assistants (PDAs), kiosks, and mobile handsets, have user interface devices, also known as human interface devices (HIDs). One user interface device that has become more common is the touch sensor pad (also commonly referred to as a touchpad). The touch-sensor board of a basic laptop emulates the functionality of a personal computer (PC) mouse. Touch sensor boards are usually embedded in PC notebooks for built-in portability. The touch sensor pad replicates mouse X/Y movement by using two defined axes that contain an array of sensor elements that detect the position of one or more conductive objects, such as a finger. Mouse left/right button clicks can be replicated via the two mechanical buttons located near the touchpad, or via tap commands on the touch sensor pad itself. The touch sensor pad provides a user interface device for performing functions such as positioning a pointer or selecting an item on the display. These touch sensor pads may include multi-dimensional sensor arrays for detecting motion in multiple axes. The sensor array may include a one-dimensional sensor array that detects motion in one axis. Sensor arrays can also be two-dimensional, detecting motion in two axes.

Another user interface device that has become more common is the touch screen. A touch screen, also known as a touch screen, touch window, touch panel or touch screen panel, is a transparent display overlay that is usually pressure sensitive (resistive or piezoelectric), electrically sensitive (capacitive), acoustically sensitive (surface acoustic wave ( SAW)) or photosensitive (infrared). The effect of such an overlay is to allow the display to be used as an input device, removing the keyboard and/or mouse as the primary input devices for interacting with the display's content. These displays can be connected to a computer, or connected to a network as a terminal. Touch screens have become familiar in retail devices, on point-of-sale systems, on ATMs, on mobile handsets, on kiosks, on game consoles, and on PDAs, where a stylus is sometimes used for Manipulating the Graphical User Interface (GUI) and entering data. Users can touch the touch screen or touch sensor pad to manipulate data. For example, a user can apply a single touch to select an item from a menu by touching the touchscreen surface with a finger.

One type of touch sensing array includes a first set of spaced apart linear electrodes and a second set of electrodes arranged at right angles and separated by a dielectric layer. The resulting intersections form a two-dimensional array of capacitors, known as inductive elements. A touch sensing array can be scanned in several ways, one of which (mutual capacitance sensing) allows a single capacitive element to be measured. Another method (self-capacitance sensing) can measure the entire sensor strip, or even the entire sensor array, with less information about the exact location, but perform one read operation.

When the two-dimensional array of capacitors is placed in close proximity, a means for sensing touch is provided. The proximity of a conductive object (such as a finger or a stylus) to the touch-sensing array causes a change in the total capacitance of the sensing elements near the conductive object. Changes in these capacitances can be measured to produce a "two-dimensional map" indicating where touches on the array occurred.

One way to measure this change in capacitance is to form a circuit that includes a signal driver (such as an AC current or voltage source ("transmit (TX) signal)) applied in a multiplexed fashion to each horizontally aligned conductor. At each vertically aligned electrode, the charge accumulated on the intersection of each capacitor is sensed and similarly scanned synchronously with the applied current/voltage power supply. Then, a charge-to-voltage converter (i.e., receive or "RX signal" This charge is measured in form of ), which is sampled and held for an A/D converter into digital form for input to the processor. The processor in turn traces the capacitive map and determines the location of the touch.

Conventional capacitive sensing receiver circuits suffer from a number of deficiencies. The change in capacitance due to the touch of a conductor is usually small. As a result, many voltages appearing at the ADC represent the baseline capacitance of the array of sensing elements, which creates a large DC component. The capacitance change induced by touch is only 1% of the baseline capacitance. In addition, the source circuit is often noisy, complicating accurate capacitance change measurements and yielding low signal-to-noise ratio (SNR).

Brief description of the drawings

Embodiments of the present invention will be more readily understood from the following detailed description of exemplary embodiments set forth in consideration of the accompanying drawings, in which like reference numerals refer to like elements, wherein:

1 shows a block diagram of one embodiment of an electronic system that includes a processing device that may be configured to measure capacitance from a flexible touch-sensitive surface and calculate or detect the magnitude of force applied to the flexible touch-sensitive surface.

Figure 2 shows a block diagram of one embodiment of a capacitive touch sensor array and a capacitive sensor that converts measured capacitances into coordinates.

3 illustrates an electrical block diagram of one embodiment of an active integrating circuit configured to receive an RX signal from a receive electrode to measure capacitance of the touch-sensing array of FIG. 2 .

4 is a block diagram of components of an active integrator, a baseline compensation circuit, and a sample-and-hold (S/H) circuit employed in the active integration circuit of FIG. 3, according to an embodiment.

5 is a schematic diagram of one embodiment of components of an S/H circuit employed in the active integrating circuits of FIGS. 3 and 4 .

6A and 6B are schematic diagrams of one embodiment illustrating the operation and relative timing of the various switches and signals associated with the active integrator of FIG. 4 and the S/H circuit of FIG. 5, respectively.

7 is a graph of one embodiment of a spectral response of a response channel of a capacitive sensor of the touch-sensing array of FIG. 1 employing the components of FIGS. 4 and 5 .

8 is a flowchart of one embodiment of a method of operating an active integrator and an S/H circuit of an active integrating circuit for measuring capacitance of a touch sensing array.

FIG. 9 is a flowchart illustrating in more detail the steps of continuously integrating the response signal of FIG. 8 .

FIG. 10 is another embodiment of FIG. 2 showing the capacitive sensor of FIG. 2 configured to provide a calibration unit configured to provide self-calibration of the capacitive sensor.

detailed description

Embodiments of the present invention provide active integrators configured to measure the capacitance of a touch-sensing array or a portion of an array, such as a single bar. The active integrator is configured to receive a response signal from the touch-sensing array indicating the presence or absence of electrical conductors on the touch-sensing array. The response signal is generally assumed to be provided by a touch-sensing array driven by an AC current/voltage supply. As a result, the response signal includes a positive part and a negative part. Embodiments described herein employ active integrators and supporting circuitry to continuously integrate the response signal. This continuous integration characteristic is primarily a result of the nature of the switched capacitance of the active integrator. One possible advantage of employing switched capacitor active integrators may be improved SNR over conventional designs. In one embodiment, when the switching frequency is matched to the fundamental frequency and phase of the response signal, the output signal has a narrow passband centered at the fundamental frequency of the response signal, resulting in substantially improved SNR. This method is called full-wave demodulation.

In one embodiment, the active integrator includes an operational amplifier (opamp) coupled to a pair of feedback capacitors. One feedback capacitor is configured to store charge in response to a positive portion of the response signal, and a second feedback capacitor is configured to store charge in response to a negative portion of the response signal. The first feedback capacitor and the second feedback capacitor may be configured to be variable to allow sensitivity calibration of the touch sensitive receiver. In one embodiment, the active integrator is coupled to a sample-and-hold (S/H) circuit configured to full-wave demodulate the output signal of the active integrator by means of one or more switches. The first capacitor is configured to hold a positive signal on the output of the active integrator when the positive portion of the response signal is present, and the second capacitor is configured to hold the active integrator when the negative portion of the response signal is present. Negative signal on output.

1 illustrates a block diagram of one embodiment of an electronic system 100 that includes a processing device 110 that may be configured to measure capacitance from a flexible touch-sensitive surface and calculate or detect the magnitude of force applied to the flexible touch-sensitive surface. . Electronic system 100 includes a touch-sensitive surface 116 (eg, a touch screen, or touchpad) coupled to processing device 110 and host 150 . In one embodiment, touch-sensitive surface 116 is a two-dimensional user interface that uses sensor array 121 to detect touches on surface 116 .

In one embodiment, sensor array 121 includes sensor elements 121(1)-121(N) (where N is a positive integer) arranged in a two-dimensional matrix (also referred to as an XY matrix). Sensor array 121 is coupled to pins 113(1)-113(N) of processing device 110 via one or more analog buses 115 that carry multiplexed signals. In this embodiment, each sensor element 121(1)-121(N) is represented as a capacitor. The self-capacitance of each sensor in sensor array 121 is measured by capacitive sensor 101 in processing device 110 .

In one embodiment, capacitive sensor 101 may include a relaxation oscillator or other device for converting capacitance into a measurement. Capacitive sensor 101 may also include a counter or timer to measure the oscillator output. Capacitive sensor 101 may further include software components to convert count values (eg, capacitance values) into sensor element detection decisions (also referred to as switch detection decisions) or relative magnitudes. In another embodiment, capacitive sensor 101 includes an active integration circuit 300 as will be described below.

It should be noted that various methods of measuring capacitance are known, such as current versus voltage phase shift measurements, resistor-capacitor charge timing, capacitive bridge dividers, charge transfer, successive approximation, sigma-delta modulators, charge accumulation circuits, Field effect, mutual capacitance, frequency shift, or other capacitance measurement algorithms. It should be noted, however, that instead of evaluating raw counts against a threshold, capacitive sensor 101 may evaluate other measurements to determine user interaction. For example, in a capacitive sensor 101 with a sigma-delta modulator, the capacitive sensor 101 evaluates the ratio of the pulse width of the output, instead of raw counts above or below some threshold.

In one embodiment, processing device 110 also includes processing logic 102 . The operations of processing logic 102 may be implemented in firmware, alternatively, they may be implemented in hardware or software. Processing logic 102 may receive signals from capacitive sensors 101 and determine the state of sensor array 121, such as whether an object (e.g., a finger) is detected on sensor array 121, or in proximity to sensor array 121 (e.g., to determine the presence of an object ), when an object is detected on the sensor array (eg, to determine the position of the object), track the motion of the object, or other information related to the object detected on the touch sensor.

In another embodiment, instead of executing the operations of processing logic 102 in processing device 110 , processing device 110 may send raw data or partially processed data to host 150 . As shown in FIG. 1 , host 150 may include decision logic 151 that performs some or all of the operations of processing logic 102 . The operations of decision logic 151 may be implemented in firmware, hardware, software, or a combination thereof. Host 150 may include a high-level application programming interface (API) in applications 152 that performs routines on received data, such as compensation for sensitivity differences, other compensation algorithms, baseline update routines, start-up and/or initialization routines, Interpolation operation, or scaling operation. The operations described with respect to processing logic 102 may be implemented in decision logic 151 external to processing device 110, in applications 152, or in other hardware, software, and/or firmware. In some other implementations, the processing device 110 is the host computer 150 .

In another implementation manner, the processing device 110 may further include a non-sensing action block 103 . This block 103 may be used to process and/or receive/send data to and from the host 150 . For example, additional components (eg, keyboards, keypads, mice, trackballs, LEDs, displays, or other external devices) may be implemented to operate with processing device 110 along with sensor array 121 .

In one embodiment, the electronic system 100 is implemented in a device that includes a touch-sensitive surface 116 as a user interface, such as a handheld electronic device, a cellular phone, a cellular phone, a notebook computer, a personal computer, a personal data assistant (PDA), a self-service Terminals, keyboards, televisions, remote controls, monitors, handheld multimedia devices, handheld video players, gaming equipment, control panels for home or industrial applications, or other computer peripherals or input devices. Alternatively, electronic system 100 may be used in other types of devices. It should be noted that the components of the electronic system 1000 may include all of the components described above. Alternatively, electronic system 100 may include only some of the components described above, or include additional components not listed herein.

FIG. 2 shows a block diagram of one embodiment of a capacitive touch sensor array 121 and a capacitive sensor 101 that converts measured capacitances into coordinates. Coordinates are calculated based on the measured capacitance. In one embodiment, sensor array 121 and capacitive sensor 101 are implemented in a system such as electronic system 100 . Sensor array 121 includes a matrix 225 of N×M electrodes (N receive electrodes and M transmit electrodes), which also includes transmit (TX) electrodes 222 and receive (RX) electrodes 223 . Each electrode in matrix 225 is connected to capacitive sensing circuit 201 through demultiplexer 212 and multiplexer 213 .

Capacitive sensor 101 includes multiplexer control 211 , demultiplexer 212 and multiplexer 213 , clock generator 214 , signal generator 215 , demodulation circuit 216 and analog-to-digital converter (ADC) 217 . ADC 217 is also coupled to touch coordinate converter 218 . Touch coordinate converter 218 outputs signals to processing logic 102 .

In one implementation, the processing logic 102 may be a processing core 102 . The processing cores may reside on a common carrier substrate, such as, for example, an integrated circuit ("IC") die substrate, a multi-chip module substrate, or the like. Alternatively, the components of processing core 102 may be one or more separate integrated circuits and/or discrete components. In an exemplary embodiment, processing core 102 is configured to provide intelligent control for programmable system-on-chip The on-chip programmable system is manufactured by Cypress Semiconductor Corporation, San Jose, California. Alternatively, processing core 102 may be one or more other processing devices known to those skilled in the art, such as a microprocessor or central processing unit, controller, special purpose processor, digital signal processor ("DSP"), special purpose Integrated Circuits ("ASICs") and Field Programmable Gate Arrays ("FPGAs"), among others. In one embodiment, processing core 102 and other components of processing device 110 are integrated into the same integrated circuit.

It should be noted that the embodiments described herein are not limited to configurations such that processing core 102 is coupled to host computer 150, but may include systems that measure capacitance on touch-sensing array 121 and send raw data to a host computer, wherein, Raw data is applied to the main computer for analysis. In fact, the processing done by the processing core 102 can also be done in the host. The host may be a microprocessor, for example, or other types of processing devices as would be understood by those of ordinary skill in the art having the benefit of this disclosure.

Components of the electronic system 100 other than the touch-sensing array 121 may be integrated into the IC of the processing core 102, or, in a separate IC. Alternatively, a description of electronic system 100 may be generated and compiled for incorporation into other integrated circuits. For example, behavioral level code or portions thereof describing electronic system 100 may be generated using a hardware descriptive language, such as VHDL or Verilog, and stored on a machine-accessible medium (eg, CD-ROM, hard disk, floppy disk, etc.). Additionally, behavioral level code can be compiled into register transfer level ("RTL") code, netlist, or even circuit layout and stored to a machine-accessible medium. Behavioral level code, RTL code, netlist, and circuit layout all represent various levels of abstraction that describe electronic system 100 .

It should be noted that the components of the electronic system 100 may include all of the components described above. Alternatively, the electronic system 100 may include only some of the above-described components.

In one embodiment, electronic system 100 is used in a notebook computer. Alternatively, the electronic device may be used in other applications such as mobile handsets, personal data assistants ("PDAs"), keyboards, televisions, remote controls, monitors, handheld multimedia devices, handheld video players, handheld gaming devices, or control panel.

The transmit and receive electrodes may be arranged in the electrode matrix 225 such that each transmit electrode overlaps and passes through each receive electrode to form a crossed array, while remaining electrically isolated from each other. Thus, each transmit electrode can capacitively couple each receive electrode. For example, transmit electrode 222 capacitively couples receive electrode 223 at the point where transmit electrode 222 and receive electrode 223 overlap.

The clock generator 214 provides a clock signal to the signal generator 215 , and the signal generator 215 generates a TX signal 224 to provide to the transmitting electrodes of the touch sensing array 121 . In one embodiment, signal generator 215 includes a set of switches that operate according to a clock signal from clock generator 214 . The switch can generate the TX signal 224 by periodically connecting the output of the signal generator 215 to a first voltage and then to a second voltage, wherein the first voltage and the second voltage are different. In another embodiment, the active integrating circuit 300 is coupled to the signal generator 215 as will be described below. Those of ordinary skill in the art will appreciate that signal generator 215 may provide TX signal 224, which may be any periodic signal having positive and negative components, including, for example, sine waves, rectangular waves, triangular waves, and the like.

The output of the signal generator 215 is connected to a demultiplexer 212 which allows the TX signal 224 to be applied to any one of the M electrodes of the touch sensing array 121 . In one embodiment, multiplexer control 211 controls demultiplexer 212 such that TX signal 224 is applied to each transmit electrode 222 in a controlled sequence. The demultiplexer 212 may also be used to ground, float, or connect additional signals to other transmit electrodes to which the TX signal 224 is not currently applied.

Because of the capacitive coupling between the transmit and receive electrodes, the TX signal 224 applied to each transmit electrode induces a current in each receive electrode. For example, when TX signal 224 is applied to transmit electrode 222 via demultiplexer 212 , TX signal 224 induces RX signal 227 on receive electrodes of matrix 225 . Using the multiplexer 213 , the RX signal 227 on each of the receive electrodes can then be measured sequentially in order to sequentially connect each of the N receive electrodes to the demodulation circuit 216 .

By using demultiplexer 212 and multiplexer 213 to select each available combination of TX electrodes and RX electrodes, the mutual capacitance associated with each intersection between TX electrodes and RX electrodes can be sensed. To improve performance, multiplexer 213 may also be segmented to allow more than one receive electrode in matrix 225 to be routed to additional demodulation circuitry 216 . In an optimized configuration, where there is an instance of a 1-to-1 correspondence of demodulation circuit 216 and receive electrodes, multiplexer 213 may not be present in the system.

When an object, such as a finger, comes close to the electrode matrix 225, the object reduces the mutual capacitance between only some of the electrodes. For example, if a finger is placed near the intersection of transmit electrode 222 and receive electrode 223 , the presence of the finger will reduce the mutual capacitance between electrodes 222 and 223 . Accordingly, a touchpad can be determined by identifying the transmit electrodes to which the TX signal 224 is applied when a reduced mutual capacitance is measured on one or more receive electrodes, and identifying one or more receive electrodes with reduced mutual capacitance. position of the upper finger.

By determining the mutual capacitance associated with each intersection of electrodes in matrix 225, the location of one or more touch contacts can be determined. This determination can be continuous, in parallel, or can occur more frequently in commonly used electrodes.

In alternative embodiments, other methods for detecting the presence of fingers or electrical conductors that induce an increase in capacitance at one or more electrodes, possibly arranged in a grid or other pattern, may be used. For example, a finger placed near an electrode of a capacitive sensor may introduce additional capacitance to ground, which increases the total capacitance between the electrode and ground. The location of the finger can be determined from the location of one or more electrodes where increased capacitance is detected.

The induced current signal 227 is rectified by the demodulation circuit 216 . The current output rectified by demodulation circuit 216 may then be filtered by ADC 217 and converted into digital code. In one embodiment, the demodulation circuit may include an active integration circuit 300 which will be described below.

The digital code is converted by touch coordinate converter 218 into touch coordinates representing the location of the input on touch sensor array 121 . The touch coordinates are sent to processing logic 102 as an input signal. In one embodiment, the input signal is received at an input of the processing logic 102 . In one embodiment, the input may be configured to receive capacitance measurements indicative of a plurality of row coordinates and a plurality of column coordinates. Optionally, the input may be configured to receive row and column coordinates.

In one embodiment, a system for tracking the location of contacts on a touch-sensitive surface can determine a force magnitude for each contact based on capacitive measurements from an array of capacitive sensors. In one embodiment, a capacitive touch-sensing system that is also capable of determining the force applied to each of a plurality of contacts on the touch-sensing surface may be constructed of a flexible material, such as PMMA, and may be constructed between a capacitive sensor array and There is no shading between the LCD display panels. In such an embodiment, the capacitance change of the sensor elements may be caused by the displacement of the sensor elements closer to the VCOM plane of the LCD display panel.

FIG. 3 shows an electrical block diagram of one embodiment of an active integrating circuit 300 configured to receive the RX signal 227 from the receiving electrodes to measure the touch sensing array 121 of FIG. 2 or a portion of the array (e.g., single band) capacitor. The active integration circuit 300 includes: a calibration unit 321 , an active integrator 326 , a sample-and-hold (S/H) circuit 340 and a sequencer circuit 345 . The multiplexer 213 of FIG. 2 is coupled to a first input 324 of an active integrator 326 . Baseline compensation circuit 328 is coupled to first input 324 of active integrator 326 through one or more switches 330 . The virtual-ground VY is coupled to the second input 338 of the active integrator 326 . The output 339 of the active integrator 326 is coupled to a sample-and-hold (S/H) circuit 340 . S/H circuit 340 is differentially coupled to ADC 217 through first and second outputs 342 , 344 . A central control circuit, hereinafter referred to as the sequencer circuit 345, generally controls (as represented by reference numeral "A") all switching and activities throughout the touch screen subsystem (TSS), including those described below in connection with FIGS. 6B and 8 describe the active integrating circuit 300. Calibration unit 321 provides self-calibration of capacitive sensor 101, and is coupled to both RX signal 227 and multiplexer 213 via one or more switches 331, and via one or more A switch (not shown) is coupled to the TX signal 227 and the demultiplexer 212 .

4 is a block diagram of the components of active integrator 326, baseline compensation circuit 328, and S/H circuit 340 employed in active integration circuit 300 of FIG. In one embodiment, the active integrator 326 may be a switched capacitor integrator including an operational amplifier 446 having a negative input 450 , a positive input 452 , and an output 454 . A first integrating capacitor 456 (also labeled C _INTP ) is coupled between output 454 and negative input 450 via switches 458a-458d. A second integrating capacitor 460 (also labeled C _INTN ) is coupled between output 454 and negative input 450 via a plurality of switches 462a-462d.

FIG. 5 is a schematic diagram of the components of one embodiment of the S/H circuit 340 employed in the active integrating circuit 300 of FIGS. 3 and 4 . In one embodiment, the S/H circuit 340 includes a first S/H capacitor 466a (also denoted C _SHP ) that is switched at S/H via a switch 468a (also denoted "shp"). The input terminal of the H buffer 467a is coupled or decoupled from the output terminal 454 of the operational amplifier 446 . The S/H circuit 340 also includes a second S/H capacitor 466b (also labeled C _SHN ), which is connected to the S/H buffer 467b via a switch 468b (also labeled "shn"). The input is coupled or decoupled from the output 454 of the operational amplifier 46 . The outputs of S/H buffers 467a, 467B and the S/H buffers themselves are coupled or decoupled to positive and negative inputs 342, 344 of differential ADC 217 via switches 469a-469d labeled "adc_;". A plurality of switches 470a-470d labeled "shpp", "shnn", "!shp&&!adc_sampling" and "!shn&&!adc_sampling" configure the S/H buffers for purposes that will be described below. Inputs 471a, 471b labeled "bufp_pdb" and "bufn_pdb" to the S/H buffers 467a, 467B, respectively, are used to power on or off each of the S/H buffers 467a, 467a, 467B, for purposes described below. 467B.

Referring to Figures 3-5, the sequencer 345 as a whole completely controls all switches and activities in the entire touch screen subsystem (TSS). This includes activating the TX signal that is applied to the touch sense array 121 (e.g., going high or low), the switches in the active integrator 326 (e.g., p1, p2, p1p, p2p), the baseline control switch pwc1/ pwc2, IDAC value and S/H circuit 340 and so on. Using the sequencer 345, all activity in the RX and TX circuits occurs in a fully synchronized manner, as will be described below in conjunction with FIG. sequencer circuit 345 as The processing device is implemented as part of a custom universal digital block (UDB) configured for all switches in the active integrator 326 according to the timing diagrams of FIGS. 6a and 6b. Timing is provided. As used herein, a UDB is a collection of free logic (PLD) and structured logic (datapath) optimized to generate all common embedded peripherals and custom functions for a specific application or design. UDBs can be used to implement various general and specific digital logic devices, including but not limited to Field Programmable Gate Arrays (FPGAs), Programmable Array Logic (PALs), and Complex Programmable Logic Devices (CPLDs).

In the embodiment depicted in FIG. 4 , the baseline compensation circuit 328 includes a current output digital-to-analog converter (IDAC) 472 coupled between ground and a gain block 474 . Gain block 474 is coupled to input negative input 450 of operational amplifier 446 through a pair of switches 476a-476b, also labeled pwc1 and pwc2, respectively. Switch 476a ( pwc1 ) is configured to apply negative current I _DACN to negative input 450 of operational amplifier 446 to cancel the positive baseline charge resulting from the response signal appearing on the output of touch-sensing array 121 through the operation of pwc1 . Likewise, switch 476b (pwc2) is configured to apply I _DACP to negative input 450 of operational amplifier 446 so that the negative baseline charge resulting from the presence of the response signal on the output of touch-sensing array 121 is canceled by the operation of pwc2.

In one embodiment, baseline compensation circuit 328 is used to minimize the baseline bias of the response signal present at the differential inputs of ADC 217 in order to maximize the number of output bits representing capacitance changes caused by the proximity of electrical conductors to touch-sensing array 121 . As a result, the dynamic range of the capacitive sensor 101 can be improved.

Tolerances associated with the design and manufacture of the sensor panel can cause significant variations in the baseline capacitance of some sensing elements, even within a single touch-sensing array 121 . This can further reduce the dynamic range of the ADC 217 since the fixed charge from any sense line is nothing more than a baseline charge that carries no information about the touch event. Thus, instead of using a single, fixed value in the baseline compensation circuit (ie, IDAC472), this value can be programmed in real time to compensate for the actual baseline charge of the currently detected sense line. Optimal settings can be determined in a "self-calibration" routine, either at manufacturing time throughout the touch system, or during power-up of the final product.

6A and 6B illustrate one embodiment of the operation and relative timing of the various switches and signals associated with the active integrator 326 of FIG. 4 and the S/H circuit 340 of FIG. 5, respectively. The switches 458a-458d, 462a-462d coupled to the active integrator 326 and the switches 468a, 468b, 469a-469d, and 470a-470d coupled to the S/H circuit 340 are all timed to continue continuously with substantially no "dead time" Integrate the positive and negative parts of the response signal. Once the negative part of the response signal has been integrated, there is virtually no delay in switching to capacitor C _INTP before the positive part of the response signal can be integrated on capacitor C _INTP . Additionally, once the positive or negative signal has been integrated, the integrated signal may be applied to C _SHP and then C _SHN via switches 468a, 468b, 469a-469d, and 470a-470d coupled to S/H circuit 340, respectively. The “differential” output of the S/H circuit 340 is thus configured as the integrated response signal of the full wave rectified input such that the same polarity of the input signal is always presented to the differential inputs 342 , 344 of the ADC 217 .

7 illustrates one embodiment of the spectral response of the response channel of capacitive sensor 101 of touch-sensing array 121 of FIG. 1 employing the components of FIGS. 4 and 5 . Since the active integrator 326 can substantially synchronously drive the S/H circuit 340 while integrating, and one of the capacitors 456, 460 can be held or reset while the other integrates continuously with substantially no dead time, compared to using The integrated channel of a single capacitor and non-time/polarity coordinating S/H circuit 340, the resulting channel has a narrowband frequency response 780 whose response 782 is also shown in FIG. The narrowband frequency response has a peak 784 corresponding to the fundamental frequency of the input signal (ie, TX signal 224). As a result, the SNR is significantly improved compared to conventional designs.

FIG. 8 is a flowchart 800 of one embodiment of a method of operating active integrator 326 and S/H circuit 340 of active integration circuit 300 for measuring the capacitance of touch-sensing array 121 . At block 802, the active integrator 326 receives a response signal from the touch-sensing array 121 having a positive portion and a negative portion (e.g., a periodic response signal having a negative portion followed by a positive portion, such as a sine wave, rectangular wave, triangle wave, etc.) . The response signal indicates the presence or absence of the conductor on the touch-sensing array 121 . At block 804, the active integrator 326 continuously integrates the response signal (in a full-wave rectified manner, see FIG. 8 below). Blocks 802 and 804 are thus repeated indefinitely for each period of the response signal as long as the response signal exists.

FIG. 9 is a flow diagram illustrating the continuously integrating response signal block 804 of FIG. 8 in more detail. Referring now to FIGS. 4 , 5 and 9 , at block 902 , charge is accumulated on the first integrating capacitor 456 , C _INTP , in response to the positive portion of the response signal. More specifically, when the TX signal 227 is activated from high to low, the corresponding switches 458a, 458b (p1/p1p) in the active integrator 326 are closed and the switches 462a, 462b (p2/p2p) are open. open. The input charge may then be integrated across capacitor 456 (C _INTP ), such that the voltage across capacitor 456 produces an increasing voltage at the output of the integrator (ie, node 454 ). The input 450 of the integrating capacitor is held constantly at Vx, which is the same as Vy (ie, node 452 ), which does not change during operation.

At block 904, charge in response to the negative portion of the response signal is accumulated on the second integrating capacitor 460, C _INTN . More particularly, after all signals have stabilized, the TX signal 227 is directed by the sequencer 345 to apply a low-to-high transition, while the switches 458a, 458b (p1/p1p) are open and the switches 462a, 462b (p2/ P2P) is closed. This connects capacitor 460 (C _INTN ) to active integrator 326, while capacitor 456 (C _INTP ) is floating, thus temporarily holding its charge (the charge on capacitor 456 cannot leak). Additionally, after the incoming charge has been integrated, at block 906 the cycle begins again, switching capacitor 456 (C _INTP ) back to active integrator 326 to collect the next charge packet, and so on. Thus, positive charge packets accumulate on capacitor 456 (C _INTP ), while negative charge packets accumulate on capacitor 460 (C _INTN ).

At blocks 908, 910, output _sampling capacitor _466a ( C _SHP ) and 466b (C _SHN ) have been connected to/from the integrator output 454 via corresponding non-overlapping closing/opening of a pair of switches 468a-468b (shp) and a pair of switches 470a-470b (shn), respectively. output 454 is removed. As a result, output sampling capacitors 466a (C _SHP ) and 466b (C _SHN ) carry the same voltage across them as corresponding integrating capacitors 456 , 460 , respectively.

After a predetermined number of cycles N, at block 912, the downstream ADC 217 of FIG. 2 is directed by the sequencer 345 to measure the differential voltage between capacitors 466a (C _SHP ) and 466b (C _SHN ) (referred to as the "sub-integration"), at block 912 At this point, at block 914, capacitors 456, 460, 466a, and 466b are reset, and the entire process begins anew. The differential voltage represents the difference in positive and negative charges on capacitors 456 (C _INTP ) and 460 (C _INTN ), respectively. As a result, the two half cycles of any TX pulse add together, which is equivalent to full-wave rectification. Although the two half-cycles are integrated in discrete steps, with very short interruptions in between to accommodate switching of the integrating capacitor, this operation is essentially called "continuous" integration.

More specifically, S/H circuit 340 operates in three phases to generate a differential voltage to ADC 217 . These three stages include sampling from integrator circuit 326 , holding the sampled charge on S/H circuit 340 and driving ADC 217 . The following steps describe the associated signals.

Each of the S/H buffers 467a, 467b samples 454 the integrator output for the previous Tx clock cycle. Positive S/H buffer 467a samples positive integrating capacitor 456 (C _INTP ) (typically, when Tx has its last high edge), and negative S/H buffer 467a samples negative integrating capacitor 460 (C _INTP ) (at Tx with its last low edge). The first signal to be converted is the buffer power on signal 471a, 471b (bufp_pdb and bufn_pdb). The S/H buffers 467a, 467b are driven dynamically so they only consume current during the sampling phase (shp) and during the driving ADC phase (adc_sample). In sampling mode, operating switch 470a (shpp) puts S/H buffer 467a into unity gain mode and sets V _Y to the right of sampling capacitors 466a (C _SHP ) and 466b (C _SHN ). Switch 468a (shp) is operated to sample the positive input (from active integrator 326 ) on sample/hold capacitor 466a ( _CSHP ). When these two signals return to 0, the S/H buffers 467a, 467b are powered down via inputs 471a, 471b (bufp_pdb and bufn_pdb), one node of sample/hold capacitor 468a (C _SHP ) is via switch 470b (!shp && !adc _sample) to V _Y , and the second node is floating. This allows "holding" of the sampled positive integrator voltage on C _SHP . A similar operation samples and holds the negative integrator voltage on sample/hold capacitor 466b (C _SHN ). Just before the adc_sample conversion, the S/H buffers 467a, 467b are powered up again (bufp_pdb and bufn_pdb), C _SHP and C _SHN are placed in the feedback surrounding their respective buffers, and the SAR capacitors inside the ADC 217 C _ADCP and C _ADCN charge up to the values stored on C _SHP and C _SHN .

Continuous integration with time overlap at the input and output of active integrator 326 results in faster panel scan times, which can also reduce operating current. The reduced operating current reduces battery drain, which is especially important in battery-operated systems with touch-sensing arrays.

In another embodiment of the active integrating circuit 300, the single-input, dual-output S/H circuit 340 can be eliminated, and the ADC 217 can be replaced with a sufficiently fast single-input ADC. With a sufficiently fast AC, the ADC can quickly sample the positive-going and negative-going signals from integrator circuit 326 directly on output 454, and then processing core 102 can digitally subtract the two signals.

FIG. 10 is another embodiment of FIG. 2 showing capacitive sensor 101 configured to provide a calibration unit 321 configured to provide self-calibration of capacitive sensor 101 and via selection circuit 1092 and multiple Demultiplexer 212 is coupled between input TX signal 227 and touch-sensing array 121 . The calibration unit 321 includes a first capacitor 1094 (also denoted CFM), a second capacitor 1096 (also denoted CM), and a switch 1098 in series with the second capacitor 1096 . The calibration unit 321 is configured to facilitate calibrating the capacitive sensor 101 by using capacitors 1094, 1096 to simulate the absence and presence of electrical conductors. The calibration unit 321 can be used to calibrate mutual capacitance sensing and self capacitance sensing. For example, an open switch 1098 can simulate a touch event, and a closed switch 1098 can simulate a no-touch event (ie, because the value of the mutual capacitance actually decreases during a touch), respectively. Using these on-chip capacitors, touch-like signals can be generated, allowing the determination (and subsequent calibration) of each channel's gain from front to back. After calibration, all channels exhibit the same overall gain to the actual touch signal, which significantly improves the calculation accuracy of the finger touch position. Methods for gain calibration may include programming the actual capacitance value (or digital value 1004) of each integrating capacitor.

In self-calibration mode, each channel 1000a-1000n of the touch-sensing array 121 can also pass through the scan channel 1000a via the demultiplexer 212, the multiplexer 213, the active integrator 326, the sample-hold circuit 340 and the ADC - 1000n are further calibrated one at a time, where ADC 217 is then digitally connected to processing core 102 . In one embodiment, a selected one of channels 1000a-1000n is continuously integrated by active integrator 326, sample-and-hold circuit 340, and ADC 217 according to the method shown in FIG. The selection circuit 802 and the two or more gain correction values 804 are simulated in software within the processing core 102 for digital calibration within and between the same or different touch-sensing arrays 121 at the factory or during run-time operation of the capacitive sensor 101 The channel variance caused by component variation of . Some or all of the components 1002, 1004 may be implemented using other techniques as would be understood by one of ordinary skill in the art having the benefit of this disclosure.

Returning again to FIG. 10, the resulting calibration values 1004 can be stored in memory and can be applied as a "digital gain calibration" factor to each output of channels 1000a-1000n. Digital gain calibration allows touch location accuracy to less than approximately about 0.2 mm.

Returning to FIG. 4, in one embodiment, the first integrating capacitor 456 and the second integrating capacitor 460 may be variable/programmable capacitors to allow correction of a second degree of gain for canceling channel gain variance.

Embodiments of the invention described herein include various operations. These operations may be performed by hardware components, software, firmware or a combination thereof. As used herein, the term "coupled" may mean directly coupled or indirectly coupled through one or more intermediate components. Any signals described herein that are provided on different buses may be time multiplexed with other signals and provided on one or more common buses. Additionally, the interconnections between circuit components or blocks may be shown as buses or as single signal lines. Each bus can optionally be one or more single signal lines, and each single signal line can optionally be a bus.

Certain embodiments may be implemented as a computer program product, which may include instructions stored on a computer-readable medium. These instructions can be used to program a general or special purpose processor to perform specified operations. A computer-readable medium includes any mechanism for storing or transmitting information in a form (eg, software, processing application) readable by a machine (eg, a computer). Computer-readable storage media may include, but are not limited to, magnetic storage media (e.g., floppy disks); optical storage media (e.g., CD-ROMs); magneto-optical storage media; read-only memory (ROM); ); erasable programmable memory (eg, EPROM and EEPROM); flash memory, or other types of media suitable for storing electronic instructions. Computer readable transmission media include, but are not limited to, electrical, optical, acoustic or other forms of propagated signals (eg, carrier waves, infrared signals, digital signals, etc.), or other types of media suitable for transmitting electronic instructions.

Additionally, certain embodiments may be practiced in a distributed computing environment, where the computer readable medium is stored and/or executed by more than one computer system. In addition, information passed between computer systems can either be pulled or pushed across the transmission media connecting the computer systems.

Although methods of operation herein are shown and described in a particular order, the order of operations for each method may be changed such that certain operations may be performed in reverse order or such that certain operations may be performed at least in part concurrently with other operations implement. In another embodiment, instructions or sub-operations of different operations may be performed in an intermittent and/or alternating manner.

In the foregoing specification, the invention has been described with reference to specific exemplary embodiments. It will, however, be evident that various modifications and changes can be made therein without departing from the broader spirit and scope of the invention as claimed in the appended claims. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.

Claims (20)

1. A circuit comprising: an active integrator configured to measure the capacitance of a touch-sensing array, wherein the active integrator is configured to receive a response signal from the touch-sensing array having a positive portion and a negative portion, wherein the response signal represents presence or absence of electrical conductors on the touch sensing array, and wherein the active integrator is configured to continuously integrate the response signal, integrating the positive portion of the response signal using a first integrating capacitor and The negative portion of the response signal is integrated using a second integrating capacitor. 2. The circuit of claim 1, wherein the active integrator comprises a switched capacitor integrator. 3. The circuit of claim 1, wherein the active integrator is configured to have a passband centered on a fundamental frequency of the response signal. 4. The circuit of claim 1, wherein: the first integrating capacitor is coupled to the output of the active integrator and to the first input of the active integrator; and The second integrating capacitor is coupled to the output of the active integrator and the first input of the active integrator. 5. The circuit of claim 4, further comprising: a first switch configured to connect or disconnect the first integrating capacitor between the output terminal and the first input terminal, and A second switch connected to connect or disconnect the second integration capacitor between the output terminal and the first input terminal. 6. The circuit of claim 5, wherein the first switch is closed in response to the positive portion and opened in response to the negative portion, and Wherein the second switch is closed in response to the negative portion and opened in response to the positive portion. 7. The circuit of claim 1 , further comprising a baseline compensation circuit coupled to the first input of the active integrator and configured to minimize a DC ( DC) component. 8. The circuit of claim 7, wherein the baseline compensation circuit comprises: Digital to Analog D/A Converter, a gain circuit coupled to the D/A converter; and a first switch and a second switch, each coupled to the gain circuit and the first input of the active integrator, wherein said first switch is configured to apply a negative signal to said first input of said active integrator when said positive portion is present, and wherein said second switch is configured to apply a negative signal to said first input of said active integrator when said negative A positive signal is applied to the first input of the active integrator when part is present. 9. The circuit of claim 1 , further comprising a sample-and-hold circuit coupled to an output of the active integrator, wherein an output signal of the active integrator is applied to the sample-and-hold circuit. 10. The circuit of claim 9, wherein the sample-and-hold circuit comprises: a first capacitor and a first switch connected in series and coupled between the output of the active integrator and the first input of an analog-to-digital converter ADC; and, a second capacitor and a second switch connected in series and coupled between the output of the active integrator and the second input of the ADC, wherein the first switch is configured to cause the first capacitor to maintain a positive signal on the output of the active integrator when the positive portion is present, and wherein the second switch is configured to cause the second capacitor to maintain a negative signal on the output of the active integrator when the negative portion is present. 11. The circuit of claim 1, further comprising a self-calibration circuit coupled between an input signal source and the touch-sensing array. 12. The circuit of claim 11 , wherein the self-calibration circuit comprises: a first capacitor coupled between the input signal source and the touch-sensing array; a second capacitor coupled between the input signal source and the touch-sensing array; and switch, which is in series with the second capacitor, Wherein, the switch is configured to insert or remove the first capacitor to simulate the presence or absence of a touching object. 13. The circuit of claim 1 , further comprising a mode selection circuit coupled to the active integrator and configured to allow the active integrator to operate in self-capacitance sensing mode and mutual capacitance Inductive mode for integration. 14. A method for measuring capacitance of a touch sensing array comprising: receiving a response signal from the touch-sensing array at an active integrator, the response signal having a positive portion and a negative portion; and continuously integrating the response signal using the active integrator to measure a change in capacitance, integrating the positive portion of the response signal using a first integrating capacitor and integrating the negative portion of the response signal using a second integrating capacitor, Wherein, the capacitance change indicates the presence or absence of a conductor. 15. The method of claim 14, wherein continuously integrating the response signal further comprises: (a) accumulating charge responsive to said positive portion using said first integrating capacitor coupled between an input terminal and an output terminal of said active integrator; (b) accumulating charge responsive to said negative portion using said second integrating capacitor coupled between said input and said output of said active integrator; and (c) Repeat steps (a) and (b) for a predetermined number of cycles. 16. The method according to claim 15, wherein said steps (a) and (b) further comprise: sample-holding the charge responsive to the positive portion on a third capacitor coupled between the output of the active integrator and the first input of an analog-to-digital converter ADC; and The charge responsive to the negative portion is sample-held on a fourth capacitor coupled between the output of the active integrator and the second input of the ADC. 17. The method according to claim 16, wherein said steps (a) and (b) further comprise: (d) measuring a differential voltage between the combination of said third capacitor and said fourth capacitor; (e) discharging the charge on each of the first integrating capacitor, the second integrating capacitor, the third capacitor and the fourth capacitor; and (f) Repeat steps (a)-(e). 18. The method of claim 14, further comprising: applying baseline compensation to the first input of the active integrator, wherein the applying includes: applying a negative signal to the first input when the positive portion is present; and A positive signal is applied to the first input when the negative portion is present. 19. An apparatus for measuring capacitance of a touch sensing array comprising: A processing device configured to detect the presence of an electrical conductor on the touch-sensing array, wherein the processing device includes a capacitance sensing circuit including an active sensor configured to measure a capacitance of the touch-sensing array. an integrator, wherein the active integrator is configured to receive a response signal from the touch-sensing array having a positive portion and a negative portion, wherein the response signal is representative of the electrical conductor on the touch-sensing array present or absent, and wherein the active integrator is configured to continuously integrate the response signal, integrate the positive portion of the response signal using a first integrating capacitor and integrate the response using a second integrating capacitor the negative part of the signal. 20. The apparatus of claim 19, wherein: the first integrating capacitor is coupled to the output of the active integrator and to the first input of the active integrator; and The second integrating capacitor is coupled to the output of the active integrator and the first input of the active integrator.