Technology directed to capacitive touch-sensing channels, including a capacitive-sensing converter based on a sigma-delta modulator, is described. One sigma-delta modulator includes a comparator, a first integrator coupled to receive an incoming signal from the input node and to provide a first output signal, a second integrator, coupled in parallel to the first integrator, to receive the incoming signal and to provide a second output signal, and switching circuitry. The switching circuitry is configured to selectively couple the first integrator between the input node and the comparator to provide the first output signal to the comparator or selectively couple the second integrator between the input node and the comparator to provide the second output signal to the comparator.

BACKGROUND

A touch sensor may be used to detect the presence and location of an object or the proximity of an object within a touch-sensitive area of the touch sensor. For example, touch-sensing circuitry may detect the presence and location of a touch object proximate to a touch sensor disposed in connection with a display screen. There are a number of different types of touch sensors. The types of touch sensor may include resistive touch sensors, surface acoustic wave touch sensors, capacitive touch sensors, inductive touch-sensing, and so forth. The different touch sensors may detect different types of objects.

Most touch-sensing applications require high-sensitivity to support a thick overlay over the touch sensor, operation of the touch sensor using a glove, or high-distance hover recognition at noisy conditions, such as caused by a nearby liquid crystal display (LCD), inductive loads switching, radio emissions, or the like. Moreover, the emission of the touch sensor is limited, which limits the excitation energy of the touch sensor to achieve a sufficient signal-to-noise ratio (SNR).

DETAILED DESCRIPTION

The following description sets forth numerous specific details such as examples of specific systems, components, methods, and so forth, in order to provide a good understanding of various embodiments of the techniques described herein for capacitive touch-sensing channels, including a capacitive-sensing converter based on a sigma-delta modulator whose structure is modified to obtain an accumulating property of a sensing result, giving sensing resolution that is proportional to an integration duration. As described above, most of the touch-sensing applications require high-sensitivity. As described herein the embodiments can provide an increased immunity to external noise by using a sinusoidal demodulation window together with sinusoidal excitation, as well as increase the sensing resolution by increasing the integration duration. It will be apparent to one skilled in the art, however, that at least some embodiments may be practiced without these specific details. In other instances, well-known components, elements, or methods are not described in detail or are presented in a simple block diagram format in order to avoid unnecessarily obscuring the techniques described herein. Thus, the specific details set forth hereinafter are merely exemplary. Particular implementations may vary from these exemplary details and still be contemplated to be within the spirit and scope of the present invention.

Described herein are various embodiments of techniques for capacitive sensing. The embodiments may provide a sense unit (also referred to as a touch sensor) that may be used in connection with capacitive-sensing circuitry to detect different types of objects. In one embodiment, the sense unit can be used for mutual capacitive sensing or self-capacitance sensing. In one embodiment, the capacitive sensing circuitry (also referred to herein as “capacitive-sensing circuitry” or “sensing circuitry”) may use a capacitive touch-sensing channel in a way that it can measure capacitance of the sense element (e.g., a single electrode with respect to a ground potential or between a receive (RX) electrode and a transmit (TX) electrode), as described in more detail herein. The sensing circuitry may also be configured to detect inductance of a sense element, such as to detect ferrous and non-ferrous metal objects proximate to the sense unit using inductive sensing techniques. Examples of devices that may use capacitive sensing may include, without limitation, automobiles, home appliances (e.g., refrigerators, washing machines, etc.), personal computers (e.g., laptop computers, notebook computers, etc.), mobile computing devices (e.g., tablets, tablet computers, e-reader devices, etc.), mobile communication devices (e.g., smartphones, cell phones, personal digital assistants, messaging devices, pocket PCs, etc.), connectivity and charging devices (e.g., hubs, docking stations, adapters, chargers, etc.), audio/video/data recording and/or playback devices (e.g., cameras, voice recorders, hand-held scanners, monitors, etc.), body-wearable devices, and other similar electronic devices.

Reference in the description to “an embodiment,” “one embodiment,” “an example embodiment,” “some embodiments,” and “various embodiments” means that a particular feature, structure, step, operation, or characteristic described in connection with the embodiment(s) is included in at least one embodiment of the invention. Further, the appearances of the phrases “an embodiment,” “one embodiment,” “an example embodiment,” “some embodiments,” and “various embodiments” in various places in the description do not necessarily all refer to the same embodiment(s).

The description includes references to the accompanying drawings, which form a part of the detailed description. The drawings show illustrations in accordance with exemplary embodiments. These embodiments, which may also be referred to herein as “examples,” are described in enough detail to enable those skilled in the art to practice the embodiments of the claimed subject matter described herein. The embodiments may be combined, other embodiments may be utilized, or structural, logical, and electrical changes may be made without departing from the scope and spirit of the claimed subject matter. It should be understood that the embodiments described herein are not intended to limit the scope of the subject matter but rather to enable one skilled in the art to practice, make, and/or use the subject matter.

FIG.1is a functional diagram of a capacitive touch-sensing channel100based on an accumulating first-order Sigma-Delta converter according to one embodiment. Capacitive touch-sensing channel100includes a sigma-delta modulator104coupled to an input node103. Sigma-delta modulator104can be a first-order sigma-delta modulator. Input node103is coupled to a touch sensor102. In one embodiment, touch sensor102includes a transmit (TX) electrode and a receive (RX) electrode, as represented as an equivalent circuit inFIG.1. In another embodiment, touch sensor102includes a single electrode. Alternatively, other types of touch sensors can be used. An output105of sigma-delta modulator104is coupled to a counter106, which is coupled to a demodulator108. In one embodiment, a waveform generator110generates an excitation signal107that is applied to touch sensor102, such as the TX electrode. Demodulator108is also configured to receive excitation signal107to demodulate an output109of counter106. An accumulator112is coupled to an output111of demodulator108and a decimator114is coupled to an output113of accumulator112. Decimator114outputs a digital result115, such a digital count value, representing a capacitance of touch sensor102.

As illustrated inFIG.1, sigma-delta modulator104includes a comparator120, a first integrator122, a second integrator124, and switching circuitry126. First integrator122is coupled to comparator120and is configured to receive an incoming signal from input node103and a reference voltage and to provide a first output signal. Second integrator124is coupled to comparator120, in parallel with first integrator122. Second integrator124is configured to receive the incoming signal at input node103and to provide a second output signal. Switching circuitry126is configured to selectively couple first integrator122between input node103and comparator120in order to provide the first output signal to comparator120or selectively couple second integrator124between input node103and comparator120to provide the second output signal to comparator120.

During operation, the incoming signal, in the form of a current comes into one of first integrator122or second integrator124and is balanced by a feedback-loop formed by a single-bit digitizer output, output105, from comparator120. In one embodiment, the feedback is expressed as −G, where G is expressed as follows:

G≥Vex·C⁢x⁢mC⁢int·F⁢t⁢xF⁢mod
The input signal balancing procedure forms a bit-stream on output105that is input into counter106. Counter106is an integrator in the digital form. Counter106reflects the digitized excitation signal scaled proportionally to the capacitance of touch sensor102. Operation of counter106can be expressed as follows:

Demodulator108multiplies the counter output and a digitized reference signal117that is coherent with excitation signal107. The demodulated digitalized signal, at output111, is integrated by accumulator112to get a magnitude of the sensed signal at output113. Decimator114forms digital result115of the sensing during an integer number of excitation signal periods (Ntx). The components of capacitive touch-sensing channel100form an accumulating first-order Sigma-Delta converter that converts a capacitance of touch sensor102into a digital value that represents the capacitance. As described in more detail below, the accumulating first-order Sigma-Delta converter gives a property of a quantization error accumulation when samples are accumulated during several periods of excitation signal107.

It should be noted that the channel transfer function of capacitive touch-sensing channel100is linear for a linear sweep of the sensed capacitance of touch sensor102. In conventional sigma-delta converters, a quantization error of the sensed signal magnitude does not change if a conversion lasts longer and the dithering allows a reduction of the quantization error. That is, the capacitance touch-sensing channel100measures a signal magnitude. The signal-shape distortions can be reduced with additional filtering, but the resolution for the signal magnitude does not change in this way as it remains constant. One conversion of an excitation period (also referred to as a Tx period) defines a quantization step value due to the symmetry of the positive and negative half-periods' shape. The shape gives the same magnitude of the quantization error at the end of each half-period but different polarity. Finally, the quantization error at the end of the excitation signal period is equal to zero. Each following conversion must be considered as a separate conversion without the history from the previous conversion. The accumulation of conversions samples during the following periods of the excitation signal narrows down the channel pass-band but does not increase the resolution. Dithering is needed to make a result of each period conversion with a random part larger than one balancing step. In this case, the following accumulation of Ntx periods of the excitation signal gives the resolution increasing by sqrt(Ntx) times (√{square root over ((Ntx))}). In this solution, it is not possible to implement an anti-aliasing filter to prevent saturation by high-frequency noise. The channel frequency response can be affected by external noise, narrowing the channel pass-band down if the integration time (the decimation factor) increases. For example, when a high-frequency noise signal is injected into the channel, the injected noise generates a current on the input of the sigma-delta modulator that is bigger than the balancing current. This can significantly distort a conversion if an impulse noise impacts the touch sensor (for example an LCD noise). The saturation can be prevented if an anti-aliasing filter reduces the magnitude of the noise high-frequency components. The conventional sigma-delta converter, however, does not implement an anti-aliasing filter. Conventional solutions use higher-order modulators that can also saturate. The saturation occurs when the signal variation during the sampling period is bigger than a balancing signal. The conventional solutions require conversion of the sensor current into a voltage, followed by a filter. A high-order sigma-delta modulator can be used to get a sufficient overload ability, but the channel resolution decreases proportionally.

In contrast, the capacitive touch-sensing channel100is based on a sigma-delta modulator104whose bit-stream is integrated by counter106that is coherently demodulated by multiplying with sine data coherent with excitation signal107, and the bit-stream is finally accumulated by accumulator112. Sigma-delta modulator104includes an additional integrator, second integrator124, which is in parallel to a main integrator, first integrator122. The integrators are connected to the incoming signal and comparator120using switching circuitry126. For example, a first switch S1and a second switch S2connect the incoming signal to comparator120through two branches. The switches S1and S2operate synchronously to form two branches from touch sensor102to comparator120. One branch is through first integrator122when excitation signal107rises and another branch is through second integrator124when excitation signal107falls. The balancing feedback-loop is connected to the active branch with a third switch S3that operates synchronously with switches S1and S2.

In this manner, the integrators store the quantization error formed at the end of their active phase and each of the following active phases starts at the quantization condition of the previous active phase of the other integrator. This gives a property of the quantization error accumulation when samples are accumulated during several periods of the excitation signal107, as illustrated inFIGS.2A-B.

In another embodiment, the capacitive touch-sensing channel100can demodulate the sigma-delta modulator's bit stream by multiplying the bitstream with cosine data. In this case, the first digital integrator106can be removed. The cosine data can be multiplied by +1 and −1. The multiplication can be substituted by adding or subtracting the sine data. This method can provide an advantage of a wider channel baseband, such as two times wider the channel passband.

FIGS.2A-2Bare waveform diagrams200of the accumulating first-order Sigma-Delta converter according to one embodiment. In waveform diagram200, an excitation signal202(labeled “Vtx”) is a sine wave that rises and falls. Sine table208is the sine data that is coherent with excitation signal202. While excitation signal202is rising, a switch control signal204(labeled as “θa”) is in a first state (e.g., low state or logical 0). While excitation signal202is falling, switch control signal204is in a second state (e.g., high state or logical 1). Waveform diagram200shows an output current210of an attenuator, such as illustrated and described below with respect toFIG.4A. Waveform diagram200also shows a balancing current signal206(labeled “Ibal”) that increases and decreases in frequency through a period of excitation signal202. Balancing current signal206represents the current used to balance the integrator based on the feedback-loop formed by the single-bit digitizer output, output105, from comparator120. Waveform diagram200shows signals212on the integrators, including quantization error214of the sensed magnitude. As described above, the signaling inFIGS.2A-Ballows the integrators of the accumulating first-order Sigma-Delta converter to store the quantization error formed at the end of their active phase and each of the following active phases start at the quantization error condition of the previous active phase of the integrator, resulting in quantization error accumulation. Waveform diagram200also shows an output216of the counter, and output218of the demodulator.FIG.2Bshows a zoomed-in view of excitation signal202, switch control signal204and balancing current signal206. The signaling inFIGS.2A-Ballows the capacitive sensing channel to be narrow-band and have a resolution that is proportional to the duration of the sensing period. Operation of the capacitive touch-sensing channel100based on an accumulating first-order Sigma-Delta converter can achieve an increased immunity to external noise by using a sinusoidal demodulation window in connection with a sinusoidal excitation. While conventional attempts do not allow increasing the resolution by increasing the sensing period (also referred to as sensing duration), capacitive touch-sensing channel100allows the resolution to be increase by increasing the sensing period. Capacitive touch-sensing channel100can be combine properties of a conventional capacitive touch-sensing channel that uses a sinusoidal excitation and a double-slope charge balancing converter based on a charge-transfer method. Capacitance touch-sensing channel100, however replaces the double-slope charge balancing converter with an accumulated sigma-delta modulator as described herein.

FIG.3is a waveform diagram300of a noise transfer function according to one embodiment. Waveform diagram300illustrates noise sources for a sine synchronous demodulator302and for a rectangular window synchronous demodulator304. For sine excitation, the sinusoidal shape of the excitation signal generates a single tone emission which can be placed into a frequency range without strong limits for the emission. In order to have low harmonics content, a complex synthesizer can be used. A single lobe noise transfer function results in better immunity to noise and better SNR in high-sensitivity modes. The Fast Fourier Transform (FFT) of channel samples allows the ability to find a silent band for the frequency hopping, resulting in reliable operation in a noisy environment. Demodulation of the channel samples can be done by multiplying the samples with sine values in digital form. A processing element can be used to multiply the channel samples with the sine values in digital form. Alternatively, modifications to an existing channel engine can be made to multiply the channel samples. The channel engine can be firmware executed by a processing device coupled to the demodulator.

The following description is directed to an implementation of the functional operations described above with respect toFIGS.1-2. For example, capacitive touch-sensing channel100can include two integrators that are each built using a current-to-current converter, as illustrated in in the accumulating first-order Sigma-Delta converter ofFIGS.4A-4C.

FIGS.4A-Care block diagrams of an accumulating first-order Sigma-Delta converter400(hereinafter “converter400”) according to one embodiment. Converter400is hereinafter referred to as converter400for ease of description. Converter400is an analog-to-digital converter (ADC) that converts a current or charge from a touch sensor into a digital value. The ADC is a first-order converter because it has a first-order sigma-delta modulator402that measures feedback, representing quantization error, used for continuous balancing integrators. First-order sigma-delta modulator402is referred to below as sigma-delta modulator402for ease of description. A counter404is used to store a digital representation of the input signal. The output of counter404is multiplied and accumulated by a multiplication and accumulation circuit (MAC)406, which is illustrated and described below with respect toFIG.4C.

In one embodiment, sigma-delta modulator402can include two integrators that each includes an operational amplifier and an integrator capacitor. Alternatively, sigma-delta modulator402can include an attenuator408, as illustrated inFIG.4A, that allows an output current to stay in a suitable range, making it possible to use a unity-value balancing source.

As illustrated, sigma-delta modulator402includes attenuator408coupled to an input node401and a bias voltage403. Attenuator408includes an amplifier (e.g., trans-impedance operational amplifier410) that is common to a first integrator and a second integrator. Sigma-delta modulator402also includes a first integrator capacitor412coupled to a first node405and a second integrator414capacitor coupled to a second node407. Sigma-delta modulator402also includes a first current source416, a second current source418, a comparator420, and a flip-flop422coupled to an output of comparator420and coupled to an input of counter404. An output of the flip-flop422is part of a balancing feedback loop424coupled to the switching circuitry. As illustrated inFIG.4A, the switching circuitry includes: a first switch426coupled to attenuator408, first node405, and second node407; a second switch428coupled to comparator420, first node405, and second node407; a third switch430coupled to a third node409, first node405, and second node407; a fourth switch432coupled to third node409, first current source416, and second current source418. First switch426, second switch428, and third switch430are configured to operate synchronously. These switches can be controlled by a first control signal411(labeled “θAcc”) that is dependent on an excitation signal413. When excitation signal413is rising, first control signal411is low, causing first switch426and second switch428to couple an incoming signal to comparator420via first node405, which is coupled to first integrator capacitor412. When excitation signal413is falling, first control signal411is high, causing first switch426and second switch428to couple an incoming signal to comparator420via second node407, which is coupled to second integrator414. Third switch430is controlled by first control signal411to connect the balancing feedback loop424to first node405when excitation signal413is rising and to second node407when excitation signal413is falling. Fourth switch432is controlled by the output of flip-flop422. The fourth switch432couples either first current source416or second current source418to third node409to balance the integrators. The balancing feedback loop424includes a balancing current signal206that controls the current sources to balance incoming signal. Comparator420compares the incoming signal, which includes the balanced feedback, against a voltage reference415. Flip-flop422is clocked using a clock signal429(labeled as “Fmod”). The same clock signal is used by counter404. As described above, the balancing current signal that is fed back to control fourth switch432represents the current used to balance the integrators based on the balancing feedback loop424formed by a single-bit digitizer output417that represents an output from comparator420that is sampled by flip-flop422. The signaling of the balancing feedback loop424allows the integrators of the sigma-delta modulator402to store a quantization error formed at the end of their active phase and each of the following active phases start at the quantization error condition of the previous active phase of the integrator, resulting in quantization error accumulation. The counter404counts the single-bit digitizer output417over a sensing period and outputs a digital count value419to MAC406, described below with respect toFIG.4C.

In one embodiment, excitation signal413is generated by a waveform generator434. Waveform generator434generates excitation signal413as a sinusoidal wave (also referred to as a sine wave). Input node401can be coupled to a touch sensor436, including a first electrode438coupled to waveform generator434and a second electrode440coupled to input node401. The switching circuitry is configured to form a first branch between touch sensor436and comparator420and a second branch between touch sensor436and comparator420. The first branch is through the first integrator when excitation signal413rises and the second branch is through the second integrator when excitation signal413falls. The switching circuitry is further configured to couple balancing feedback loop424to the first branch when excitation signal413rises and to the second branch when excitation signal413falls. The first integrator is configured to store a quantization error formed at an end of a first active phase of the first integrator, and wherein the second integrator is configured to start at the quantization error at a start of a second active phase of the second integrator for a quantization error accumulation. In one embodiment, waveform generator434is controlled by control data421(labeled “sine table”). The control data can be stored in a sine wave table. The control data421is digital data that is coherent with excitation signal413. The control data421is also used by the digital demodulator, described below with respect toFIG.4C.

In another embodiment, the sigma-delta modulator includes a comparator, a first integrator coupled to receive an incoming signal from the input node and to provide a first output signal, a second integrator, coupled in parallel to the first integrator, to receive the incoming signal and to provide a second output signal, and switching circuitry to selectively couple the first integrator between the input node and the comparator to provide the first output signal to the comparator or selectively couple the second integrator between the input node and the comparator to provide the second output signal to the comparator. In a further embodiment, the switching circuitry includes a first switch coupled to provide the incoming signal to the first integrator or the second integrator and a second switch coupled to provide the first output signal to the comparator or the second output signal to the comparator. The first switch and the second switch are configured to operate synchronously.

In another embodiment, the sigma-delta modulator includes a balancing feedback loop coupled to the switching circuitry and the switching circuitry includes a first switch coupled to provide the incoming signal to the first integrator or the second integrator, a second switch coupled to provide the first output signal to the comparator or the second output signal to the comparator, and a third switch coupled to provide a balancing feedback signal from the balancing feedback loop to the incoming signal provided to the first integrator or the incoming signal provided to the second integrator. In this embodiment, the first switch, the second switch, and the third switch are configured to operate synchronously.

As illustrated inFIG.4B, converter400includes a current-to-current converter (also referred to as an attenuator408) that is based on a trans-impedance operational amplifier410with a 100% feedback loop and an output stage amplifier442. That is, the trans-impedance operational amplifier410has a feedback loop. The attenuator408keeps an output current to be in a suitable range, making it possible to use a unity-value balancing source for continuously balancing.

As illustrated inFIG.4C, converter400is coupled to MAC406. MAC406can be one implementation of demodulator108, accumulator112and decimator114ofFIG.1. MAC406includes a multiply circuit444that is coupled to counter404and an accumulation circuit446coupled to multiply circuit444. Multiply circuit444is configured to demodulate digital count value419that is output by counter404by multiplying digital count value419by control data421. The output of the multiply circuit444is a demodulated signal423. Control data421is the sine data coherent to excitation signal413from waveform generator434. Accumulation circuit446is configured to accumulate demodulated signal423by adding a current output425of the multiply circuit444to an accumulated value427that is already stored in a register448. An output of accumulation circuit is stored in register448as an updated accumulated value429that is output from MAC406.

FIG.4Dis a schematic diagram of the current-to-current converter408according to one embodiment. As illustrated inFIG.4D, the trans-impedance operational amplifier410can include an output stage450of a first set of transistors and can be complemented with an additional output stage452of a second set of transistors that mirrors a current generated by the output stage450of the trans-impedance operational amplifier410. The output current amplification or attenuation can be achieved by changing the number of transistors in the mirroring stage. The attenuation tuning allows keeping the output current in a suitable range making it possible to use a unity-value balancing source.

FIG.4Eis a schematic diagram of a current-to-current converter with low pass filters454according to one embodiment. As illustrated inFIG.4E, low-pass filters (LPFs)454can be added into the path of the driving signals. The LPFs454can suppress high-frequency noise components of the incoming current. The LPFs can operate as anti-aliasing filter.

FIG.5is a touch system500having an array502of electrodes and multiple capacitive touch-sensing channels504according to one embodiment. Touch system500includes an analog front-end (AFE) a capacitive touch-sensing controller, the controller being coupled to the array502. The AFE includes a waveform generator506coupled to a first multiplexer circuit508, a second multiplexer circuit510coupled to the multiple capacitive touch-sensing channels504. Waveform generator506can be a direct digital synthesizer (DDS) that receive digital input, referred to as control data or sine data, and generates an excitation signal. The DDS can generates a DDS-based sine wave. The sine wave is different than a rectangular excitation signal as done conventionally. The excitation signal can be applied to any one of the electrodes of array502via the first multiplexer circuit508. It should be noted that the first multiplexer circuit508can connect the direct output or the inverse output of waveform generator506to any sensor TX line according to a multiphase pattern. Any one of the multiple capacitive touch-sensing channels504can be coupled to any one of the electrodes of the array502via the second multiplexer circuit510. Each of the multiple capacitive touch-sensing channels504can include an accumulated sigma-delta converter512and a MAC514. Accumulated sigma-delta converter512is similar to accumulated sigma-delta converter400ofFIGS.4A-4C. MAC514is similar to the MAC406ofFIGS.4A-4C. As described herein, the accumulated sigma-delta converter512generates samples and the samples are multiplied with sine data coherent to excitation for demodulation by the MAC514. Waveform generator506forms a half-period signal to drive the quantization error accumulation in the accumulated sigma-delta converter512.

In another embodiment, a system includes a touch sensor having a first electrode and a second electrode and a capacitance touch-sensing controller coupled to the touch sensor. The capacitance touch-sensing controller includes a waveform generator coupled to the first electrode. The waveform generator generates an excitation signal, sine data coherent to the excitation signal, and a control signal indicative of the excitation signal rising or falling. A sensing channel is coupled to the second electrode at an input node. The sensing channel includes an accumulated sigma-delta analog-to-digital converter (ADC) to generate a digital value representing a capacitance of the touch sensor. The accumulated sigma-delta ADC can include a comparator, a first integrator coupled to receive an incoming signal from the input node and to provide a first output signal, a second integrator, coupled in parallel to the first integrator, to receive the incoming signal and to provide a second output signal, and switching circuitry to selectively couple the first integrator between the input node and the comparator to provide the first output signal to the comparator or selectively couple the second integrator between the input node and the comparator to provide the second output signal to the comparator. In a further embodiment, the accumulated sigma-delta ADC further includes a balancing feedback loop coupled to the switching circuitry. The switching circuitry can include a first switch coupled to provide the incoming signal to the first integrator or the second integrator, a second switch coupled to provide the first output signal to the comparator or the second output signal to the comparator, and a third switch coupled to provide a balancing feedback signal from the balancing feedback loop to the incoming signal provided to the first integrator or the incoming signal provided to the second integrator. The first switch, the second switch, and the third switch are configured to operate synchronously.

In another embodiment, the accumulated sigma-delta ADC includes a first-order sigma-delta modulator, including an attenuator coupled to the input node and a bias voltage. The attenuator can include an amplifier that is common to the first integrator and the second integrator. The accumulated sigma-delta ADC further includes a first integrator capacitor coupled to a first node, a second integrator capacitor coupled to a second node a first current source, a second current source, and a flip-flop coupled to an output of the comparator and coupled to an input of the counter. An output of the flip-flop is part of a balancing feedback loop coupled to the switching circuitry. In this embodiment, the switching circuitry includes: a first switch coupled to attenuator, the first node, and the second node; a second switch coupled to the comparator, the first node, and the second node; a third switch coupled to a third node, the first node, and the third node; and a fourth switch coupled to the third node, the first current source, and the second current source. The first switch, the second switch, and the third switch are configured to operate synchronously. The fourth switch is controlled by the output of the flip-flop.

In one embodiment, the first integrator is configured to store a quantization error formed at an end of a first active phase of the first integrator, and the second integrator is configured to start at the quantization error at a start of a second active phase of the second integrator for a quantization error accumulation.

In another embodiment, the accumulated sigma-delta ADC includes a first-order sigma-delta modulator that includes a first integrator capacitor, a second integrator capacitor, and a current-to-current converter. The current-to-current converter can include a trans-impedance operational amplifier with a feedback loop and a set of transistors of an output stage, coupled to the trans-impedance operational amplifier. The set of transistors mirrors a current signal generated by the trans-impedance operational amplifier. In a further embodiment, the current-to-current converter can further includes a set of LPFs coupled between the trans-impedance operational amplifiers and the set of transistors. The set of LPFs filter high-frequency components of the current signal. In this embodiment, the switching circuitry can be configured to form a first integrator by coupling the first integrator capacitor into a first branch between the current-to-current converter and the comparator and to form a second integrator by coupling the second integrator capacitor into a second branch between the current-to-current converter and the comparator. In a further embodiment, the switching circuitry is configured to allow the incoming signal through the first branch when an excitation signal rises, and allow the incoming signal through the second branch when the excitation signal falls. The switching circuitry can also be further configured to couple a balancing feedback loop to the first branch when the excitation signal rises and to the second branch when the excitation signal falls.

In another embodiment, the capacitive touch-sensing channel can further include a multiply-accumulation circuit that includes a register to store an accumulated value and a multiply circuit coupled to a counter of the modulator. The multiply-accumulation circuit includes a multiply circuit that demodulates an output of the counter by multiplying the output of the counter by sine data coherent to an excitation signal from a waveform generator. The multiply-accumulation circuit also includes an accumulation circuit coupled to the multiply circuit. The accumulation circuit accumulate a demodulated signal by adding a current output of the multiply circuit to the accumulated value to obtain an updated accumulated value and storing the updated accumulated value in the register.

FIG.6is a touch system600having an array602of electrodes, multiple capacitive touch-sensing channels604, and a processing device618according to one embodiment. Touch system600includes a waveform generator606coupled to a multiplexer circuit608. The multiplexer circuit608may represent first multiplexer circuit508and second multiplexer circuit510ofFIG.5. The multiplexer circuit608is used to couple the waveform generator606to any one or more electrodes of the array602, as well as couple any one or more electrodes of the array602to one of the multiple capacitive touch-sensing channels604. Waveform generator606can be a DDS that receive digital input, referred to as control data or sine data, and generates an excitation signal. The control data is also sent to the capacitive touch-sensing channels604. As described herein, the excitation signal is a sine wave. It should be noted that the multiplexer circuit608can connect the direct output or the inverse output of waveform generator606to any sensor TX line according to a multiphase pattern. Any one of the multiple capacitive touch-sensing channels604can be coupled to any one of the electrodes of the array602via the multiplexer circuit608. Each of the multiple capacitive touch-sensing channels604can include an accumulated sigma-delta converter612and a MAC614. Accumulated sigma-delta converter612is similar to accumulated sigma-delta converter400ofFIGS.4A-4C. MAC614is similar to the MAC406ofFIGS.4A-4C. Because a multiphase pattern can be used, the capacitive touch-sensing channels604can include a deconvolution circuit616coupled to an output of MACs614. As described herein, the accumulated sigma-delta converter612generates samples and the samples are multiplied with sine data coherent to excitation for demodulation by the MAC614. The deconvolution circuit616can perform deconvolution on the sampled data. Waveform generator606forms a half-period signal to drive the quantization error accumulation in the accumulated sigma-delta converter612.

Touch system600can also include processing device618that receives the digital output from the multiple capacitive touch-sensing channels604. Processing device618can be a processor, a controller, hardware circuits that can perform further processing of the digital data. In one embodiment, processing device618executes firmware that includes post processing logic, communication logic, mutual capacitance mapping, self-capacitor vector generator, or the like. Processing device618can include a state machine. The processing device618can output data to a host620after processing the digital data. Touch system600can include other components, such as control circuitry to control the multiplexer circuit608, a sequencer to sequence through the electrodes of the array602, a baseline compensation circuit, or the like.

FIG.7is a method of operating an accumulating first-order Sigma-Delta converter according to one embodiment. The method700may be performed by processing logic that comprises hardware (e.g., circuitry, dedicated logic, programmable logic, microcode, etc.), software, firmware, or a combination thereof. In one embodiment, the method700may be performed by any of the processing devices described herein. In one embodiment, the method700is performed by capacitive touch-sensing channel100ofFIG.1. In another embodiment, the method700is performed by accumulating first-order Sigma-Delta converter400ofFIG.4A-4E. In another embodiment, the method700is performed by a device that includes the capacitive touch-sensing channel and a processing device coupled to the capacitive touch-sensing channel.

The method700begins by the processing logic receiving, by sigma-delta modulator of a capacitive-sensing channel, an incoming signal from a touch sensor (block702). The sigma-delta modulator comprising a comparator, a first integrator, and a second integrator. The processing logic selectively couples, by switching circuitry of the capacitive-sensing channel, the incoming signal to the comparator through a first integrator in a first branch when an excitation signal rises (block704). The processing logic selectively couples, by the switching circuitry, the incoming signal to the comparator through a second integrator in a second branch when the excitation signal falls (block706). The processing logic generates, by the comparator, an output signal (block708). The processing logic selectively couples, by the switching circuitry, a balancing feedback loop from the output signal of the comparator to the first branch when the excitation signal rises and to the second branch when the excitation signal falls (block710). The processing logic generates a count of the output signal (block712). The processing logic demodulates the count by multiplying the count by sine data coherent to the excitation signal to obtain a demodulated signal (block714). The processing logic accumulates the demodulated signal to obtain a quantization error accumulation (block716). The processing logic down-samples the quantization error accumulation to obtain a digital value (block718), and the method700ends. The digital value is indicative of a capacitance associated with the touch sensor.

In a further embodiment, the processing logic selectively couples the incoming signal to the comparator through the first integrator by controlling a first switch and a second switch to couple the input node and the comparator to a first node coupled to a first integrator capacitor. The processing logic selectively couples the incoming signal to the comparator through the second integrator by controlling the first switch and the second switch to couple the input node and the compactor to a second node coupled to a second integrator capacitor. In a further embodiment, the processing logic selective couples the balancing feedback loop from the output signal of the comparator to the first branch and to the second branch by controlling a third switch to couple a third node to the first node or the second node and controlling a fourth switch to couple a first current source or a second current source to the third node based on the output signal.

Embodiments descried herein may also relate to an apparatus for performing the operations herein. This apparatus may be specially constructed for the required purposes, or it may comprise a general-purpose computer selectively activated or reconfigured by a computer program stored in the computer. Such a computer program may be stored in a non-transitory computer-readable storage medium, such as, but not limited to, any type of disk including floppy disks, optical disks, CD-ROMs and magnetic-optical disks, read-only memories (ROMs), random access memories (RAMs), EPROMs, EEPROMs, magnetic or optical cards, flash memory, or any type of media suitable for storing electronic instructions. The term “computer-readable storage medium” should be taken to include a single medium or multiple media (e.g., a centralized or distributed database and/or associated caches and servers) that store one or more sets of instructions. The term “computer-readable medium” shall also be taken to include any medium that is capable of storing, encoding, or carrying a set of instructions for execution by the machine and that causes the machine to perform any one or more of the methodologies of the present embodiments. The term “computer-readable storage medium” shall accordingly be taken to include, but not be limited to, solid-state memories, optical media, magnetic media, any medium that is capable of storing a set of instructions for execution by the machine and that causes the machine to perform any one or more of the methodologies of the present embodiments.

The above description sets forth numerous specific details such as examples of specific systems, components, methods, and so forth, in order to provide a good understanding of several embodiments of the present disclosure. It is to be understood that the above description is intended to be illustrative and not restrictive. Many other embodiments will be apparent to those of skill in the art upon reading and understanding the above description. The scope of the disclosure should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.