Finger detection with auto-baseline tracking

A device and system for automatically tracking a baseline input into a biometric sensor is provided. The device and system include a processing system with an amplifier having an input terminal and an output terminal for producing an output signal based on the input signal received by the input terminal. The processing system further includes at least one signal conditioning element coupled to the input terminal of the amplifier and configured to condition a compensation signal, and the processing system further includes a control circuit that adjusts one or more signal conditioning parameters of the at least one signal conditioning element based on the output signal of the amplifier. The at least one input signal received by the input terminal includes a combination of the at least one compensation signal and a signal from a first set of one or more receiver electrodes of the biometric sensor.

FIELD OF DISCLOSURE

This invention generally relates to electronic sensing, and more particularly, to capacitive fingerprint sensing.

BACKGROUND OF THE INVENTION

Biometric authentication systems are used for authenticating users of devices incorporating the authentication systems. Among other things, biometric sensing technology can provide a reliable, non-intrusive way to verify individual identity for authentication purposes.

Fingerprints, like various other biometric characteristics, are based on unalterable personal characteristics and thus are a reliable mechanism to identify individuals. There are many potential applications for utilization of biometric and fingerprints sensors. For example, electronic fingerprint sensors may be used to provide access control in stationary applications, such as security checkpoints. Electronic fingerprint sensors may also be used to provide access control in portable applications, such as portable computers, personal data assistants (PDAs), cell phones, gaming devices, navigation devices, information appliances, data storage devices, and the like. Accordingly, some applications, particularly portable applications, may require electronic fingerprint sensing systems that are compact, highly reliable, and inexpensive.

Constantly scanning a fingerprint sensor array to capture an image may unnecessarily consume power when there is no corresponding fingerprint to be imaged. To minimize power consumption, a fingerprint presence detection system is sometimes used to detect the presence of a finger before entering a higher power fingerprint imaging mode.

In view of the above, there is a need for a finger presence detection system of a fingerprint sensor that provides an accurate indication of finger presence over a sensor. These and other advantages of the disclosure, as well as additional inventive features, will be apparent from the description of the disclosure provided herein.

BRIEF SUMMARY OF THE DISCLOSURE

One embodiment provides a processing system for automatically tracking a baseline input into a biometric sensor. The processing system includes an amplifier having at least one input terminal and an output terminal for producing an output signal based on at least one input signal received by the at least one input terminal. The processing system further includes at least one signal conditioning element coupled to the at least one input terminal of the amplifier and configured to condition at least one compensation signal. And the processing system further includes a control circuit that adjusts one or more signal conditioning parameters of the at least one signal conditioning element based on the output signal of the amplifier. Wherein the at least one input signal received by the input terminal includes a combination of the at least one compensation signal and a signal from a first set of one or more receiver electrodes of the biometric sensor.

Another embodiment includes an electronic system for capacitive sensing. The electronic system includes a capacitive sensor configured to capacitively sense an input object in proximity to a plurality of electrodes. Wherein the plurality of electrodes includes a first set of one or more transmitter electrodes capacitively coupled to a first set of one or more receiver electrodes and forming a first signal path for a first sensor input signal. The plurality of electrodes further includes a second set of one or more transmitter electrodes capacitively coupled to a second set of one or more receiver electrodes and forming a second signal path for a second sensor input signal with opposite phase to the first sensor input signal. The electronic system further includes a processing system configured for automatically tracking a baseline value of the first input signal and the second input signal. The processing system includes a first compensation path including a first signal conditioning element, wherein the first compensation path transmits a first compensation signal. The processing system further includes a second compensation path including a second signal conditioning element, wherein the second compensation path transmits a second compensation signal with opposite phase to the first compensation signal. The electronic system further includes an amplifier including a first input terminal, a second input terminal and an output terminal, wherein the first sensor input signal and the first compensation signal are combined into a first amplifier input signal input on the first input terminal and the second sensor input signal and the second compensation signal are combined into a second amplifier input signal input on the second input terminal, and the output terminal produces an output signal based on the first amplifier input signal and the second amplifier input signal. The electronic system further includes a control circuit configured to adjust one or more signal conditioning parameters of the first signal conditioning element and the second signal conditioning element.

Yet another embodiment includes a processing system for automatically tracking a baseline input into a sensor. The processing system includes an amplifier having at least one input terminal and an output terminal for producing an output signal based on at least one input signal received by the at least one input terminal. The processing system further includes at least one signal conditioning element coupled to the at least one input terminal of the amplifier and configured to condition at least one compensation signal. The processing system further includes a control circuit that adjusts one or more signal conditioning parameters of the at least one signal conditioning element based on the output signal of the amplifier. The processing system further includes at least one comparator configured to compare the output signal to at least one baseline threshold. Wherein the control circuit adjusts the one or more signal conditioning parameters in a first direction when the output signal is higher than the at least one baseline threshold and adjusts the one or more signal conditioning parameters in a second direction when the output signal is lower than the at least one baseline threshold. Wherein the at least one input signal received by the input terminal includes a combination of the at least one compensation signal and a signal from a first set of one or more receiver electrodes of the sensor.

While the disclosure will be described in connection with certain preferred embodiments, there is no intent to limit it to those embodiments. On the contrary, the intent is to cover all alternatives, modifications and equivalents as included within the spirit and scope of the disclosure as defined by the appended claims.

DETAILED DESCRIPTION OF THE DISCLOSURE

One way to detect presence of a finger or other input object is to use one or more dedicated presence sensing elements on the input device. For example, in a fingerprint sensor, finger presence sensing electrodes may be used in addition to the electrodes of a sensing array that are used to capture an image of a fingerprint in a sensing region of the input device.

Another way to detect presence of a finger or other input object is to re-use selected sensor electrodes of the sensor array as presence sensing electrodes for presence detection. This embodiment may allow space to be saved by avoiding a need for dedicated presence sensing electrodes, as well as allowing for more accurate presence detection by using electrodes for presence detection that coincide with the sensor array.

A drawback to using presence sensing electrodes, either dedicated or not, is that typically, in certain implementations of the input device, the presence sensing electrodes may be disposed underneath a cover layer. Because the electrodes are disposed underneath the cover layer, a portion of an electric field utilized to detect the presence of an input object, such as a fingerprint when the input device is configured as a fingerprint sensor, will not be exposed outside of the cover layer. Also, this portion of the electric field not exposed outside of the cover layer will increase as a cover layer thickness increases. As such, any such signal indicating the presence of an input object will not have high gain to amplify the signal. Accordingly, a sensitivity of the input device will be affected by the thickness of the cover layer.

For example, in certain embodiments, an input device of a smart phone may include presence sensing electrodes for detecting an input object, such as a fingerprint. Regardless of whether the presence sensing electrodes are either dedicated presence sensing electrodes or selected sensor electrodes of the sensor array, the presence sensing electrodes may be disposed under a cover lens of the smart phone. As such, a portion of the electric field utilized to detect the presence of the fingerprint will not be exposed outside of the cover lens, which will reduce the sensitivity of the input device.

An additional drawback to using presence sensing electrodes for presence detection of an input object is that the sensor electrodes and their associated circuitry, such as one or more amplifiers, are exposed to temperature fluctuations within the device. As the temperature of the device changes, the output of the one or more amplifiers may be affected. This may cause drift in the output of the one or more amplifiers and make the detection of the input object more difficult.

To address the above discussed drawbacks, signal conditioning elements are added in parallel to the presence sensing electrodes. The signal conditioning elements may be added in parallel regardless of whether the presence sensing electrodes are dedicated or not. By doing so, the sensitivity of an output signal from the presence sensing electrodes will be increased such that the signal can be amplified with high gain. Further, one or more comparator devices may be implemented on the output of the presence sensing electrodes so to compare the output to threshold values in order to monitor and correct any drift experienced from temperature or other negative environmental factors.

Turning now to the figures,FIG. 1is a block diagram of an electronic system or device100that includes an input device such as sensor102and processing system104, in accordance with an embodiment of the disclosure. As used in this document, the term “electronic system” (or “electronic device”) broadly refers to any system capable of electronically processing information. Some non-limiting examples of electronic systems include personal computers of all sizes and shapes, such as desktop computers, laptop computers, netbook computers, tablets, web browsers, e-book readers, and personal digital assistants (PDAs). Additional example electronic devices include composite input devices, such as physical keyboards and separate joysticks or key switches. Further example electronic systems include peripherals such as data input devices (including remote controls and mice), and data output devices (including display screens and printers). Other examples include remote terminals, kiosks, and video game machines (e.g., video game consoles, portable gaming devices, and the like). Other examples include communication devices (including cellular phones, such as smart phones), and media devices (including recorders, editors, and players such as televisions, set-top boxes, music players, digital photo frames, and digital cameras). Additionally, the electronic device100could be a host or a slave to the sensor102.

Sensor102can be implemented as a physical part of the electronic device100, or can be physically separate from the electronic device100. As appropriate, the sensor102may communicate with parts of the electronic device100using any one or more of the following: buses, networks, and other wired or wireless interconnections. Examples include I2C, SPI, PS/2, Universal Serial Bus (USB), Bluetooth, RF, and IRDA.

The device100may utilize any combination of sensor components and sensing technologies to detect user input in the sensing region. The device100comprises one or more sensing elements for detecting user input. For example, the device100may use capacitive techniques, where voltage or current is applied to create an electric field. Nearby input objects cause changes in the electric field, and produce detectable changes in capacitive coupling that may be detected as changes in voltage, current, or the like.

One exemplary capacitive technique utilizes “mutual capacitance” (or “trans-capacitance”) sensing methods based on changes in the capacitive coupling between sensor electrodes. In various embodiments, an input object near the sensor electrodes alters the electric field between the sensor electrodes, thus changing the measured capacitive coupling. In one implementation, a mutual capacitance sensing method operates by detecting the capacitive coupling between one or more transmitter sensor electrodes (also “transmitter electrodes” or “TX electrodes”) and one or more receiver sensor electrodes (also “receiver electrodes” or “RX electrodes”). Transmitter sensor electrodes may be modulated relative to a reference voltage to transmit transmitter signals. The reference voltage may be a substantially constant voltage in various embodiments, or the reference voltage may be system ground. The transmitter electrodes are modulated relative to the receiver electrodes to transmit transmitter signals and to facilitate receipt of resulting signals. A resulting signal may comprise effect(s) corresponding to one or more transmitter signals, and/or to one or more sources of environmental interference (e.g. other electromagnetic signals).

It will be appreciated that embodiments of this disclosure are also usable in environments utilizing “self-capacitance” techniques. “Self capacitance” (or “absolute capacitance”) sensing methods are based on changes in the capacitive coupling between sensor electrodes and an input object. In various embodiments, an input object near the sensor electrodes alters the electric field near the sensor electrodes, thus changing the measured capacitive coupling. In one implementation, an absolute capacitance sensing method operates by modulating sensor electrodes with respect to a reference voltage (e.g. system ground), and by detecting the capacitive coupling between the sensor electrodes and input objects. In another implementation, an absolute capacitance sensing method operates by modulating a drive ring or other conductive element that is ohmically or capacitively coupled to the input object, and by detecting the resulting capacitive coupling between the sensor electrodes and the input object. The reference voltage may by a substantially constant voltage or a varying voltage and in various embodiments, the reference voltage may be system ground.

In certain embodiments, sensor102is a biometric sensor utilizing one or more various electronic sensing technologies to capture an image of a biometric pattern, such as a fingerprint, palm print, handprint, or vein pattern of a user. In certain embodiments, the biometric sensor is a capacitive fingerprint sensor which utilizes mutual capacitance sensing techniques between sensor electrodes in a second mode to detect presence of a finger or other biometric object in a sensing area. In a fingerprint sensor embodiment, for example, upon detection of a finger, the fingerprint sensor may utilize a full array of sensor electrodes in a first mode to capture an image of a fingerprint in the sensing area using mutual capacitance or self-capacitance sensing techniques. By way of example, the sensor electrodes used to detect presence of a finger in the second mode may be separate presence sensing electrodes, or they may be a selected subset of the electrodes used to capture the image of the fingerprint.

Turning now to the processing system104fromFIG. 1, basic functional components of the electronic device100utilized during capturing and storing a user fingerprint image are illustrated. The processing system104includes a processor(s)106, a memory108, a template storage110, an operating system (OS)112and a power source(s)114. Each of the processor(s)106, the memory108, the template storage110, the operating system112and power source114are interconnected physically, communicatively, and/or operatively for inter-component communications.

As illustrated, processor(s)106is configured to implement functionality and/or process instructions for execution within electronic device100and the processing system104. For example, processor106executes instructions stored in memory108or instructions stored on template storage110. Memory108, which may be a non-transitory, computer-readable storage medium, is configured to store information within electronic device100during operation. In some embodiments, memory108includes a temporary memory, an area for information not to be maintained when the electronic device100is turned off. Examples of such temporary memory include volatile memories such as random access memories (RAM), dynamic random access memories (DRAM), and static random access memories (SRAM). Memory108also maintains program instructions for execution by the processor106.

Template storage110comprises one or more non-transitory computer-readable storage media. The template storage110is generally configured to store enrollment views for fingerprint images for a user's fingerprint. The template storage110may further be configured for long-term storage of information. In some examples, the template storage110includes non-volatile storage elements. Non-limiting examples of non-volatile storage elements include magnetic hard discs, optical discs, floppy discs, flash memories, or forms of electrically programmable memories (EPROM) or electrically erasable and programmable (EEPROM) memories.

The processing system104also hosts an operating system112. The operating system112controls operations of the components of the processing system104. For example, the operating system112facilitates the interaction of the processor(s)106, memory108and template storage110.

The processing system104includes one or more power sources114to provide power to the electronic device100. Non-limiting examples of power source114include single-use power sources, rechargeable power sources, and/or power sources developed from nickel-cadmium, lithium-ion, or other suitable material.

Fingerprint sensors are sometimes referred to as swipe sensors or placement sensors depending on their principle of operation. Typically, swipe sensors capture an image that is larger than the sensing area by capturing a series of scans of the fingerprint as the user swipes or otherwise moves their finger over the sensing area. A processing system then reconstructs the scans into a larger swipe image. Since the image is reconstructed from a series of scans, this allows the sensing array to be made small, such as a small two-dimensional array or even as small as a single linear array, while still capturing a series of scans that can be reconstructed into a larger area image. Placement sensors typically capture an image that corresponds to the size of the sensing area by capturing scans of the fingerprint as it is placed or otherwise held over the sensing area. Usually, placement sensors include a two dimensional sensor array that can capture a sufficient area of the fingerprint in a single scan, allowing the fingerprint image to be captured without the user having to move the finger during the image capture process.

FIGS. 2-3Billustrate exemplary embodiments of the sensor102in accordance with the disclosure contained herein.FIG. 2illustrates an embodiment of the sensor102configured as a capacitive sensor that includes a plurality of electrodes200, including a set of transmitter electrodes206and a receiver electrode204, arranged in a linear one dimensional capacitive gap array. A pixel is formed at the capacitive coupling between each of the transmitter electrodes206and the receiver electrode204. In the depicted embodiment, the sensor electrodes200are operated in a first mode to capture an image of a fingerprint by driving transmitter signals onto each of the transmitter signals206, and detecting resulting signals at the receiver electrode204that correspond to the transmitter signals. In one implementation, the transmitter signals are driven onto each of the transmitter electrodes206one at a time, in a sequence one after another. In another implementation, transmitter signals are driven onto multiple transmitter electrodes simultaneously, using a code division multiplexing (CDM) or time-division multiplexing (TDM) sensing scheme.

Additionally, in the embodiment illustrated inFIG. 2, a reference receiver electrode208is illustrated. The reference receiver electrode208is located farther away from the transmitter electrodes206and is utilized as a reference to a differential measurement, such that noise is rejected at a differential output into differential amplifier210. Additionally, while the embodiment illustrated inFIG. 2shows only a single linear sensor array for imaging a sensing area in the first mode, it is possible to utilize an additional linear array with a corresponding receiver electrode and set of transmitter electrodes, which can be correlated to the illustrated linear array to assist with image reconstruction.

In certain embodiments of the sensor102ofFIG. 2, the sensor electrodes200may also be operated in a second mode. In the second mode, transmitter signals are driven onto one or more selected transmitter electrodes of the transmitter electrodes206, and resulting signals corresponding to the transmitter signals are detected at a set of one or more other transmitter electrodes selected from the set of transmitter electrodes206. In this mode, it is possible to detect mutual capacitance between parallel extending electrodes by operating one or more selected transmitter electrodes as receivers, which may be useful to detect presence of a finger over the sensor, among other uses. This may also allow the presence of the finger or another input object to be detected with less power consumption than used in imaging with the full sensor array102in the first mode.

In the illustrated embodiment, the transmitter electrodes206and the receiver electrode204are coplanar with each other, and an array of pixels are formed at the array of capacitive gaps202between the ends of each transmitter electrode206and the receiver electrode204. In another embodiment, the receiver electrode204and the transmitter electrodes206are configured to overlap, and the receiver electrode204and transmitter electrodes206are formed in the same or separate substrates separated by an insulator at each location where they overlap.

Moreover, while the illustrated embodiment depicts a plurality of transmitter electrodes206coupled to a common receiver electrode204to form a sensor array, in another embodiment, it is possible to use a similar construction having the receiver electrodes and transmitter electrodes reversed, so that a plurality of receiver electrodes are capacitively coupled to a common transmitter electrode to form the sensor array.

FIGS. 3A-3Billustrate additional embodiments of sensor102. As shown, sensor102is configured as a capacitive sensor that includes a plurality of electrodes300which form a two-dimensional array of pixels. In the sensor102ofFIGS. 3A-3B, rows of receiver electrodes302overlap columns of transmitter electrodes304to form a pixel based on a capacitive coupling at each overlap location. In one implementation, the receiver electrodes302and transmitter electrodes304are formed on the same substrate. In another implementation, they are formed on different substrates. In either case, some dielectric may separate the set of transmitter electrodes304and the set of receiver electrodes302at each overlap location, and one of the sets may be closer to a sensing area where a finger or other object is placed. In one implementation, the receiver electrodes302are disposed closer to a sensing area of the capacitive sensor102, and selected receiver electrodes are operated in a low power mode to detect a presence of a finger.

In the embodiment illustrated inFIG. 3A, the transmitter electrodes304and receiver electrodes302are depicted as sets of bars and stripes, respectively. The transmitter electrodes304each extend parallel to each other, and the receiver electrodes302also extend parallel to each other, in a different direction from the transmitter electrodes to form a two-dimensional array of pixels. In the illustrated embodiment, the transmitter electrodes and receiver electrodes extend perpendicular to each other. The transmitter electrodes and receiver electrodes may be formed, for example, on separate respective substrates, or opposing sides of the same substrate, and in either case the substrate material may separate the transmitter electrodes304and receiver electrodes302to form capacitive gaps between them at each overlap location.

In the embodiment illustrated inFIG. 3B, the transmitter electrodes304and receiver electrodes302are depicted as forming a diamond sensor pattern. In this embodiment, rows of receiver electrodes overlap columns of transmitter electrodes. Specifically, each of the transmitter electrodes304is made up of a set of interconnected diamonds, and each of the transmitter electrodes extends parallel to each other. Similarly, each of the receiver electrodes302is made up of a set of interconnected diamonds, and each of the receiver electrodes extends parallel to each other, perpendicular to the transmitter electrodes. In the illustrated embodiment, the receiver electrodes overlap the transmitter electrodes at narrower portions along the length of each electrode. The diamond sensor pattern ofFIG. 3Bmay be formed in a variety of ways. For example, the receiver electrodes and transmitter electrodes may be formed in the same layer, on the same side of an insulating substrate. Small amounts of dielectric may be used over the narrower portions of the transmitter electrodes304, so that the diamonds of each receiver electrode can be interconnected with conductive material over the transmitter electrodes, without creating ohmic contact between the receiver electrodes and transmitter electrodes. The diamond pattern may also be formed with the receiver electrodes and transmitter electrodes on separate substrates, or opposing sides of the same substrate, as described above.

It will be appreciated that other sensor array patterns are possible without departing from the principles described herein. For example, other electrode shapes besides diamond patterns, bars, and stripes are possible without departing from certain principles described herein. Similarly, other electrode orientations besides perpendicular rows and columns are possible without departing from certain principles described herein. It will also be appreciated that transmitter signals can be driven onto each of the transmitter electrodes304and resulting signals can be detected at each of the receiver electrodes302using a variety of modulation schemes in order to capture an image of the sensing area. In one implementation, the transmitter signals are driven onto each of the transmitter electrodes304one at a time, in a sequence one after another. In another implementation, transmitter signals are driven onto multiple transmitter electrodes simultaneously and/or resulting signals are detected at each of the receiver electrodes simultaneously, using a code division multiplexing (CDM) or time-division multiplexing (TDM) sensing scheme.

Turning now toFIGS. 4 and 5, arrangements of electrodes and associated circuitry utilized for presence detection of an input object, such as a finger, are illustrated. In various embodiments, the electrodes and circuitry may be arranged in a single end drive configuration, such as that shown inFIG. 4, or in a differential drive configuration in which opposite phase transmitter signals are driven onto separate electrodes, such as that shown inFIG. 5. Also, in either case the electrodes and circuitry may be arranged to take a single end measurement (not pictured), or may be arranged to take a differential measurement, as shown in bothFIGS. 4 and 5.

In one embodiment, the illustrated electrode and circuit arrangements may depict a selected subset of electrodes200,300from the sensor102re-used for finger presence detection, in accordance with principles described herein. In another embodiment, the electrode and circuit arrangements ofFIGS. 4 and 5may depict dedicated electrodes utilized for finger presence detection, in accordance with principles described herein. Regardless of whether the arrangement of electrodes and the associated circuitry is configured to re-use selected sensor electrodes from sensor102or are dedicated for finger presence detection and separate from the sensor102, the techniques and principles disclosed herein and described in relation toFIGS. 4-11are applicable.

Moreover,FIGS. 4 and 5depict the electrodes as parallel bars/stripes. However, it will be appreciated that the techniques described herein can also be applied to parallel diamond shaped electrodes, e.g., as shown inFIG. 3B, or other sensor patterns, without departing from the scope of the principles described therein.

FIG. 4illustrates a capacitive sensor400configured for finger presence detection. The capacitive sensor400includes electrodes and circuitry arranged in a single end drive configuration and configured to take a differential measurement. In this regard, transmit electrodes Tx402and404are driven with a transmitter signal from a single transmitter410, and receive electrodes Rx+406and Rx−408are configured to detect resulting signals. In the illustrated embodiment, the single end drive signal is driven onto two electrodes, Tx402and404for signal enhancement, and the receive electrodes also include two electrodes, Rx+406and Rx−408. However, in different implementations, it is possible to drive and/or detect on more or fewer electrodes, e.g., based on the dimensions of the sensor pattern and the electric fields extending above the sensor pattern that can be affected by a finger touch.

Receive electrodes Rx+406and Rx−408are configured to detect resulting signals corresponding to the transmitter signal driven onto Tx404and Tx402. The resulting signals are provided to an amplifier412with Rx+406connected to a positive input416of the amplifier412and Rx−408connected to a negative input414of the amplifier412. Additionally, in the illustrated embodiment, Rx+406is closer than Rx−408to the electrodes Tx402and404that are driven with the transmitter signal. This arrangement generates an imbalanced differential signal provided to the amplifier412. This imbalanced signal has the beneficial effect of removing more noise from the differential measurement being performed by the amplifier412compared to a single end measurement (not pictured). In this configuration, common mode noise coupled onto Rx+406and Rx−408is removed in output418of the amplifier412.

Accordingly, output418is a low noise gain signal that correlates to an amount of energy capacitively coupled from Tx402and404to Rx+406. The amount of energy coupled from Tx−402and404to Rx+406is affected by the presence of a biometric object such as a fingerprint. When a biometric object is present in the sensing area of a capacitive sensor400, the output418will be less in value than when no biometric object is present. In this regard, the output418of the capacitive sensor400can be utilized for finger presence detection.

Turning now toFIG. 5, a capacitive sensor500that provides finger presence detection is illustrated, according to a particular embodiment. In this embodiment, the capacitive sensor includes electrodes and circuitry arranged in a differential drive configuration and configured to take a differential measurement. The capacitive sensor500includes an electrode arrangement550, which includes one or more transmitter electrodes selected to transmit input signals and further includes one or more receiver electrodes selected to receive the input signals from the transmitter electrodes. In the illustrated embodiment, transmitter signals are driven onto at least four electrodes, illustrated as a first Tx−502, a second Tx−504, a first Tx+506and a second Tx+508. The pair of electrodes Tx−502and504are driven with transmitter signals having opposite phase to the pair of electrodes Tx+506and508. In addition, the electrodes Rx−520and Rx+518are disposed between their respective pairs of electrodes driven with transmitter signals. Specifically, Rx−520is disposed between the first Tx−502and the second Tx−504, and Rx+518is disposed between the first Tx+506and the second Tx+508.

FIG. 5further illustrates a first transmitter510and a second transmitter512arranged in a differential drive configuration. The first transmitter510couples a first transmit signal514onto the first Tx+506and the second Tx+508, and the second transmitter512couples a second transmit signal516that is similar to the first transmit signal but with opposite phase onto the first Tx−502and the second Tx−504. In the illustrated embodiment, the second transmitter512is configured as an inverter. Because there are two transmit electrodes for each receive electrode, the first resulting signal level provided to a positive input path526and the second resulting signal level provided to a negative input path524of an amplifier522are typically greater relative to a signal level that would be achieved if each opposite phase signal were driven onto only a single electrode. Further, since differential drive is used and the inputs to the amplifier correspond to receiver signals that result from the opposite phase transmitter signals, further signal enhancement may be achieved compared to a single-end drive implementation, as illustrated inFIG. 4.

An output of the amplifier522is a differential measurement between the first resulting signal and the second resulting signal and results in a low noise gain signal that correlates to an amount of energy capacitively coupled from Tx−502and504to Rx−520and from Tx+506and508to Rx+518. The amount of energy coupled from Tx−502and504to Rx−520and from Tx+506and508to Rx+518is affected by the presence of a biometric object such as a finger. When a biometric object is present at the capacitive sensor500, the output of the amplifier522will be less in value than when no biometric object is present. In this regard, the output of the capacitive sensor500can be utilized to detect for the presence of a biometric object, such as a finger.

Regardless of whether a single end drive configuration, such as that shown inFIG. 4, or a differential drive configuration, such as that shown inFIG. 5, is utilized for finger presence detection, the output of the capacitive sensor400,500is compared to a threshold signal level to determine if the processing system104may maintain operation of the capacitive sensor102and the device100, in general, in the second mode (i.e., low power) or return operation to the first mode (i.e., higher power).

As discussed above, the transmitter410and amplifier418in the single end drive embodiment ofFIG. 4and transmitters510and512and amplifier522in the differential drive embodiment ofFIG. 5are exposed to environmental conditions within the device100, such as temperature fluctuations. For instance, as the temperature of the device100changes, the output of the amplifiers418or522may be affected. In certain embodiments, the threshold used to determine the presence of the biometric object is set in reference to a baseline output of the amplifier418or the amplifier522when the biometric object is not present at the capacitive sensor400,500. As such, the environmental conditions within the device100may cause unwanted changes in this baseline output. Accordingly, unwanted changes in the baseline output may cause false readings provided by the capacitive sensor400,500. A false reading from the capacitive sensor400,500may cause the device100to improperly wake up the device100and/or return the sensor102to the first mode (i.e., higher power) even when a biometric object is not present at a sensing region of the capacitive sensor400,500.

Techniques and principles disclosed herein and described in relation toFIGS. 6-11provide for tracking any unwanted changes in the baseline output of the capacitive sensor400,500to avoid improper finger presence detection functionality. Turning now toFIG. 6, a sensor circuit600with auto-baseline tracking functionality is illustrated according to an embodiment of the disclosure. The sensor circuit600compensates for any drift in the output of the finger presence detection system of the device100, and may be implemented as part of various types of finger presence sensing systems. For instance, the sensor circuit600may be implemented in a presence sensing system using dedicated electrodes separate from the sensor102(seeFIG. 1), or the sensor circuit600may be implemented in a presence sensing system that re-uses selected sensor electrodes of the sensor102. Further, various components of the sensor circuit600may be implemented as part as the processing system104.

In the illustrated embodiment, the sensor circuit600is shown as being implemented with the capacitive sensor500(seeFIG. 5) and includes the electrode arrangement550illustrated in circuit diagram form. As illustrated, the electrode arrangement550includes two input paths, the negative input path524(seeFIG. 5) driven by the second transmitter512and the positive input path526driven by the first transmitter510. Regarding the negative input path524, capacitor Cm represents a capacitance between the receive electrode Rx−520and the two transmit electrodes, Tx−502,504, and capacitor Crx2grepresents a capacitance between Rx−520and ground. Regarding the positive input path526, capacitor Cm simulates a capacitance between the receive electrode Rx+518and the two transmit electrodes, Tx+506,508, and capacitor Crx2grepresents a capacitance between Rx+518and ground.

In the illustrated embodiment, sensor circuit600further includes a first compensation path602and a second compensation path604. While the illustrated embodiment shows two compensation paths, the disclosure contemplates greater or fewer compensation paths. Generally, for each input path, such as input paths524and526, discussed above, there will be a corresponding compensation path. However, this one-to-one relationship is not required, in as much as in certain embodiments, some subset of inputs may not include a corresponding compensation path.

As illustrated, compensation paths602and604include a respective transmitter608and614for driving a baseline input signal onto the compensation paths602and604. The baseline input signal may be similar to the first and second transmit signal514and516(seeFIG. 5). Transmitter608drives the baseline input signal, or just input signal, onto the first compensation path602. The input signal is similar to the input signal provided by transmitter512but with opposite phase. Transmitter614drives the input signal onto the second compensation path604. The input signal provided by transmitter614is similar to the input signal provided by the transmitter610but with opposite phase.

Compensation path602includes one or more signal conditioning elements644, and compensation path604includes one or more signal conditioning elements646. Generally, signal conditioning elements644and646can include any element or device that will function to condition the baseline input signal driven by transmitter608or transmitter614. For instance, as a nonexhaustive list of devices, the one or more signal conditioning elements644and646may each include any combination of one or more of a capacitor, a variable capacitor, an attenuator such as a digitally controlled attenuator, and a digital-to-analog converter (DAC). In embodiments of the disclosure where the signal conditioning elements644and646include a DAC, the transmitters608and614may optionally be removed from the sensor circuit600.

Further, each signal conditioning element includes one or more signal conditioning parameters. Signal conditioning parameters are either variable settings for the signal conditioning elements or an intrinsic property of the signal conditioning element. For instance, signal conditioning parameters for a capacitor may include a capacitance and phase of the capacitor. Similarly, signal conditioning parameters for a variable capacitor will include a variable capacitance and phase. Signal conditioning parameters for an attenuator may include a resistance or a variable resistance for a digitally controlled attenuator. Also, signal conditioning parameters for a DAC may include any DAC settings that control an output magnitude of the DAC.

In the illustrated embodiment, the signal conditioning elements644include a digitally controlled attenuator610and a compensation capacitor612, and signal conditioning elements646include a digitally controlled attenuator616and a compensation capacitor618. Accordingly, in the illustrated embodiment, the signal conditioning parameters for both the signal conditioning elements644and646include an attenuation factor for the digitally controlled attenuators610and616and a capacitance and phase of the compensation capacitors612and618.

The output of the compensation path602results in a compensation signal620, and the output of the compensation path604results in a compensation signal624. Compensation signal620is a version of the input signal from the transmitter608but conditioned by the one or more signal conditioning elements in the compensation path602. Compensation signal624is a version of the input signal from the transmitter614but conditioned by the one or more signal conditioning elements in the compensation path604.

In the illustrated embodiment, sensor circuit600includes an amplifier522that takes a differential measurement and produces an amplified output signal. The amplifier522includes a negative input622, which receives a combination of the compensation signal620and a signal present on the negative input path524(seeFIG. 5). The amplifier522further includes a positive input626, which receives a combination of the compensation signal624and a signal present on the positive input path526(seeFIG. 5). Based on the combination of the signal present on the negative input path524and the compensation signal620and the combination of the signal present on the positive input path526and the compensation signal624, the amplifier522generates an output signal628.

Additionally, in the illustrated embodiment, as arranged, the compensation capacitor612will function to subtract a portion of a voltage of the input signal of the negative input path524, and the compensation capacitor618will function to subtract a portion of a voltage of the input signal of the positive input path526. In embodiments where the sensor circuit600is implemented underneath a cover layer, such as a cover glass layer of a display, the input signal of both the negative input path524and the positive input path526generate an electric field in the electrode arrangement550. Due the electrode arrangement being implemented underneath the cover layer, only a portion of the electric field is exposed beyond the cover layer. Accordingly, that portion of the electric field not exposed beyond the cover layer is subtracted out by the compensation capacitors612and618in conjunction with their respective attenuators610and616, thereby allowing a high gain to be applied by amplifier522.

The above discussion of a voltage of the input signal entering the amplifier522can be represented with a voltage equation. As an example, for the illustrated embodiment ofFIG. 6, the following equation (1) can be used to determine the voltage entering the amplifier522.
Vin=2(A*Cm−Acomp*Ccomp)/Crx2g(1)

In the above equation (1), it is assumed that the magnitude of Cm and Crx2gfor both the negative input path524and the positive input path526are equal. A is the amplitude of the input signal transmitted by each of the transmitters608,512,510and614, where the amplitude of the input signal from each transmitter is assumed to be equal to simplify the above equation (1). Cm is the value of the capacitance between the receive electrode Rx+518and the two transmit electrodes, Tx+506,508(seeFIG. 5) or the capacitance between the receive electrode Rx−520and the two transmit electrodes, Tx−502,504, where both Cm are assumed to be equal to simplify the above equation (1). Acomp is an amplitude of the signal into the compensation capacitor612after the attenuator610, or the amplitude of the signal into the compensation capacitor618after the attenuator616, where both Acomp are assumed to be equal values to simplify the above equation (1). Ccomp is the capacitance of either the compensation capacitor612or compensation capacitor618, wherein both Ccomp are assumed to be equal to simplify the above equation (1). Crx2gis the capacitance between Rx+518and ground or Rx−520and ground, where both Crx2gare assumed to be equal to simplify the above equation (1). As can be seen in the above equation, the Acomp*Ccomp portion is subtracted from the A*Cm portion. Accordingly, in embodiments where sensor circuit600is implemented underneath a cover layer, this subtraction of Acomp*Ccomp from A*Cm increases the sensitivity of the output signal628by applying a high gain only to the portion of the electric field of the electrode arrangement550exposed beyond the cover layer.

As an aside, the above discussion regarding the equation (1) can be modified for other types of signal conditioning elements644and646. For instance, in certain embodiments, the compensation capacitors612and618could be replaced by variable capacitors, and the digitally controlled attenuators610and616could be replaced by another type of attenuator or a DAC. Further, a first DAC could replace both the compensation capacitor612and the digital controlled attenuator610for the compensation path602, and a second DAC could replace both the compensation capacitor318and the digital controlled attenuator616for the compensation path604. In doing so, equation (1) would need to be modified to represent the actual signal conditioning elements utilized.

The sensor circuit600includes a comparator network606, which includes one or more comparators that receive the output628and compare it to a threshold value. The embodiment of the sensor illustrated inFIG. 6includes four operational amplifiers, each arranged as an individual comparator630,632,634and636. Each positive input of each comparator630,632,634and636receives the output signal628and compares it to a threshold value. The comparator630receives the output signal628at its positive terminal and receives an error threshold648at its negative terminal for indicating when an error occurs in sensor circuit600. The comparator632receives the output signal628at its positive terminal and receives a high baseline threshold650at its negative terminal for indicating when the output signal628crosses the high baseline threshold. The comparator634receives the output signal628at its positive terminal and receives a low baseline threshold652at its negative input for indicating when the output signal628crosses the low baseline threshold652. The comparator636receives the output signal628at its positive terminal and receives a presence threshold654at its negative terminal for indicating when an object, such as a biometric object (fingerprint), is present on a sensing area of the sensor circuit600.

The high baseline threshold650is a boundary value on the high side indicating when the output signal628may be drifting higher than desired, and the low baseline threshold652is a boundary value on the low side indicating when the output signal628may be drifting lower than desired. The presence threshold654is a threshold value indicating the presence of an object on a sensing area of the sensor circuit600. As such, if the output signal628decreases below the presence threshold654, then the sensor circuit600will indicate that the biometric object is present. The error threshold648is set at a level above which the output signal628will typically reach during normal operation. Accordingly, if the output signal628goes above the error threshold648, the sensor circuit600will indicate the occurrence of an error.

It will be appreciated that the output signal628does not have to be provided to the positive input of the comparators630,632,634and636. In another embodiment, the output signal628could be provided to the negative inputs of the comparators630,632,634and636, and the positive input could receive the associated threshold.

Additionally, each comparator of the comparator network606is configured to sample the output signal628with a certain sampling period or window. This sampling window can be wide enough so as to ensure capturing of a peak or valley of the output signal628.

The sensor circuit600further includes a control circuit638. The control circuit may be implemented as digital logic, as illustrated inFIG. 6. The digital logic638receives each output from the comparator network606. Based on the outputs of the comparator network606, the digital logic638, via connections640and642, will adjust variable signal conditioning parameters from certain signal conditioning elements of each compensation path602and604. In this regard, the digital logic638can tune the output signal628such that it remains within the boundary set by the high baseline threshold650and the low baseline threshold652input into comparators632and634, respectively. In situations where the object, such as a biometric object like a fingerprint, is present at the sensing region of the sensor circuit600, the output signal628will exceed the presence threshold654at the comparator636and the digital logic638will indicate that the object is present. Further, if the output signal628exceeds the error threshold648, then the digital logic638will indicate the occurrence of an error.

In an alternative embodiment, rather than adjusting the signal conditioning parameters, the digital logic638could set the error and presence thresholds648,654according to the changing output signal628. In this embodiment, a difference between a baseline value of the output signal628, determined when the object is not present at the sensor circuit600, and the error and presence thresholds648,654would be maintained by changing the error and presence thresholds648,654. Accordingly, when the object is present at the sensor circuit600, the sensor circuit600would still indicate the presence of the object.

Generally, the digital logic638adjusts the compensation signals slower than the comparator630or comparator636indicating that the output signal628has crossed one of the error or presence thresholds648,654. In this manner, the digital logic will be able to correct movement in the baseline input signal when the object is not present but not affect sensor circuit600from being able to detect the presence of the object at the sensor circuit600.

Further, in certain embodiments, the digital logic638can adjust the various signal conditioning parameters independently between the compensation path602and the compensation path604. Accordingly, in these embodiments, the digital logic638can condition the output signal628in a variety of ways in order to have the output signal628closely track the desired baseline level.

In the illustrated embodiment, the digital logic638can adjust the attenuation factor of the digitally controlled attenuator610by using connection640and can adjust the attenuation factor of the digitally controlled attenuator616by using the connection642. In other embodiments, the digital logic638may adjust signal compensation parameters for a variable capacitor and/or a DAC in a similar manner. Regardless, the digital logic638is able to adjust a level of the output signal628by adjusting the signal compensation parameters of the signal conditioning elements644and646such that the sensor circuit600is able to perform auto-baseline tracking and correction. For instance, in the illustrated embodiment, the digital logic638will adjust the attenuation factor of the attenuators610and616in accordance with equation (1) so to maintain the output signal628within the desired range, as indicated by the high and low baseline thresholds.

FIG. 7illustrates a plot700of output signal628in relation to the high and low baseline thresholds650,652and the error and presence thresholds648,654. As illustrated, the output signal628illustrates a condition where no object is present at the sensor circuit600(seeFIG. 6). Accordingly, the output signal628is based on the baseline input signal transmitted at each of transmitter608,512,510and614. However, due to temperature and other environmental effects, the output signal628fluctuates. The comparator network606, in particular comparators632and634will indicate when the output signal628crosses one of the high or low baseline thresholds650,652, respectively. If the output signal628crosses one of the high or low baseline thresholds650,652, then the digital logic638will adjust one or more signal conditioning parameters of one or more signal conditioning elements644and646in order to bring the output signal628back to being within the high or low baseline thresholds650,652. In doing so, the digital logic638may change an attenuation value of an attenuator, such as a digitally controlled attenuator610and616; the digital logic638may change a capacitance of a variable capacitor; or the digital logic638may change the DAC settings so to change an output level of a DAC. For instance, if the output signal628drifts higher than the high baseline threshold650, then the digital logic638may increase the attenuation of the signal conditioning parameters for one or more of the signal conditioning elements644and646. Or, if the output signal628drifts lower than the low baseline threshold652, then the digital logic638may decrease the attenuation of the signal conditioning parameters for one or more of the signal conditioning elements644and646.

Returning toFIG. 6, the following is an exemplary description of the operation of the sensor circuit600. A baseline input signal is input onto the transmitter608, the transmitter512, the transmitter510and the transmitter614. The illustrated embodiment performs a differential measurement at the amplifier522based on the differential drive signal from transmitters510,512. Accordingly, two of the four transmitters, specifically, transmitters512and614are arranged as inverters so to invert a phase of the baseline input signal, while the other two transmitters514and608are arranged as amplifiers or a buffer in order to maintain the same timing as the signal from transmitters512and614. Transmitter608transmits the baseline input signal onto the compensation path602; transmitter512inverts the phase of the baseline input signal and transmits it onto the negative input path524of the electrode arrangement550(seeFIG. 5); transmitter510transmits the baseline input signal onto the positive input path526of the electrode arrangement550; and transmitter614inverts the phase of the baseline input signal and transmits it onto the compensation path604.

The baseline input signal affected by the negative input path524(seeFIG. 5) of the electrode arrangement550is combined with the baseline input signal affected or conditioned by the signal conditioning elements644of the compensation path602and the resulting signal is input into the negative input622of the amplifier522. The baseline input signal affected by the positive input path526(seeFIG. 5) of the electrode arrangement550is combined with the baseline input signal affected or conditioned by the signal conditioning elements646of the compensation path604and the resulting signal is input into the positive input626of the amplifier522. The amplifier522then performs a differential measurement between the signals input on the negative input622and the positive input626in order to obtain the output signal628.

While the foregoing describes circuitry for a differential drive and a differential measurement embodiment, in other embodiments, the circuit may be adapted for single end drive and/or a single end measurement. For example, if single end drive is used, the transmitter512which provides an opposite phase transmit signal may be omitted. Furthermore, if single end measurement is used, circuitry for providing a negative input path622to the amplifier522may be omitted.

The output signal628is then provided to the comparator network606, which in the illustrated embodiment, includes the four comparators630,632,634and636. The comparators630and636compare the output signal628to an error and presence thresholds648,654, respectively. The comparator636determines whether the object, such as a fingerprint, is present at the sensor circuit600, and the comparator630determines whether an error has occurred in the sensor circuit600. The comparators632and634compare the output signal628against high and low baseline thresholds650,652, respectively, in order to determine whether output signal628has crossed either of the high or low baseline thresholds650,652.

The output of the comparator network606is provided to the digital logic638. In certain embodiments, the digital logic638will indicate the presence of the object when the presence threshold654is exceeded and indicate the occurrence of an error when the error threshold648is exceeded. Further, the digital logic638will adjust one or more signal conditioning parameters of one or more signal conditioning elements644and646of one or more of compensation paths602or604when the output signal628exceeds one of the high or low baseline thresholds650,652input into the comparators632and634, respectively. In this manner, the sensor circuit600is able to provide finger presence detection functionality with auto-baseline tracking to compensate for temperature and other environmental effects degrading the measurement of the input baseline signal by the electrode arrangement550.

FIG. 8illustrates sensor circuit800, which operates similarly to sensor circuit600(seeFIG. 6). However, sensor circuit800includes comparator network806, which is different from comparator network806in that the comparator634is not needed for sensor circuit800. In this embodiment, digital logic838is configured to control signal conditioning parameters of the signal conditioning elements644and646such that the output signal628tracks a single baseline threshold850input into comparator832. Accordingly, whenever the comparator832indicates that the output signal628crosses the baseline threshold850input onto the negative input of the comparator832, the digital logic838will track a magnitude and direction of the output signal difference from the baseline threshold850. The digital logic838will then make appropriate adjustments to the signal conditioning parameters of the signal conditioning elements644and646in order to compensate for the magnitude and direction of the output signal628to bring it closer to the baseline threshold850. For instance, if the output signal628drifts higher than the baseline threshold850, then the digital logic838may increase the attenuation of the signal conditioning parameters for one or more of the signal conditioning elements644and646. Or, if the output signal628drifts lower than the baseline threshold850, then the digital logic838may decrease the attenuation of the signal conditioning parameters for one or more of the signal conditioning elements644and646.

FIG. 9illustrates plot900showing the output signal628in reference to the single baseline threshold850and the error and presence thresholds648,654. The error and presence thresholds648,654function in the same manner as discussed in reference toFIGS. 6 and 7. As illustrated, the output signal628shows a condition where no input object is present at a sensing area of the sensor circuit800(seeFIG. 8). Accordingly, the output signal628is based on the baseline input signal transmitted at each of transmitter608,512,510and614. However, due to temperature and other environmental effects, the output signal628fluctuates. The comparator network806, in particular comparator832, will indicate a magnitude and direction of the output signal628in reference to the baseline threshold850. The digital logic838will adjust one or more signal conditioning parameters of one or more signal conditioning elements644and646in order to bring the output signal628back to the baseline threshold850. For instance, the digital logic838may change an attenuation value of an attenuator, such as a digitally controlled attenuator; the digital logic838may change a capacitance of a variable capacitor; or the digital logic838may change the DAC settings so to change an output level of a DAC.

FIG. 10illustrates a flow chart1000providing a method of providing finger presence detection functionality with auto-baseline tracking. At step1002a baseline input signal is input into a finger presence detection sensor, such as sensor circuit600or sensor circuit800(seeFIGS. 6 and 8). At step1004, a comparator network606,806will track an output signal628against certain threshold values. And, at step1006, a control circuit, such as digital logic638,838, of the sensor circuit600,800will adjust one or more signal conditioning parameters of one or more signal conditioning elements644,646within the compensation paths602,604in order for the output signal to closely track the baseline threshold.

FIG. 11illustrates a flow chart1100for having the digital logic638,838(seeFIGS. 6 and 8) or other control circuit enter a fast adjustment mode. The fast adjustment mode allows the digital logic638,838to make greater changes to the signal conditioning parameters so to have a greater effect on a level of the output signal628. Accordingly, if the finer adjustments of the signal conditioning parameters are not fast enough to bring an output signal heavily affected by temperature or other environmental effects back closer to the desired threshold, the digital logic638,838can make larger adjustments to the signal conditioning parameters.

At step1102, the digital logic638,838(seeFIGS. 6 and 8) counts a number of adjustments and direction of the adjustments made by the digital logic638,838. At step1104, the digital logic638,838determines whether a number of successive adjustments in a same direction exceed a threshold count. If the number of successive adjustments in the same direction do not exceed the threshold count, then, at step1106, the digital logic638,838maintains normal operation. However, if the number and direction of adjustments exceeds the threshold value, then, at step1108, the digital logic638,838will enter the fast adjustment mode such that it can appropriately adjust the quickly degrading output signal628. For instance, the digital logic638,838may be configured to make larger adjustments to the signal conditioning parameters, e.g., by incrementing a signal conditioning parameter value by a larger amount, while in the fast adjustment mode.

The embodiments and examples set forth herein were presented in order to best explain the present disclosure and its particular application and to thereby enable those skilled in the art to make and use the invention. However, those skilled in the art will recognize that the foregoing description and examples have been presented for the purposes of illustration and example only. The description as set forth is not intended to be exhaustive or to limit the invention to the precise form disclosed.