WO2023021633A1

WO2023021633A1 - Dac-generated driven shield and voltage reference

Info

Publication number: WO2023021633A1
Application number: PCT/JP2021/030263
Authority: WO
Inventors: Paul Vincent; Brent Quist; Steve NOALL; Wayne Liu
Original assignee: Alps Alpine Co., Ltd.
Priority date: 2021-08-18
Filing date: 2021-08-18
Publication date: 2023-02-23
Also published as: KR20240026198A

Abstract

A capacitive sensor system includes a capacitive sensor including a sensing electrode and a shield electrode, a regulator circuit configured to regulate a voltage of the capacitive sensor and to generate an output signal based on an output of the capacitive sensor, a reference signal generator configured to generate a reference signal, supply the reference signal to the capacitive sensor via the regulator circuit, and supply the reference signal to the shield electrode as a driven shield signal, and an offset control module configured to generate an offset signal to modify an output current of the output signal.

Description

DAC-GENERATED DRIVEN SHIELD AND VOLTAGE REFERENCE

　　The present disclosure relates to capacitive sensors, and more particularly to a reference signal generator for a capacitive sensor.

　　The background description provided here is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.

　　An electronic device may implement a capacitive sensor configured to sense contact between an object (e.g., a finger) and a surface, such as a surface of the electronic device, and generate a sensed signal indicative of the sensed contact. For example, a reference signal generator (e.g., a waveform generator) is configured to generate and output a reference or control signal to the capacitive sensor. The sensed signal corresponds to changes in an amplitude and/or phase of the reference signal based on whether an object is contacting the sensor. Accordingly, the presence or absence of an object contacting the sensor can be determined based on the amplitude or phase of the sensed signal.

　　The electronic device may include a driven shield circuit that receives the sensed signal and outputs a driven shield signal based on the sensed signal. The driven shield signal is provided to a driven shield of the capacitive sensor to prevent capacitive coupling between external objects and the capacitive sensor. In some examples, the driven shield signal is coupled to the driven shield to match a voltage of the driven shield to a voltage of the capacitive sensor. An example implementation of a driven shield circuit is described in PTL 1.

[PTL 1]　　United States Patent No. 5,166,679

　　Technical problems associated with conventional driven shield circuits include long settling times of a voltage of the capacitive sensor and clipping of the sensed signal. A capacitive sensor system according to the present disclosure provides a solution to these technical problems by using a same reference signal supplied to the capacitive sensor as a driven shield signal and generating an offset signal in accordance with a rise timing of the reference signal to reduce the clipping of the sensed signal.

　　For example, capacitive sensor system according to the present disclosure includes a capacitive sensor including a sensing electrode and a shield electrode, a regulator circuit configured to regulate a voltage of the capacitive sensor and to generate an output signal based on an output of the capacitive sensor, a reference signal generator configured to generate a reference signal, supply the reference signal to the capacitive sensor via the regulator circuit, and supply the reference signal to the shield electrode as a driven shield signal, and an offset control module configured to generate an offset signal to modify an output current of the output signal.

　　In other features, the regulator circuit includes a current conveyor coupled to the capacitive sensor. The current conveyor is configured to regulate the voltage of the capacitive sensor based on the reference signal supplied by the reference signal generator. The reference signal generator includes a digital-to-analog converter. The reference signal generator generates the reference signal as a periodic signal. The capacitive sensor generates the output as a sensed signal corresponding to the reference signal as modified in accordance with at least one of contact with and proximity of an object.

　　In other features, the offset control module is coupled to an output of the regulator circuit to modify the output current of the output signal. The offset control module generates the offset signal as an offset current and the offset control module adds the offset current to the output current to reduce a magnitude of the output current. The offset control module adds the offset current to the output current to reduce the magnitude of the output current below a clipping region. The offset control module is configured to add the offset current to the output current while the output current increases from an initial value to a sensed input value.

　　In other features, the offset control module generates the offset signal such that a rise timing of the offset signal matches a rise timing of the output signal. To generate the offset signal such that the rise timing of the offset signal matches the rise timing of the output signal, the offset control module generates the offset current at a rise timing of the reference signal. The regulator circuit generates the output signal as one of a stepped input signal and a ramped input signal. The reference signal generator is configured to generate the reference signal with a predetermined slope. The regulator circuit generates the output signal having a slope that corresponds to the predetermined slope of the reference signal. The regulator circuit generates the output signal with a positive slope and the offset control module generates the offset signal with a negative slope that is complementary to the positive slope of the output signal.

　　Further areas of applicability of the present disclosure will become apparent from the detailed description, the claims and the drawings. The detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the disclosure.

According to an embodiment of the present disclosure, long settling times of a voltage of the capacitive sensor and clipping of the sensed signal can be reduced.

The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein:
FIG. 1 is an example capacitive sensor system including a driven shield circuit; FIG. 2A is an example electronic device including a capacitive sensor; FIGS. 2B is an example reference signal generator for a capacitive sensor; FIGS. 2C is an example reference signal generator for a capacitive sensor; FIG. 3 is an example capacitive sensor system implementing a switched capacitor circuit for a capacitive sensor according to the present disclosure; FIG. 4A illustrates a stepped input signal, a ramped signal, and a corresponding output signal of a capacitive sensor according to the present disclosure; and FIG. 4B illustrates a ramp input signal, a ramped offset signal, and a corresponding output signal of a capacitive sensor according to the present disclosure;

　　In the drawings, reference numbers may be reused to identify similar and/or identical elements.

　　A change in a signal supplied to a capacitive sensor (corresponding to a sensed signal) in response to a contact with an object is typically small relative to a supplied signal (i.e., a drive, control, or reference signal) and may be difficult to detect. Accordingly, different methods may be implemented to improve detection of the change. For example, a reference signal generator that provides the reference signal to the sensor may also supply a duplicate of the reference signal or a second reference signal generator can be provided to supply the duplicate of the reference signal. The duplicated signal is subtracted from the supplied signal and the result, which can be amplified to improve detection, corresponds to the sensed signal.

　　In some examples, the reference signal generator is a sinewave generator such as a Wien bridge oscillator. In other examples, the reference signal generator may be configured to supply a digital sinewave to a digital-to-analog converter (DAC) and the output of the DAC is filtered and/or amplified. In some examples, an offset control module is configured to generate an offset signal that is a duplicate of the sensed signal when there is no contact between an object and the capacitive sensor.

　　In some examples, an electronic device including the capacitive sensor may include a regulator circuit (e.g., a voltage-controlled current mode regulator) coupled to an output of the capacitive sensor. The regulator circuit is configured to regulate an output voltage corresponding to the reference signal and supplied to the capacitive sensor and generate an output current indicative of the sensed signal.

　　Referring now to FIG. 1, an example capacitive sensor system 100 includes a detection circuit 104 and a driven shield circuit 108. The detection circuit 104 is configured to generate an output signal based on a sensed signal received from a capacitive sensor 110. For example, the detection circuit 104 may include a timing circuit and/or other components configured to generate the output signal. The driven shield circuit 108 is configured to receive the sensed signal and output a driven shield signal based on the sensed signal. For example, the driven shield circuit 108 includes an operational amplifier 112 and one or more resistors R as shown, and the driven shield signal is a buffered copy of the sensed signal.

　　The driven shield signal is provided to a driven shield 116 of the capacitive sensor system. For example, the driven shield 116 may be a conductive plate, sheet, etc. (i.e., a shield electrode) arranged to prevent capacitive coupling between external objects and the capacitive sensor 110. In some examples, the driven shield signal is coupled to the driven shield 116 to match a voltage of the driven shield 116 to a voltage of the capacitive sensor 110. A capacitance associated with a parasitic capacitance between the capacitive sensor 110 and the driven shield 116 may be large relative to the sensed signal. Accordingly, any difference between the respective voltages on the capacitive sensor 110 and the driven shield 116 results in signal accumulation on an input signal path (i.e., the path of the sensed signal between the capacitive sensor 110 and the detection circuit 104). The difference between these voltages may cause clipping of the sensed signal (e.g., an output signal that is generated based on the sensed signal.

　　Further, since the voltage of the capacitive sensor 110 may vary and has an associated settling time, there is an associated delay for the voltage of the driven shield 116 to match the voltage of the capacitive sensor 110 (i.e., the voltage of the driven shield 116 "follows" the voltage of the capacitive sensor 110 with an associated delay). In other words, as the voltage of the capacitive sensor 110 settles, the driven shield 116 reacts to each change in the voltage of the capacitive sensor 110. Each time the voltage of the driven shield 116 changes in response to changes in the voltage of the capacitive sensor 110, the sensing signal may be capacitively coupled to the driven shield 116, which triggers additional settling of both the capacitive sensor 110 and the driven shield 116.

　　A capacitive sensing system according to the principles of the present disclosure generates a driven shield signal using a reference signal generator. For example, the driven shield signal corresponds to a same reference signal provided to the capacitive sensor and to a regulator circuit as described below in more detail. In some examples, the reference signal generator is a DAC configured to generate a periodic reference signal and output the reference signal to the regulator circuit, the capacitive sensor (e.g., via the regulator circuit), and the driven shield.

　　Referring now to FIG. 2A, an example electronic device 200 including a sensor module 204 corresponding to a capacitive sensor is shown. A reference signal generator (e.g., a waveform generator) 208 supplies a reference or control signal (e.g., a square wave, a sinewave, etc.) 210 to the sensor module 204. The reference signal generator 208 further supplies the reference signal 210 to a driven shield of the sensor module 204 according to the principles of the present disclosure. The reference signal generator 208 may supply the reference signal 210 to an offset control module 212 or an optional second reference signal generator 216 may provide a duplicate of the reference signal 210 to the offset control module 212.

　　The sensor module 204 modifies the reference signal 210 and generates a sensed signal 218 based on the reference signal 210 and proximity or contact with a sensed object. In other words, the sensed signal 218 corresponds to the reference signal 210 as modified in accordance with detection of (e.g., contact with and/or proximity of) an object. The sensed signal 218 is indicative of whether an object (e.g., a finger) is in contact with the sensor module 204. In some examples, the sensed signal 218 indicates a proximity of the object to the sensor module 204. The sensed signal 218 may differ (e.g., in amplitude and/or phase) from the reference signal 210 regardless of whether an object is in contact with the sensor module 204.

　　In one example, the offset control module 212 is configured to generate an offset signal 220 that is a duplicate of the sensed signal 218 without contact between an object and the sensor module 204. In other words, the offset control module 212 is configured to modify the reference signal 210 in the same manner as the sensor module 204 when there is no contact between an object and the sensor module 204 (and/or, in some examples, when the object is not sufficiently near the sensor module 204 to affect the sensed signal 218). As such, when there is no contact with an object, the sensed signal 218 and the offset signal 220 will be essentially the same (e.g., in magnitude, phase, and/or both magnitude and phase) and a difference between the sensed signal 218 and the offset signal 220 will approach zero. In other examples, the offset signal 220 may be configured to have an opposite polarity relative to the reference signal 210 and may simply be summed with the sensed signal 218.

　　Conversely, when there is contact between an object and the sensor module 204, the offset signal 220 and the sensed signal 218 will be different. A regulator circuit 224 outputs and amplifies a difference between the offset signal 220 and the sensed signal 218. The regulator circuit 224 is further configured to regulate a voltage provided to the sensor module 204 (e.g., a voltage of the reference signal 210). In some examples, the regulator circuit 224 may be a voltage-controlled current mode regulator implemented as a current conveyor. For example, a current conveyor is configured to receive an input voltage (e.g., the reference signal 210), regulate an output voltage (e.g., a voltage provided to the sensor module 204) based on the input voltage, and generate an output current (e.g., an output signal 228) indicative of the sensed signal 218.

　　The output signal 228 (e.g., an output current) of the regulator circuit 224 indicative of the sensed signal 218 may then be processed to detect contact between the sensor module 204 and an object. For example, contact may be determined based on whether an amplitude and/or a phase of the output signal 228 exceeds a respective threshold. In some examples, the sensed signal 218 may be indicative of a proximity of the object to the sensor module 204 regardless of whether the object is in direct contact with the sensor module 204. In these examples, the sensed signal 218 may be further indicative of a distance between the object and the sensor module 204. The output signal 228 is provided to an output signal path (e.g., an output signal path of a capacitive sensor system of the electronic device 200). For example, the output signal 228 is provided to an ADC via a sample-and-hold circuit.

　　The reference signal generator 208 is configured to generate a periodic signal such as a square wave, a sinewave, etc. For example, the reference signal generator 208 may include a DAC. In some examples, the reference signal generator 208 implements a Wien bridge oscillator. In other examples, the reference signal generator 208 may generate a digital sinewave that is subsequently converted to an analog sinewave. For example, as shown in FIGS. 2B and 2C, the reference signal generator 208 may include a digital sinewave generator 232. In FIG. 2B, an analog DAC 236 converts the digital sinewave to an analog signal, which is then filtered and amplified (or, in some examples, attenuated) using an LPF 240 having gain control capabilities. For example, an amplitude control signal is provided to the LPF 240 to control the gain. The LPF 240 may be a first order filter to reduce cost or, in some examples, may be a second, third, or higher order filter. In other examples, a band pass filter may be used.

　　Conversely, in FIG. 2C, a multiplier 244 is provided between the digital sinewave generator 232 and the analog DAC 236. The amplitude control signal is provided to the multiplier 244 to control the gain. The output of the multiplier (corresponding to the amplified digital sinewave) is provided to the analog DAC 236. The analog signal output by the analog DAC 236 is filtered using an LPF 248 without gain control.

　　Referring now to FIG. 3, an example capacitive sensor system 300 (e.g., a capacitive sensor system 300 for the electronic device 200) including a sensor module 304 (e.g., a capacitive sensor) and configured to supply a driven shield signal 308 to a driven shield 310 according to the present disclosure is shown. A reference signal generator 312 outputs a control or reference signal 316 (e.g., a modulated waveform such as a square wave, an analog sinewave, a digital waveform such as a digital sinewave that is converted to an analog sinewave, etc.). In some examples, the reference signal generator 312 is a DAC configured to generate the reference signal 316. For example, the reference signal generator 312 may be a DAC configured to receive a digital input (e.g., a 4-bit digital input) and convert the digital input to an analog signal. The DAC may be further configured to vary the analog signal such that a magnitude of the reference signal 316 is ramped, stepped, etc. as described below in more detail.

　　 The reference signal 316 is supplied to the sensor module 304 via a regulator circuit 318 and, in some examples, to an offset control module 320. The offset control module 320 is configured to generate an offset signal 324 based on the reference signal 316. The offset signal 324 is provided to the regulator circuit 318. The reference signal 316 is further coupled to the driven shield 310. In other words, the reference signal 316 according to the present disclosure is supplied to the driven shield 310 as the driven shield signal 308.

　　The sensor module 304 corresponds to a capacitive sensor (e.g., a capacitive touch circuit) including one or more parasitic capacitances (e.g., parasitic capacitance Crg) that modify an amplitude and phase of the reference signal 316 provided to the regulator circuit 318. In some examples, the reference signal 316 or another signal may be coupled to the one or more sensing electrodes 328 through a capacitance Crs. When an object (e.g., a finger 332) approaches (i.e., becomes within a proximity of) and/or contacts the sensing electrode 328 of the sensor module 304, a capacitance 336 of the finger 332 further modifies the amplitude and phase of a sensed signal 340 that is based on the reference signal 316. Accordingly, the sensor module 304 generates, as an output voltage, the sensed signal 340 indicative of whether an object such as the finger 332 is in contact with the sensor module 304 or, in some examples, a proximity of the finger 332 to the sensor module 304. More specifically, the sensed signal 340 is modified in accordance with the detected capacitance 336.

　　The regulator circuit 318 detects and outputs an indication of the sensed signal 340. In some examples, the regulator circuit 318 detects and outputs a difference between the sensed signal 340 and the offset signal 324. For example, the offset control module 320 is configured such that the sensed signal 340 and the offset signal 324 (i.e., respective amplitudes and phases of the sensed signal 340 and the offset signal 324) are the same when there is no contact between the sensor module 304 (e.g., the sensing electrode 328) and an object such as the finger 332. In some examples, the offset control module 320 is configured to adjust a phase and an amplitude of the offset signal 324 such that the output of the regulator circuit 318 (e.g., an output signal 344, corresponding to an output current) becomes substantially zero when there is no contact between the sensor module 304 (the sensing electrode 328) and the object.

　　In other words, the offset control module 320 is configured to adjust the phase and the amplitude of the offset signal 324 such that the phase and the amplitude of the offset signal 324 respectively coincide with a phase and an amplitude of the sensed signal 340 when the sensing electrode 328 of the sensor module 304 does not sense the proximity of or contact with an object. For example, the parasitic capacitances Crs and Crg modify the amplitude and phase of the reference signal 316. The offset control module 320 adjusts the amplitude and the phase of the offset signal 324 to compensate for the changes to the reference signal 316 caused by the capacitances Crs and Crg. The output of the regulator circuit 318 (i.e., the output signal 344) indicates whether there is contact between the sensor module 304 and the finger 332 or, in some examples, a proximity of the finger 332 to the sensor module 304 based on the sensed signal 340.

　　The regulator circuit 318 receives the reference signal 316 (e.g., as an input voltage of the regulator circuit 318) from the reference signal generator 312. The regulator circuit 318 is configured to regulate a voltage coupled to the sensor module 304 (e.g., a voltage of the reference signal 316). In some examples, the voltage coupled to the sensor module 304 corresponds to a voltage coupled to an optional sensor input/output (I/O) pad 348 that is further coupled to an output of the sensor module 304. Accordingly, the sensed signal 340 corresponds to an output voltage of the regulator circuit 318 that is generated based on the reference signal 316 and is further modified by the detected capacitance 336. The regulator circuit 318 generates the output signal 344 (e.g., the output current) indicative of the sensed signal 340.

　　A sample-and-hold circuit 350 receives the output signal 344 as an input voltage or signal. The reference signal 316 (and, accordingly, the output signal 344) is a periodic signal, such as a sinewave, a square wave, etc. Therefore, the input voltage of the sample-and-hold circuit 350 varies. The sample-and-hold circuit 350 is configured to sample values of the input voltage and generate an output voltage 352 corresponding to an average of the sampled values of the input voltage (i.e., an input voltage average). In this manner, the output voltage 352 is indicative of the output signal 344 and, accordingly, the sensed signal 340. An ADC 356 converts samples of the output voltage 352 to digital values for further processing by the capacitive sensor system 300.

　　As described above, the reference signal generator 312 according to the present disclosure supplies the reference signal 316 as the driven shield signal 308 to the driven shield 310. In other words, the voltage of the driven shield 310 does not follow the voltage of the sensing electrode 328. Instead, the reference signal 316 determines the voltage of the driven shield 310, which in turn drives the settling of the voltage of the sensing electrode 328.

　　Further, the reference signal generator 312 is configured to control the voltage of the reference signal 316 (and, correspondingly, the voltage of the driven shield signal 308). For example, the reference signal generator 312 controls a magnitude, slope, timing, etc. of the reference signal 316. Similarly, the offset control module 320 is configured to control the offset signal 324 based in part on the reference signal 316. The offset control module 320 according to the present disclosure is further configured to control the offset signal 324 to reduce clipping in the output signal 344. For example, the offset control module 320 generates the offset signal 324 such that a rise timing of the offset signal 324 matches a rise timing of the reference signal 316 and, correspondingly, matches a rise timing of the output signal 344. In other words, the offset control module 320 provides an output current of the output signal 344 at the rise timing of the reference signal 316.

　　As shown in FIG. 4A, the output signal 344 (e.g., an output current corresponding to the sensed signal 340 as described above) may be generated as a stepped input signal 400. The stepped input signal 400 may sharply increase (step up) from an initial value (e.g., 0 volts) to a sensed input value. However, in some examples, input values outside of a certain range (e.g., above 5 volts and below 5 volts, as indicated by respective clipping thresholds 404 and 408) may be clipped. In other words, input values that exceed either of the clipping

thresholds

404 and 408 may not be accurately detected and received. Accordingly, input values in

respective clipping regions

412 and 416 are clipped.

　　Conversely, the offset signal 324 (e.g., an offset current) may be generated as a ramped offset signal 420. The ramped offset signal 420 decreases from an initial value (e.g., 0 volts) as the stepped input signal 400 increases. Since the ramped offset signal 420 (as the offset signal 324) is coupled to the output of the regulator circuit 318 as described above and shown in FIG. 3, the ramped offset signal 420 modifies the output signal 344. For example, the ramped offset signal 420 as generated by the offset control module 320 is added to (i.e., summed with) the output signal 344 while the output signal 344 increases from the initial value to the sensed input value. Since the ramped offset signal 420 has a polarity opposite corresponding stepped input signal 400, the ramped offset signal 420 reduces the magnitude of the stepped input signal 400 below the clipping region 412. For example, the stepped input signal 400 as modified by the ramped offset signal 420 is shown at 424. A portion of the stepped input signal 400 may still be clipped as shown at 428.

　　As shown in FIG. 4B, the output signal 344 may be generated as a ramped input signal 432. The ramped input signal 432 may gradually increase (ramp up) from an initial value (e.g., 0 volts) to a sensed input value. Input values in

respective clipping regions

412 and 416 are clipped as described above. The ramped offset signal 420 reduces the magnitude of the ramped input signal 432 below the clipping region 412. For example, the ramped input signal 432 as modified by the ramped offset signal 420 is shown at 436. The ramped offset signal 420 as generated by the offset control module 320 is added to (i.e., summed with) the output signal 344 while the output signal 344 increases from the initial value to the sensed input value.

　　The ramped offset signal 420 may be provided at a rise time of the reference signal 316 such that a rise time of the ramped offset signal 420 generally matches a rise time of the ramped input signal 432. For example, as shown, a negative slope of the ramped offset signal 420 is complementary to a positive slope of the ramped input signal 432. For example, the reference signal generator 312 is configured to generate the reference signal 316 with a predetermined slope such that the ramped input signal 432 also has the predetermined slope. Conversely, the offset control module 320 is configured to generate the ramped offset signal 420 to have a slope that is complementary (e.g., opposite to) the predetermined slope of the ramped input signal 432. In other words, as the ramped input signal 432 increases, the ramped offset signal 420 decreases to offset the increase of the ramped input signal 432. Accordingly, the ramped input signal 432 as modified by the ramped offset signal 420 is maintained below the clipping region 412 and is not clipped.

　　In this manner, the reference signal generator 312 is configured to control the voltage of the reference signal 316 (and, correspondingly, the voltage of the driven shield signal 308 and the sensed signal 340) such that the output signal 344 is configured to reduce or eliminate clipping. For example, the reference signal generator 312 may be configured such that a magnitude of the reference signal 316 increases (e.g., steps or ramps up) over time. In one example, if the reference signal 316 is a periodic signal such as a sine wave or a square wave, the magnitude of the periodic signal may be controlled to ramp up at a rate corresponding to the slope of the ramped input signal 432 (e.g., a predetermined gradient or slope). Conversely, the offset control module 320 is configured to generate the offset signal 324 as a ramped signal to prevent the output signal 344 from reaching a clipping region. In one example, the offset control module 320 is configured such that the slope of offset signal 324 is matched (e.g., complementary) to the slope of the output signal 344.

　　The foregoing description is merely illustrative in nature and is in no way intended to limit the disclosure, its application, or uses. The broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent upon a study of the drawings, the specification, and the following claims. It should be understood that one or more steps within a method may be executed in different order (or concurrently) without altering the principles of the present disclosure. Further, although each of the embodiments is described above as having certain features, any one or more of those features described with respect to any embodiment of the disclosure can be implemented in and/or combined with features of any of the other embodiments, even if that combination is not explicitly described. In other words, the described embodiments are not mutually exclusive, and permutations of one or more embodiments with one another remain within the scope of this disclosure.

　　Spatial and functional relationships between elements (for example, between modules, circuit elements, semiconductor layers, etc.) are described using various terms, including "connected," "engaged," "coupled," "adjacent," "next to," "on top of," "above," "below," and "disposed." Unless explicitly described as being "direct," when a relationship between first and second elements is described in the above disclosure, that relationship can be a direct relationship where no other intervening elements are present between the first and second elements, but can also be an indirect relationship where one or more intervening elements are present (either spatially or functionally) between the first and second elements. As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A OR B OR C), using a non-exclusive logical OR, and should not be construed to mean "at least one of A, at least one of B, and at least one of C."

　　In the figures, the direction of an arrow, as indicated by the arrowhead, generally demonstrates the flow of information (such as data or instructions) that is of interest to the illustration. For example, when element A and element B exchange a variety of information but information transmitted from element A to element B is relevant to the illustration, the arrow may point from element A to element B. This unidirectional arrow does not imply that no other information is transmitted from element B to element A. Further, for information sent from element A to element B, element B may send requests for, or receipt acknowledgements of, the information to element A.

　　In this application, including the definitions below, the term "module" or the term "controller" may be replaced with the term "circuit." The term "module" may refer to, be part of, or include: an Application Specific Integrated Circuit (ASIC); a digital, analog, or mixed analog/digital discrete circuit; a digital, analog, or mixed analog/digital integrated circuit; a combinational logic circuit; a field programmable gate array (FPGA); a processor circuit (shared, dedicated, or group) that executes code; a memory circuit (shared, dedicated, or group) that stores code executed by the processor circuit; other suitable hardware components that provide the described functionality; or a combination of some or all of the above, such as in a system-on-chip.

　　The module may include one or more interface circuits. In some examples, the interface circuits may include wired or wireless interfaces that are connected to a local area network (LAN), the Internet, a wide area network (WAN), or combinations thereof. The functionality of any given module of the present disclosure may be distributed among multiple modules that are connected via interface circuits. For example, multiple modules may allow load balancing. In a further example, a server (also known as remote, or cloud) module may accomplish some functionality on behalf of a client module.

　　The term code, as used above, may include software, firmware, and/or microcode, and may refer to programs, routines, functions, classes, data structures, and/or objects. The term shared processor circuit encompasses a single processor circuit that executes some or all code from multiple modules. The term group processor circuit encompasses a processor circuit that, in combination with additional processor circuits, executes some or all code from one or more modules. References to multiple processor circuits encompass multiple processor circuits on discrete dies, multiple processor circuits on a single die, multiple cores of a single processor circuit, multiple threads of a single processor circuit, or a combination of the above. The term shared memory circuit encompasses a single memory circuit that stores some or all code from multiple modules. The term group memory circuit encompasses a memory circuit that, in combination with additional memories, stores some or all code from one or more modules.

　　The term memory circuit is a subset of the term computer-readable medium. The term computer-readable medium, as used herein, does not encompass transitory electrical or electromagnetic signals propagating through a medium (such as on a carrier wave); the term computer-readable medium may therefore be considered tangible and non-transitory. Non-limiting examples of a non-transitory, tangible computer-readable medium are nonvolatile memory circuits (such as a flash memory circuit, an erasable programmable read-only memory circuit, or a mask read-only memory circuit), volatile memory circuits (such as a static random access memory circuit or a dynamic random access memory circuit), magnetic storage media (such as an analog or digital magnetic tape or a hard disk drive), and optical storage media (such as a CD, a DVD, or a Blu-ray Disc).

　　The apparatuses and methods described in this application may be partially or fully implemented by a special purpose computer created by configuring a general purpose computer to execute one or more particular functions embodied in computer programs. The functional blocks, flowchart components, and other elements described above serve as software specifications, which can be translated into the computer programs by the routine work of a skilled technician or programmer.

　　The computer programs include processor-executable instructions that are stored on at least one non-transitory, tangible computer-readable medium. The computer programs may also include or rely on stored data. The computer programs may encompass a basic input/output system (BIOS) that interacts with hardware of the special purpose computer, device drivers that interact with particular devices of the special purpose computer, one or more operating systems, user applications, background services, background applications, etc.

　　The computer programs may include: (i) descriptive text to be parsed, such as HTML (hypertext markup language), XML (extensible markup language), or JSON (JavaScript Object Notation) (ii) assembly code, (iii) object code generated from source code by a compiler, (iv) source code for execution by an interpreter, (v) source code for compilation and execution by a just-in-time compiler, etc. As examples only, source code may be written using syntax from languages including C, C++, C#, Objective-C, Swift, Haskell, Go, SQL, R, Lisp, Java (registered trademark), Fortran, Perl, Pascal, Curl, OCaml, Javascript (registered trademark), HTML5 (Hypertext Markup Language 5th revision), Ada, ASP (Active Server Pages), PHP (PHP: Hypertext Preprocessor), Scala, Eiffel, Smalltalk, Erlang, Ruby, Flash (registered trademark), Visual Basic (registered trademark), Lua, MATLAB, SIMULINK, and Python (registered trademark).

100: capacitive sensor system
104: detection circuit
108: driven shield circuit
110: capacitive sensor
112: operational amplifier
116: driven shield
200: electronic device
204: sensor module
208: reference signal generator
210: reference signal
212: offset control module
216: optional second reference signal generator
218: sensed signal
220: offset signal
224: regulator circuit
228: output signal
232: digital sinewave generator
236: analog DAC
240: LPF
244: multiplier
248: LPF
300: capacitive sensor system
304: sensor module
308: driven shield signal
310: driven shield
312: reference signal generator
316: reference signal
318: regulator circuit
320: offset control module
324: offset signal
328: sensing electrode
332: finger
336: capacitance
340: sensed signal
344: output signal
352: output voltage
356: ADC
400: stepped input signal
404: clipping threshold
412: clipping region
420: ramped offset signal
432: ramped input signal

Claims

　　　　A capacitive sensor system, comprising:
　　　　a capacitive sensor including a sensing electrode and a shield electrode;
　　　　a regulator circuit configured to regulate a voltage of the capacitive sensor and to generate an output signal based on an output of the capacitive sensor;
　　　　a reference signal generator configured to
　　　　　　generate a reference signal,
　　　　　　supply the reference signal to the capacitive sensor via the regulator circuit, and
　　　　　　supply the reference signal to the shield electrode as a driven shield signal; and
　　　　an offset control module configured to generate an offset signal to modify an output current of the output signal.
　　　　The capacitive sensor system of claim 1, wherein the regulator circuit includes a current conveyor coupled to the capacitive sensor, wherein the current conveyor is configured to regulate the voltage of the capacitive sensor based on the reference signal supplied by the reference signal generator.
　　　　The capacitive sensor system of claim 1, wherein the reference signal generator includes a digital-to-analog converter.
　　　　The capacitive sensor system of claim 1, wherein the reference signal generator generates the reference signal as a periodic signal.
　　　　The capacitive sensor system of claim 1, wherein the capacitive sensor generates the output as a sensed signal corresponding to the reference signal as modified in accordance with at least one of contact with and proximity of an object.
　　　　The capacitive sensor system of claim 1, wherein the offset control module is coupled to an output of the regulator circuit to modify the output current of the output signal.
　　　　The capacitive sensor system of claim 6, wherein the offset control module generates the offset signal as an offset current, and wherein the offset control module adds the offset current to the output current to reduce a magnitude of the output current.
　　　　The capacitive sensor system of claim 7, wherein the offset control module adds the offset current to the output current to reduce the magnitude of the output current below a clipping region.
　　　　The capacitive sensor system of claim 7, wherein the offset control module is configured to add the offset current to the output current while the output current increases from an initial value to a sensed input value.
　　　　The capacitive sensor system of claim 7, wherein the offset control module generates the offset signal such that a rise timing of the offset signal matches a rise timing of the output signal.
　　　　The capacitive sensor system of claim 10, wherein, to generate the offset signal such that the rise timing of the offset signal matches the rise timing of the output signal, the offset control module generates the offset current at a rise timing of the reference signal.
　　　　The capacitive sensor system of claim 7, wherein the regulator circuit generates the output signal as one of a stepped input signal and a ramped input signal.
　　　　The capacitive sensor system of claim 7, wherein the reference signal generator is configured to generate the reference signal with a predetermined slope.
　　　　The capacitive sensor system of claim 13, wherein the regulator circuit generates the output signal having a slope that corresponds to the predetermined slope of the reference signal.
　　　　The capacitive sensor system of claim 14, wherein the regulator circuit generates the output signal with a positive slope, and wherein the offset control module generates the offset signal with a negative slope that is complementary to the positive slope of the output signal.