# Patent

## Title
Apparatus and method for reducing noise in fingerprint sensing circuits

## Abstract

An apparatus for reducing noise in fingerprint sensing circuits is disclosed in one embodiment of the invention as including a fingerprint sensing area onto which a user can apply a fingerprint. An analog front end is coupled to the fingerprint sensing area and is configured to generate an analog response signal. An analog-to-digital converter (ADC) samples the analog response signal and converts the sample to a digital value, which may be received by a digital device such as a processor or CPU. To reduce the amount of the noise that is present in the analog response signal and therefore reflected in the digital value, the digital device may be shut down while the ADC is sampling the analog response signal.

## Background
BACKGROUND
This invention relates to fingerprint sensing technology, and more particularly to apparatus and methods for reducing the effects of noise in fingerprint sensing circuits.
Fingerprint sensing technology is increasingly recognized as a reliable, non-intrusive way to verify individual identity. Fingerprints, like various other biometric characteristics, are based on unalterable personal characteristics and thus are believed to be more reliable when identifying individuals. The potential applications for fingerprints sensors are myriad. 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.
Various electronic fingerprint sensing methods, techniques, and devices have been proposed or are currently under development. For example, optical and capacitive fingerprint sensing devices are currently on the market or under development. Like a digital camera, optical technology utilizes visible light to capture a digital image. In particular, optical technology may use a light source to illuminate an individual's finger while a charge-coupled device (CCD) captures an analog image. This analog image may then be converted to a digital image.
There are generally two types of capacitive fingerprint sensing technologies: passive and active. Both types of capacitive technologies utilize the same principles of capacitance to generate fingerprint images. Passive capacitive technology typically utilizes an array of plates to apply an electrical current to the finger. The voltage discharge is then measured through the finger. Fingerprint ridges will typically have a substantially greater discharge potential than valleys, which may have little or no discharge.
Active capacitive technology is similar to passive technology, but may require initial excitation of the epidermal skin layer of the finger by applying a voltage. Active capacitive sensors, however, may be adversely affected by dry or worn minutia, which may fail to drive the sensor's output amplifier. By contrast, passive sensors are typically capable of producing images regardless of contact resistance and require significantly less power.
Although each of the fingerprint sensing technologies described above may generate satisfactory fingerprint images, each may be adversely affected by noise, interference, and other effects. For example, capacitive sensors may be particularly susceptible to noise and parasitic capacitive coupling, which may degrade the quality of the acquired fingerprint image. Accordingly, it would be an advance in the art to reduce the effects of noise, parasitic capacitive coupling, and other effects in capacitive-type fingerprint sensing circuits.



BRIEF DESCRIPTION OF THE DRAWINGS
In order that the advantages of the invention will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific examples illustrated in the appended drawings. Understanding that these drawings depict only typical examples of the invention and are not therefore to be considered limiting of its scope, the invention will be described and explained with additional specificity and detail through use of the accompanying drawings, in which:
FIG. 1 is a high-level block diagram of one embodiment of a fingerprint sensing area containing an array of fingerprint sensing elements and interfacing with a fingerprint sensing circuit;
FIG. 2 is a partial cutaway profile view of a fingerprint sensing area showing the interaction between a finger and fingerprint sensing elements in a capacitive-type fingerprint sensor, with a fingerprint ridge lying substantially over the sensor gap;
FIG. 3 is a partial cutaway profile view of a fingerprint sensing area showing the interaction between a finger and fingerprint sensing elements in a capacitive-type fingerprint sensor, with a fingerprint valley lying substantially over the sensor gap;
FIG. 4 is a high-level block diagram of one embodiment of a fingerprint sensing circuit for use with the present invention;
FIG. 5 is a timing diagram showing an example of various signals that may exist within a fingerprint sensing circuit useable with the present invention; and
FIG. 6 is a high-level block diagram showing an example of an oscillator, multipliers, dividers, and the like that may be used to generate the signals of FIG. 5.



DETAILED DESCRIPTION
The invention has been developed in response to the present state of the art, and in particular, in response to the problems and needs in the art that have not yet been fully solved by currently available fingerprint sensors. Accordingly, the invention has been developed to provide novel apparatus and methods for reducing noise in fingerprint sensing circuits. The features and advantages of the invention will become more fully apparent from the following description and appended claims and their equivalents, and also any subsequent claims or amendments presented, or may be learned by practice of the invention as set forth hereinafter.
Consistent with the foregoing, an apparatus for reducing noise in fingerprint sensing circuits is disclosed in one embodiment of the invention as including a fingerprint sensing area onto which a user can apply a fingerprint. An analog front end is coupled to the fingerprint sensing area and is configured to generate an analog response signal. An analog-to-digital converter (ADC) samples the analog response signal and converts the sample to a digital value, which may be received by a digital device such as a processor or CPU. To reduce the amount of the noise that is present in the analog response signal and therefore reflected in the digital value, the digital device may be shut down while the ADC is sampling the analog response signal.
In selected embodiments, the digital device is shut down by turning off a clock signal to the digital device. In other embodiments, the digital device is shut down by disabling the digital device or turning off power to the digital device. In certain embodiments, the digital device is configured to shut down some interval prior to the time the ADC samples the analog response signal. The interval may be selected to allow any noise to settle out of the system prior to sampling the analog response signal.
In another embodiment in accordance with the invention, a method for reducing noise in a fingerprint sensing circuit may include providing a fingerprint sensing area onto which a user can apply a fingerprint. An analog response signal associated with finger activity over the fingerprint sensing area may then be generated. The method may further include sampling the analog response signal and converting the sample to a digital value. This digital value may then be received by a digital device. To reduce the amount of noise that is present in the analog response signal during sampling and therefore reflected in the digital value, the method may further include shutting down the digital device while sampling the analog response signal.
In another embodiment in accordance with the invention, a method for reducing noise in fingerprint sensing circuits includes providing a fingerprint sensing area onto which a user can apply a fingerprint and generating an analog response signal in response to finger activity over the fingerprint sensing area. The analog response signal is then sampled and converted to a digital value. This digital value may then be received or processed by a CPU. To reduce the amount of the noise that is present in the analog response signal during sampling and reflected in the digital value, the method may further include turning off a clock signal to the CPU while sampling the analog response signal.
It will be readily understood that the components of the present invention, as generally described and illustrated in the Figures herein, may be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the systems and methods of the present invention, as represented in the Figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of selected embodiments of the invention.
Some of the functional units or method steps described in this specification may be embodied or implemented as modules. For example, a module may be implemented as a hardware circuit comprising custom VLSI circuits or gate arrays, off-the-shelf semiconductors such as logic chips, transistors, or other discrete components. A module may also be implemented in programmable hardware devices such as field programmable gate arrays, programmable array logic, programmable logic devices, or the like.
Modules may also be implemented in software for execution by various types of processors. An identified module of executable code may, for instance, comprise one or more physical or logical blocks of computer instructions which may, for instance, be organized as an object, procedure, or function. Nevertheless, the executables of an identified module need not be physically located together, but may comprise disparate instructions stored in different locations which, when joined logically together, comprise the module and achieve the stated purpose of the module.
Indeed, a module of executable code could be a single instruction, or many instructions, and may even be distributed over several different code segments, among different programs, and across several memory devices. Similarly, operational data may be identified and illustrated herein within modules, and may be embodied in any suitable form and organized within any suitable type of data structure. The operational data may be collected as a single data set, or may be distributed over different locations including over different storage devices, and may exist, at least partially, merely as electronic signals on a system or network.
Reference throughout this specification to “one embodiment,” “an embodiment,” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment may be included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment.
Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, etc. In other instances, well-known structures, or operations are not shown or described in detail to avoid obscuring aspects of the invention.
The illustrated embodiments of the invention will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. The following description is intended only by way of example, and simply illustrates certain selected embodiments of apparatus and methods that are consistent with the invention as claimed herein.
Referring to FIG. 1, in selected embodiments, a fingerprint sensor 10 useable with an apparatus and method in accordance with the invention may include a fingerprint sensing area 12 to provide a surface onto which a user can swipe a fingerprint. A dotted outline of a finger 14 is shown superimposed over the fingerprint sensing area 12 to provide a general idea of the size and scale of one embodiment of a fingerprint sensing area 12. The size and shape of the fingerprint sensing area 12 may vary, as needed, to accommodate different applications.
In certain embodiments, the fingerprint sensing area 12 may include an array of transmitting elements 16, such as a linear array of transmitting elements 16, to assist in scanning lines of “pixels” as a fingerprint is swiped across the fingerprint sensing area 12. In this embodiment, the transmitting elements 16 are shown as a linear array of conductive traces 16 connected to a fingerprint sensing circuit 18. The transmitting elements 16 are not drawn to scale and may include several hundred transmitting elements 16 arranged across the width of a fingerprint, one transmitting element 16 per pixel. A fingerprint image may be generated by scanning successive lines of pixels as a finger is swiped over the array. These lines may then be assembled to generate a fingerprint image, similar to the way a fax image is generated using line-by-line scanning.
In certain embodiments, the transmitting elements 16 are configured to sequentially emit, or burst, a probing signal, one after the other. As will be explained in more detail hereafter, the probing signal may include a burst of probing pulses, such as a burst of square waves. This probing signal may be sensed on the receiving end by a receiving element 20. Like the transmitting elements 16, the receiving element 20 is shown as a conductive trace 20 connected to the fingerprint sensing circuit 18. Although shown as a single receiving element 20, in other embodiments, pairs of receiving elements 20 may be used to differentially cancel out noise.
At the receiving element 20, a response signal may be generated in response to the probing signal. The magnitude of the response signal may depend on factors such as whether a finger is present over the fingerprint sensing area 12 and, more particularly, whether a ridge or valley of a fingerprint is immediately over the gap 22 between a transmitting element 16 and the receiving element 20. The magnitude of the signal generated at the receiving element 20 may be directly related to the RF impedance of a finger ridge or valley placed over the gap 22 between the corresponding transmitting element 16 and receiving element 20.
By using a single receiving element 20 (or a small number of receiving elements 20) and a comparatively larger number of transmitting elements 16, a receiver that is coupled to the receiving element 20 may be designed to be very high quality and with a much better dynamic range than would be possible using an array of multiple receiving elements. This design differs from many conventional fingerprint sensors, which may employ a single large transmitting element with a large array of receiving elements and receivers. Nevertheless, the apparatus and methods described herein are not limited to the illustrated transmitter and receiver design. Indeed, the apparatus and methods disclosed herein may be used with fingerprint sensors using a small number of transmitting elements and a relatively large number of receiving elements, a large number of transmitting elements and a relatively small number of receiving element, or a roughly equal number of transmitting and receiving elements.
As shown in FIG. 1, the fingerprint sensing area 12 (including the transmitting and receiving elements 16, 20) may be physically decoupled from the fingerprint sensing circuit 18. Positioning the sensing elements 16, 20 off the silicon die may improve the reliability of the fingerprint sensor 10 by reducing the sensor's susceptibility to electrostatic discharge, wear, and breakage. This may also allow the cost of the sensor 10 to be reduced over time by following a traditional die-shrink roadmap. This configuration provides a distinct advantage over direct contact sensors (sensors that are integrated onto the silicon die) which cannot be shrunk to less than the width of an industry standard fingerprint. Nevertheless, the apparatus and methods disclosed herein are applicable to fingerprint sensors with sensing elements that are located either on or off the silicon die.
Referring generally to FIGS. 2 and 3, in selected embodiments, the transmitting and receiving elements 16, 20 discussed above may be adhered to a non-conductive substrate 30. For example, the substrate 30 may be constructed of a flexible polyimide material marketed under the trade name Kapton® and with a thickness of between about 25 and 100 μm. The Kapton® polymer may allow the fingerprint sensor 10 to be applied to products such as touchpads and molded plastics having a variety of shapes and contours while providing exceptional durability and reliability.
In selected embodiments, a user's finger may be swiped across the side of the substrate 30 opposite the transmitting and receiving elements 16, 20. Thus, the substrate 30 may electrically and mechanically isolates a user's finger from the transmitting element 16 and receiving element 20, thereby providing some degree of protection from electrostatic discharge (ESD) and mechanical abrasion.
The capacitive coupling between the transmitting element 16 and the receiving element 20 may change depending on whether a fingerprint ridge or valley is immediately over the gap 22. This is because the dielectric constant of a finger is typically ten to twenty times greater than the dielectric constant of air. The dielectric constant of the ridges of a finger may vary significantly from finger to finger and person to person, explaining the significant range of dielectric constants. Because a fingerprint ridge has a dielectric constant that differs significantly from that of air, the capacitive coupling between the transmitting element 16 and receiving element 20 may vary significantly depending on whether a ridge or valley is present over the sensor gap 22.
For example, referring to FIG. 2, when a fingerprint ridge 32 is over the gap 22, the capacitive coupling between the transmitting element 16 and receiving element 20 may be increased such that the probing signal emitted by the transmitting element 16 is detected at the receiving element 20 as a stronger response signal. It follows that a stronger response signal at the receiving element 20 indicates the presence of a ridge 32 over the gap 22. On the other hand, as shown in FIG. 3, the capacitive coupling between the transmitting element 16 and receiving element 20 may decrease when a valley is present over the gap 22. Thus, a weaker response signal at the receiving element 20 may indicate that a valley 34 is over the gap 22. By measuring the magnitude of the response signal at the receiving element 20, ridges and valleys may be detected as a user swipes his or her finger across the sensing area 12, allowing a fingerprint image to be generated.
Referring to FIG. 4, in certain embodiments, a fingerprint sensing circuit 18 useable with an apparatus and method in accordance with the invention may include a transmitter clock 40 configured to generate an oscillating signal, such as an oscillating square-wave signal. Scanning logic 42 may be used to sequentially route the oscillating signal to buffer amplifiers 46, one after the other, using switches 44. The buffer amplifiers 46 may amplify the oscillating signal to generate the probing signal. As shown, the buffer amplifiers 46 may sequentially burst the probing signal 48 to each of the transmitting elements 16, one after the other. A response signal, generated in response to the probing signal 48, may be sensed at the receiving element 20 and may be routed to a variable-gain amplifier 50 to amplify the response signal. The amplified response signal may then be passed to a band pass filter 52 centered at the frequency of the transmitter clock 40.
The output from the band pass filter 52 may then be supplied to an envelope detector 54, which may detect the envelope of the response signal. This envelope may provide a baseband signal, the amplitude of which may vary depending on whether a ridge or valley is over the sensor gap 22. The baseband signal may be passed to a low pass filter 56 to remove unwanted higher frequencies. The variable-gain ampifier 50, band pass filter 52, envelope detector 54, low pass filter 56, as well as other analog components may be collectively referred to as an analog front end 57.
The output from the low pass filter 56 may be passed to an analog-to-digital converter (ADC) 58, which may convert the analog output to a digital value. The ADC 58 may have, for example, a resolution of 8 to 12 bits and may be capable of resolving the output of the low pass filter 56 to 256 to 4096 values. The magnitude of the digital value may be proportional to the signal strength measured at the receiving element 20. Likewise, as explained above, the signal strength may be related to the capacitive coupling between the transmitting element 16 and receiving element 20, which may depend on the RF impedance of the feature over the sensor gap 22.
The resulting digital value may be passed to a CPU 60 or other digital components, which may eventually pass digital fingerprint data to a host system 62. The host system 62, in selected embodiments, may process the fingerprint data using various matching algorithms in order to authenticate a user's fingerprint.
In addition to processing the digital data, the CPU 60 may control the gain of the variable-gain amplifier 50 using a digital-to-analog converter (DAC) 64. The gain may be adjusted to provide a desired output power or amplitude in the presence of variable sensing conditions. For example, in selected embodiments, the gain of the variable-gain amplifier 50 may be adjusted to compensate for variations in the impedance of different fingers. In selected embodiments, the CPU 60 may also control the operation of the scanning logic 42.
Referring to FIG. 5, a timing diagram is illustrated to describe various signals that may occur within the fingerprint sensing circuit 18 of FIG. 4. In selected embodiments, the fingerprint sensing circuit 18 may include a phase-locked loop (PLL) output 70 which may provide the master reference signal to drive many of the components in the fingerprint sensing circuit 18. In certain embodiments, the PLL output 70 may be derived from a lower frequency crystal input that is multiplied to generate the PLL output 70. As will be explained in more detail in association with FIG. 6, the PLL output 70 may be divided down to provide clocking signals to different components in the fingerprint sensing circuit 18.
A pixel clock signal 72 may control the amount of time each transmitting element 16 is transmitting the probing signal 48. For example, a rising edge 74a of the pixel clock signal 72 may cause a first transmitting element 16 in the array to begin emitting the probing signal 48. The next rising edge 74b may cause the first transmitting element 16 to cease transmitting and cause the next transmitting element 16 to begin emitting the probing signal 48. This process may continue for each transmitting element 16 in the array to generate a “line” of fingerprint data. Successive lines may be scanned in this manner and the lines may be combined to generate a fingerprint image as previously discussed. The interval between the rising edges 74a, 74b may be referred to as the “pixel period” 76.
Analog response signals 78a, 78b may represent the signal that is output from the analog front end 57 described in FIG. 4. A first analog response signal 78a may reflect the response that occurs when a ridge transitions to a valley over the sensing gap 22. A second analog response signal 78b may reflect the response that occurs when a valley transitions to a ridge over the sensing gap 22.
As shown, the first analog response signal 78a may initially have a magnitude 80a when a ridge is placed over the sensor gap 22 (reflecting greater capacitive coupling between the transmitting and receiving elements 16, 20). When the ridge is removed from the sensor gap 22 (i.e., a valley is placed over the sensor gap 22), the magnitude 82a of the signal 78a may become smaller (reflecting reduced capacitive coupling between the transmitting and receiving elements 16, 20). The gradual transition between the peak magnitudes 80a, 82a may be the result of time constants of analog components in the fingerprint sensing circuit 18. That is, due to the frequency response of analog components, such as band pass and low pass filters 52, 56, some time may be needed for the signal 78a to settle at a new level. After the signal 78a has transitioned from one peak value 80a to the other 82a, the signal 78a may reach a substantially steady state level 84a. Sampling is ideally conducted during this period.
Similarly, the second analog response signal 78b may initially have a magnitude 80b when a valley is placed over the sensor gap 22 (reflecting lower capacitive coupling between the transmitting and receiving elements 16, 20). When a ridge is placed over the sensor gap 22, the magnitude 82b of the signal 78b may become larger (reflecting increased capacitive coupling between the transmitting and receiving elements 16, 20). The signal may transition between the peak magnitudes 80b, 82b, reflecting time constants in the fingerprint sensing circuit 18, after which the signal 78b may reach a substantially steady state level 84b. 
An ADC clock signal 86 may be used to clock the ADC 58 in the fingerprint sensing circuit 18. In this example, the ADC 58 uses three clock cycles 88 to sample the voltage of the analog response signal 78a, 78b and eight clock cycles to convert the sample to a digital value. In certain embodiments, the ADC 58 may open a gate and store the sample on a capacitor for about three clock cycles. After the three clock cycles have passed, the ADC 58 may close the gate and convert the sample to a digital value. In certain embodiments, the ADC 58 may use one clock cycle to generate each bit of the digital value. The number of clock cycles used for sampling and converting is presented only by way of example and is not intended to be limiting. As shown, sampling may be performed during the steady state period 84a, 84b (i.e., the peak response period) to generate the most accurate sample.
In selected embodiments, an ADC convert pulse signal 92 may be used to instruct the ADC 58 to begin sampling. For example, when an edge, such as a rising edge 94, of the ADC convert pulse signal 92 is detected by the ADC 58, the ADC 58 may begin to sample the analog response signal 78a, 78b and convert the sample to a digital value.
A CPU clock signal 96 may be used to clock the CPU 60 and other digital components in the fingerprint sensing circuit 18. As shown, in selected embodiments, the clock signal 96 may be configured to operate during an initial portion of the pixel period 76 (the period of minimum noise sensitivity) but may be shut down during the remainder of the pixel period 76. This may be performed to reduce system noise (e.g., switching noise produced by the CPU 60 or other digital components when they change state) in the fingerprint sensing circuit 18 when the ADC 58 is sampling the analog response signal 78. This will ideally reduce the amount of noise that is reflected in the digital value, allowing a clearer fingerprint image to be generated.
The signal 98 may represent noise that is generated in the fingerprint sensing circuit 18 when the CPU clock 96 is turned on. By shutting off the CPU clock 96 at an appropriate time, noise may be allowed to settle out of the system prior to sampling the analog response signal 78. Thus, in selected embodiments, the CPU clock 96 may be shut down some interval prior to the time the ADC 58 samples the analog response signal 78 in order to allow time for noise to settle out of the system.
In alternative embodiments, instead of shutting down the CPU clock 96 while sampling the analog response signal 78, other actions may be taken to reduce noise in the fingerprint sensing circuit 18. For example, the CPU 60 itself may be disabled or shut down when sampling the analog response signal 78, and then re-enabled or re-started once sampling is complete.
Referring to FIG. 6, in order to generate the signals described in FIG. 5, in selected embodiments, an oscillator 100, such as a crystal oscillator 100, may be used to provide a stable clock signal 102. In this example, the oscillator produces a 24 MHz output signal. A frequency multiplier 104 or phase-locked loop circuit 104 may multiply the oscillator frequency to generate the PLL output 70, as previously discussed. In this example, the PLL output 70 is a 144 MHz signal. This output 70 may provide a master reference signal to drive many of the components in the fingerprint sensing circuit 18.
As mentioned, the PLL output 70 may be divided to generate clocking signals to clock different components in the fingerprint sensing circuit 18. For example, in selected embodiments, a frequency divider 106 may divide the PLL output frequency 70 to generate a probing signal frequency 108, in this example 18 MHz. This signal 108 may be divided further to generate the pixel clock signal 72, in this example a 1 MHz signal.
Similarly, the PLL output 70 may be divided to provide clock signals 96, 86 for the CPU 60 and ADC 58, respectively. In this example, a divider 112 may divide the PLL output 70 to generate a CPU clock signal 96, in this example a 48 MHz signal 96. Similarly, a divider 114 may divide the PLL output 70 to generate an ADC clock signal 86, in this example a 12 MHz signal 86.
In selected embodiments, the CPU clock 112 and ADC clock 114 may receive input from the pixel clock 72. For example, a rising or falling edge of the pixel clock 72 may cause the CPU clock 112 and ADC clock 114 to begin outputting the CPU clock signal 96 and ADC clock signal 86, respectively. In selected embodiments, the CPU clock 112 and ADC clock 114 are programmable with respect to how many clock pulses to output and a delay before outputting the clock pulses. Thus, when the CPU clock 112 and ADC clock 114 receive a rising or falling edge from the pixel clock 110, they may output a specified number of clock pulses starting at a specified time as determined by the delay. In this way, the CPU clock 96 may be shut down during selected portions of the pixel period 76, and more particularly while the ADC 58 is sampling the analog response signal 78.
It should be recognized that the components and frequencies illustrated in FIG. 6 simply represent one embodiment of an apparatus and method for implementing the invention as discussed herein. Thus, the illustrated components and frequencies are presented only by way of example and are not intended to be limiting. Similarly, the signals illustrated in FIG. 5 are presented only by way of example and are not intended to be limiting.
Similarly, apparatus and methods in accordance with the invention are applicable to a wide variety of fingerprint sensing technologies are not limited to the fingerprint sensing technology disclosed herein. Indeed, the apparatus and methods may be applicable to a wide variety of capacitive, optical, ultrasonic, and other fingerprint sensing technologies where noise may be a concern. Each of these technologies may benefit from reduced noise by turning off clocks to various components during sampling, or by shutting down or disabling selected components during sampling. Thus, apparatus and methods are not limited to any specific type of fingerprint sensor but may be applicable to a wide variety of fingerprint sensing technologies.
The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described examples are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.

## Claims

1. An apparatus for reducing noise in fingerprint sensing circuits, the apparatus comprising:
a fingerprint sensing area onto which a user can apply a fingerprint;
an analog front end coupled to the fingerprint sensing area to generate an analog response signal;
an analog-to-digital converter (ADC) to sample the analog response signal and convert the sample to a digital value;
a digital device to receive the digital value from the ADC, the digital device further configured to shut down while the ADC is sampling the analog response signal;
wherein the digital device is shut down by turning off a clock signal to the digital device; and
wherein the digital device is further configured to re-start while the ADC is converting a sample to a digital value.

2. The apparatus of claim 1, wherein the digital device is shut down by disabling the digital device.

3. The apparatus of claim 1, wherein the digital device is shut down by turning off power to the digital device.

4. The apparatus of claim 1, wherein the digital device is configured to shut down some interval prior to the time the ADC samples the analog response signal.

5. The apparatus of claim 4, wherein the interval is selected to allow the analog response signal to attain substantially steady state prior to sampling the analog response signal.

6. The apparatus of claim 1, wherein the digital device is configured to re-start after the ADC has finished sampling the analog response signal.

7. The apparatus of claim 1, wherein the digital device is a processor.

8. The apparatus of claim 7, wherein the digital device is a CPU.

9. A method for reducing noise in fingerprint sensing circuits, the method comprising:
providing a fingerprint sensing area onto which a user can apply a fingerprint;
generating an analog response signal associated with finger activity over the fingerprint sensing area;
sampling the analog response signal;
converting the sample to a digital value;
receiving the digital value at a digital device;
shutting down the digital device while sampling the analog response signal;
wherein the digital device is shut down by turning off a clock signal to the digital device; and
turning on the clock signal to the digital device while the ADC is converting a sample to a digital value.

10. The method of claim 9, wherein shutting down the digital device comprises disabling the digital device.

11. The method of claim 9, wherein shutting down the digital device comprises turning off power to the digital device.

12. The method of claim 9, wherein shutting down the digital device comprises shutting down the digital device some interval prior to sampling the analog response signal.

13. The method of claim 12, further comprising selecting the interval to allow the analog response signal to attain substantially steady state prior to sampling the analog response signal.

14. The method of claim 9, further comprising re-starting the digital device after sampling the analog response signal.

15. The method of claim 9, wherein the digital device is a processor.

16. A method for reducing noise in fingerprint sensing circuits, the method comprising:
providing a fingerprint sensing area onto which a user can apply a fingerprint;
generating an analog response signal associated with a finger being applied onto the fingerprint sensing area;
sampling the analog response signal to form an analog sample signal;
converting the analog sample signal to a digital value;
receiving the digital value at a CPU;
turning off a clock signal to the CPU while sampling the analog response signal; and

turning on the clock signal to a digital device while the ADC is converting a sample to a digital value.