WO2017143863A1 - 用于指纹传感器的信号处理电路及方法 - Google Patents

用于指纹传感器的信号处理电路及方法 Download PDF

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WO2017143863A1
WO2017143863A1 PCT/CN2016/113375 CN2016113375W WO2017143863A1 WO 2017143863 A1 WO2017143863 A1 WO 2017143863A1 CN 2016113375 W CN2016113375 W CN 2016113375W WO 2017143863 A1 WO2017143863 A1 WO 2017143863A1
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signal
output signal
analog output
processing circuit
amplifier
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PCT/CN2016/113375
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English (en)
French (fr)
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李卓
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北京集创北方科技股份有限公司
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Priority to JP2017561658A priority Critical patent/JP6632163B2/ja
Priority to US15/579,701 priority patent/US10528784B2/en
Publication of WO2017143863A1 publication Critical patent/WO2017143863A1/zh

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    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06V—IMAGE OR VIDEO RECOGNITION OR UNDERSTANDING
    • G06V40/00—Recognition of biometric, human-related or animal-related patterns in image or video data
    • G06V40/10—Human or animal bodies, e.g. vehicle occupants or pedestrians; Body parts, e.g. hands
    • G06V40/12—Fingerprints or palmprints
    • G06V40/13—Sensors therefor
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06V—IMAGE OR VIDEO RECOGNITION OR UNDERSTANDING
    • G06V40/00—Recognition of biometric, human-related or animal-related patterns in image or video data
    • G06V40/10—Human or animal bodies, e.g. vehicle occupants or pedestrians; Body parts, e.g. hands
    • G06V40/12—Fingerprints or palmprints
    • G06V40/13—Sensors therefor
    • G06V40/1306—Sensors therefor non-optical, e.g. ultrasonic or capacitive sensing
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06V—IMAGE OR VIDEO RECOGNITION OR UNDERSTANDING
    • G06V10/00—Arrangements for image or video recognition or understanding
    • G06V10/20—Image preprocessing
    • G06V10/30—Noise filtering
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06V—IMAGE OR VIDEO RECOGNITION OR UNDERSTANDING
    • G06V40/00—Recognition of biometric, human-related or animal-related patterns in image or video data
    • G06V40/10—Human or animal bodies, e.g. vehicle occupants or pedestrians; Body parts, e.g. hands
    • G06V40/12—Fingerprints or palmprints
    • G06V40/1365—Matching; Classification

Definitions

  • the present invention relates to signal processing techniques and, more particularly, to signal processing circuits and methods for fingerprint sensors.
  • the fingerprint sensor is a sensor device for imaging a fingerprint, and has been widely used in an access control system, an attendance system, a mobile terminal such as a mobile phone and a tablet computer for verifying the identity of a user.
  • the fingerprint sensor includes an optical sensor and a capacitive sensor.
  • the optical fingerprint sensor includes an optical scanning element for acquiring an optical image of the fingerprint.
  • the capacitive fingerprint sensor acquires the capacitance characteristics of each local position of the active area of the finger, thereby forming a fingerprint image by using the amount of change in the capacitance.
  • the signal acquisition and processing circuit of the optical fingerprint sensor is separated from the finger, so it has strong antistatic ability and long service life, but is easily affected by the dust in the environment and the cleanliness of the finger. Capacitive fingerprint sensors are compact in size and high in resolution, but have poor antistatic properties. Therefore, the use of capacitive fingerprint sensing in mobile terminals is more common.
  • FIG. 1 shows a schematic diagram of the operation of a capacitive fingerprint sensor.
  • the capacitive fingerprint sensor 110 includes an array of a plurality of sensing electrodes.
  • the capacitive fingerprint sensor 110 is touched at the finger 120, and a first capacitance Cf is formed between the finger and the sensing electrode. Since the fingerprint ridge of the finger 120 and the first capacitance Cf formed by the fingerprint valley are different, the signal processing circuit quantifies the difference between the fingerprint ridge and the fingerprint valley by converting the capacitance of the first capacitance Cf into an electrical signal.
  • the capacitance of the contact area is obtained by using a plurality of sensing electrodes, that is, a fingerprint image can be formed.
  • the system noise is also introduced into the signal processing circuit, so that the fingerprint image cannot be collected normally, which causes the recognition rate to be lowered or even not used normally.
  • the human body ground potential VGND1 of the finger and the reference ground potential VGND2 of the sensor are not the same.
  • the touch of the finger is equivalent to introducing a ripple in the reference ground potential VGND2 of the sensor, thereby being received by the sensing electrode and transmitted to the signal processing circuit.
  • a bias circuit is employed to adjust the operating frequency of the amplifier, and a selected operating frequency is employed to suppress common mode noise introduced by finger touch.
  • a signal processing circuit for a fingerprint sensor including an inductive element that generates an inductive signal in response to a finger touch, the signal processing circuit including: a bias circuit for generating an offset a signal for generating an analog output signal based on the sensed signal; and an analog to digital converter for converting the analog output signal to a digital output signal, wherein the signal processing circuit operates in a noise detection mode and an image An acquisition mode in which the amplifier amplifies a superimposed signal of the sensing signal and the bias signal to generate a first analog output signal, and adjusts the signal according to the first analog output signal An operating frequency of the amplifier, in the image acquisition mode, the amplifier amplifies the sensing signal to generate a second analog output signal, and generates a fingerprint image based on the second analog output signal.
  • the inductive element is a sensing electrode.
  • the signal processing circuit further includes a first AC signal source, wherein the first AC signal source is coupled to the sensing electrode to provide a first AC signal to the sensing electrode as a driving signal.
  • the bias circuit comprises a first switch, a second capacitor and a second AC signal source connected in series between the input end of the amplifier and the ground, and the second AC signal source is directed to the second
  • the capacitor provides a second AC signal in which the first switch is closed, in the image acquisition mode, the first switch is open.
  • said biasing circuit comprises a second capacitor, a first switch and a second AC signal source, connected in series between the input of said amplifier and ground, and said second capacitor and said first a second switch connected between the intermediate node of the switch and the ground, in the noise detecting mode, the first switch is closed and the second switch is turned off, and the second alternating current signal source is opposite to the second capacitor
  • a second AC signal is provided, in the image acquisition mode, the first switch is open and the second switch is closed such that the second capacitor is grounded via the second switch.
  • the signal processing circuit is further operative in an initialization mode, wherein in the initialization mode, the amplifier amplifies the bias signal to generate a third analog output signal, and according to the third analog output The signal adjusts the bias circuit such that the third analog output signal has an initial value.
  • the amplitude of the second alternating current signal is set by detecting an output of the analog to digital converter when the finger is not touched, such that the third analog output signal has the initial value .
  • the fingerprint sensor comprises a plurality of sensing elements, and the amplitudes of the second alternating current signals are respectively set for the plurality of sensing elements.
  • the third analog output signal has a first initial value and a second initial value, respectively.
  • an operating frequency of the amplifier is adjusted by detecting an output of the analog-to-digital converter when a finger is touched, thereby setting an operating frequency in the image capturing mode.
  • the amplifier has an inverting input, a non-inverting input and an output, the inductive element and the biasing circuit being coupled to the inverting input, the non-inverting input receiving a reference voltage.
  • a signal processing method for a fingerprint sensor comprising an inductive element that generates an inductive signal in response to a finger touch
  • the method comprising: the sensing signal and the bias signal Superimposing the signal to amplify to generate a first analog output signal; adjusting an operating frequency according to the first analog output signal; amplifying the sensing signal to generate a second analog output signal; and generating a fingerprint according to the second analog output signal
  • the method further comprises amplifying said bias signal to produce a third analog output signal, and converting said third analog output signal to a digital output signal, wherein said detecting said by detecting when said finger is not touched And a digital output signal, the size of the superimposed signal is set such that the third analog output signal has an initial value.
  • the fingerprint sensor comprises a plurality of sensing elements, and the magnitude of the bias signal is respectively set for the plurality of sensing elements.
  • the third analog output signal has a first initial value and a second initial value, respectively.
  • the method further comprises converting said first analog output signal to a digital output signal, wherein said digital output signal is detected by a finger touch to adjust said operating frequency to set said image acquisition mode The working frequency.
  • the bias circuit is used to superimpose the bias signal on the sensing signal, so that the operating frequency of the amplifier can be adjusted so that the analog output after the signal processing circuit and the digital value output by the analog-to-digital converter are quantized. Fluctuating within a reasonable range.
  • the signal processing circuit reduces the influence of the common mode noise on the sensing signal by selecting the operating frequency of the amplifier, thereby suppressing the common mode noise introduced by the finger touch. sound.
  • Figure 1 shows a schematic diagram of the principle of a capacitive fingerprint sensor
  • Figure 2 is a waveform diagram showing the ground potential of the human body and the reference ground potential of the sensor
  • Figure 3 shows a schematic circuit diagram of a signal processing circuit according to the prior art
  • Figure 4 shows a schematic circuit diagram of a signal processing circuit in accordance with a first embodiment of the present invention
  • Figure 5 shows a schematic circuit diagram of a signal processing circuit in accordance with a second embodiment of the present invention
  • FIG. 6 is a flow chart showing a signal processing method according to a third embodiment of the present invention.
  • Fig. 7 shows a distribution map of initialization signals of a plurality of sensing elements in a signal processing method.
  • the present invention can be embodied in various forms, some of which are described below.
  • Fig. 3 shows a schematic circuit diagram of a signal processing circuit according to the prior art.
  • the signal processing circuit 200 includes a first capacitor Cf associated with the inductive component, a first AC signal source 210, a reference voltage source 220, an amplifier 230, and an analog to digital converter 240.
  • the first end of the first capacitor Cf is coupled to the inverting input of the amplifier 230, and the second end is coupled to the first AC signal source 210.
  • Reference voltage source 220 is coupled to the non-inverting input of amplifier 230.
  • the output of amplifier 230 is coupled to the input of analog to digital converter 240.
  • a first capacitance Cf is formed between the finger and the sensing electrode of the sensor.
  • the fingerprint ridge of the finger and the first capacitance Cf formed by the fingerprint valley are different.
  • the sensing electrode of the first capacitor Cf can be formed using an integrated circuit process using a metal layer or a polysilicon layer.
  • the first AC signal source 210 generates an AC signal. For example, one or a combination of a sine wave, a square wave, and a triangular wave.
  • the sensing element is driven by the first AC signal source 210.
  • Zoom in The inverting input and the non-inverting input of the converter 230 receive the sense signal and the reference voltage Vref, respectively, and compare the two to provide an amplified signal at the output.
  • Amplifier 230 converts the change in first capacitance Cf into an induced signal to quantify the difference between the fingerprint ridge and the fingerprint valley.
  • Analog to digital converter 240 converts the amplified signal into a digital signal.
  • the finger touch sensor surface is equivalent to a noise source that is connected to the signal detection circuit through the Cf capacitor, thereby affecting the signal detection and the output of the post-processing circuit.
  • System noise can be divided into differential mode noise and common mode noise. Differential mode noise can be suppressed by increasing the power supply rejection ratio of the system. Common mode noise means that the same noise is present on both the system's power supply and the reference ground. This kind of noise has a more serious impact on system performance when the human body and the system are not in common. Common mode noise is generally a narrow bandwidth signal with certain spectral characteristics. Since it is introduced into the system with a finger touch, it cannot be distinguished from the normally received sensing signal.
  • the fingerprint recognition sensor can suppress the noise of a certain amplitude and frequency by means of digital/analog filtering, but the fingerprint recognition sensor cannot capture the fingerprint image correctly when there is a specific frequency or a high amplitude interference signal or noise exceeding the processing capability of the sensor. .
  • Fig. 4 shows a schematic circuit diagram of a signal processing circuit in accordance with a first embodiment of the present invention.
  • the signal processing circuit 300 includes a first capacitor Cf associated with the inductive component, a first AC signal source 210, a reference voltage source 220, an amplifier 230, and an analog to digital converter 240.
  • the first end of the first capacitor Cf is coupled to the inverting input of the amplifier 230, and the second end is coupled to the first AC signal source 210.
  • Reference voltage source 220 is coupled to the non-inverting input of amplifier 230.
  • the output of amplifier 230 is coupled to the input of analog to digital converter 240.
  • the signal processing circuit 300 further includes a bias circuit 310.
  • the bias circuit 310 includes a first switch S1, a second capacitor Cb, and a second AC signal source 250 that are sequentially connected in series between the inverting input of the amplifier 230 and ground.
  • the second AC signal source 250 generates an AC signal Vdrv. For example, one of a sine wave, a square wave, and a triangular wave.
  • the second AC signal source 250 generates an AC signal Vdrv of a predetermined amplitude
  • the second capacitor Cb has a predetermined capacitance value, thereby generating a predetermined amount of charge on the second capacitor Cb as a bias signal.
  • a first capacitance Cf is formed between the finger and the sensing electrode of the sensor.
  • Amplifier 230 converts the change in first capacitance Cf into an induced signal to quantify the difference between the fingerprint ridge and the fingerprint valley.
  • the signal processing circuit 300 can operate in a noise detection mode and an image acquisition mode.
  • the first switch S1 is closed, such that the amplifier 230 amplifies the superimposed signals of the induced signal and the bias signal to generate a first analog output signal, and is adjusted according to the first analog output signal.
  • the operating frequency of amplifier 230 is adjusted according to the first analog output signal.
  • the first switch S1 is turned off, such that the amplifier 230 amplifies the sensed signal to generate a second analog output signal, and generates a fingerprint image based on the second analog output signal.
  • the bias circuit 310 is operative to adjust the operating frequency of the amplifier 230 in the noise detection mode to suppress the effect of noise on the fingerprint image in the image acquisition mode.
  • amplifier 230 acts as a discrete time domain charge transfer amplifier for obtaining the sensed signal.
  • the sensing signals may be obtained using various signal processing circuits in a continuous time domain.
  • signal processing circuit 300 can also operate in an initialization mode.
  • the amplifier 230 amplifies the bias signal to generate a third analog output signal, and adjusts the bias circuit 310 according to the third analog output signal such that the third analog output signal has an initial value.
  • the first switch S1 is closed, and by detecting the output of the analog-to-digital converter when the finger is not touched, the amplitude of the alternating current signal Vdrv can be set such that the third analog output signal has The initial value.
  • Fig. 5 shows a schematic circuit diagram of a signal processing circuit in accordance with a second embodiment of the present invention.
  • the signal processing circuit 400 includes a first capacitor Cf associated with the inductive component, a first AC signal source 210, a reference voltage source 220, an amplifier 230, and an analog to digital converter 240.
  • the first end of the first capacitor Cf is coupled to the inverting input of the amplifier 230, and the second end is coupled to the first AC signal source 210.
  • Reference voltage source 220 is coupled to the non-inverting input of amplifier 230.
  • the output of amplifier 230 is coupled to the input of analog to digital converter 240.
  • the signal processing circuit 400 includes a bias circuit 410.
  • the bias circuit 410 includes a second capacitor Cb, a first switch S1 and a second AC signal source 250, which are sequentially connected in series between the inverting input terminal of the amplifier 230 and the ground, and the second capacitor Cb and the first switch S1.
  • the second switch S2 connected between the intermediate node and the ground.
  • the signal processing circuit 400 can operate in a noise detection mode and an image acquisition mode.
  • the first switch S1 is closed and the second switch S2 is turned off, and the second capacitor Cb is connected to the second AC signal source 250 via the first switch S1, so that the amplifier 230 superimposes the sensing signal and the bias signal.
  • Amplification is performed to generate a first analog output signal, and an operating frequency of the amplifier 230 is adjusted in accordance with the first analog output signal.
  • the first switch S1 is closed and the second switch S2 is closed, and the second capacitor Cb is grounded via the second switch S2 such that the amplifier 230 amplifies the induced signal to generate a second analog output signal, and according to the The second analog output signal generates a fingerprint image. Since the amount of charge on the second capacitor Cb remains constant, Therefore, the fingerprint image is not affected when the fingerprint image is acquired.
  • the bias circuit 410 is operative to adjust the operating frequency of the amplifier 230 in the noise detection mode to suppress the effect of noise on the fingerprint image in the image acquisition mode.
  • signal processing circuit 400 can also operate in an initialization mode in which amplifier 230 amplifies the bias signal to produce a third analog output signal, and in accordance with the The three analog output signals adjust the bias circuit 410 such that the third analog output signal has an initial value.
  • the first switch S1 is closed and the second switch S2 is turned off, and by detecting the output of the analog-to-digital converter when the finger is not touched, the amplitude of the alternating current signal Vdrv can be set such that the The third analog output signal has the initial value.
  • Fig. 6 is a flow chart showing a signal processing method according to a third embodiment of the present invention. The various processes of the method will be described by taking the signal processing circuit 300 shown in FIG. 4 as an example.
  • step S01 an initial value is set.
  • the first capacitor Cf is disconnected from the amplifier 230.
  • the first switch S1 is closed such that the bias circuit 310 provides a bias signal to the inverting input of the amplifier 230.
  • This step is performed by setting parameters of the circuit elements in the bias circuit 310, for example, setting the capacitance value of the second capacitor Cb and the amplitude of the alternating current signal Vdrv of the second alternating current signal source 250, and the amplified signal of the amplifier 230 is a predetermined voltage, so that the mode
  • the digital output of the digital converter 240 is approximately one-half of the full scale digital value. Taking an 8-bit analog-to-digital converter as an example, the output digital code range is (0-255).
  • step S01 in the case where only the second capacitor Cb is connected to the inverting input of the amplifier 230, the output digital value of the analog to digital converter 240 is near 128. Since the output digital value depends on the ratio between the internal components of the sensor, the output digital value can be controlled to fluctuate within a relatively accurate range based on the integrated circuit manufacturing process.
  • step S02 the finger touch is on the fingerprint sensor, and the first capacitor Cf is connected to the amplifier 230.
  • the signal of the inverting input terminal of the amplifier 230 is the sensing signal of the first capacitor Cf and the bias signal generated by the bias circuit 310.
  • Superimposed signal as shown in equation (1):
  • Vout represents the analog output voltage of the amplifier
  • A represents the amplification factor of the amplifier
  • Cf and Cb represent the capacitance values of the first capacitor and the second capacitor, respectively
  • Vn and Vdrv represent the first AC signal source and the second AC signal source, respectively.
  • the amplitude, f is the operating frequency of the amplifier, and f 0 is the frequency of the second AC signal source.
  • the signal processing circuit can be a continuous time mode of operation or a discrete time mode of operation in the form of a switched capacitor.
  • the operating frequency of the Vdrv signal is the same as the frequency of the control signal of the detection circuit or has Strong correlation (integer multiples of each other).
  • the operating frequency of the amplifier is adjusted according to noise detection to attenuate or even eliminate the influence of common mode noise.
  • the operating frequency is, for example, the operating frequency of amplifier 230.
  • the noise directly affects the detection result, and the output digital value quantized by the analog-to-digital converter deviates from the ideal interval. What is more, the excessive common mode noise directly causes the amplifier 230 and the analog-to-digital converter 240 to be saturated, so that the time for the circuit to re-establish the working state becomes longer, which affects the subsequent detection results.
  • the above process of adjusting the operating frequency includes setting the operating frequency in step S03, detecting the output digital value of the analog-to-digital converter 240 in step S04, and determining whether the output digital value of the analog-to-digital converter 240 deviates from the ideal range in step S05.
  • steps S03 through S05 are repeated.
  • the set operating frequency can attenuate or even eliminate the effects of common mode noise and thus can be used as the operating frequency in the image acquisition mode.
  • the energy of common mode noise is generally concentrated in a narrow bandwidth centered on a specific frequency, so the system finds the frequency with lower common mode noise by the frequency search method through the above noise detection method, and uses this frequency as the operating frequency of the circuit. , thereby weakening or even eliminating the effects of common mode noise.
  • the signal processing circuit proposed by the present invention can detect whether there is interference affecting image quality under the current frequency setting by detecting the output digital value of the analog-to-digital converter 240. Further, by detecting the output digital value of the analog to digital converter, the operating frequency of the amplifier is adjusted such that the operating frequency in the image acquisition mode is an ideal value.
  • a bias circuit is used to adjust the operating frequency of the amplifier 230, and a fingerprint image is acquired using the selected operating frequency, thereby suppressing the common mode noise introduced by the finger touch.
  • the fingerprint sensor may comprise a plurality of inductive elements for which the amplitude of the alternating signal Vdrv may be set to be the same.
  • the amplitude of the alternating signal Vdrv may be separately set for better detection.
  • the AC signal Vdrv in all the sensing element detecting circuits in the fingerprint sensor can be set to two amplitudes, so that the digital output values of the plurality of sensing elements are corresponding to the two numbers. value.
  • Fig. 7 shows a distribution map of initialization signals of a plurality of sensing elements in a signal processing method. Taking an 8-bit analog-to-digital converter as an example, the average value of the digital output values of the plurality of sensing elements is about 128.
  • the plurality of sensing elements are arranged in an array and can be divided into a first set of sensing elements and a second set of sensing elements in a checkerboard pattern.
  • the average value of the digital output values of the first group of sensing elements is 198, and the average value of the digital output values of the second group of sensing elements is 58. Based on the above excitation mode, the average value of all the sensing elements of the image is outputted by the statistical fingerprint, and/or the mean values of the first group of sensing elements and the second group of sensing elements are set, and a reasonable initial value is set to determine the noise intensity.
  • a process of adjusting the operating frequency of the amplifier is performed after the finger touches the fingerprint sensor and before the fingerprint image is acquired.
  • a part of the sensor central area
  • the operating frequency of the amplifier can be adjusted by detecting the output digital value of the analog-to-digital converter. Since only a portion of the sensing electrodes of the fingerprint sensor are driven and detected, the time required to adjust the operating frequency is saved.
  • the system may divide the sensor of the noise detection area into N regions.
  • the frequencies of the alternating current signals Vn applied to one end of the first capacitor Cf in the detecting circuit in each region are different from each other. Therefore, a single noise detection process can detect noise conditions at N frequencies.
  • the ideal operating frequency of the signal processing circuit can be determined by detecting the output digital values of the analog to digital converters in multiple regions.

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Abstract

一种用于指纹传感器的信号处理电路（300）及方法，所述指纹传感器包括响应手指触摸产生感应信号的感应元件。所述信号处理电路（300）包括：偏置电路（310），用于产生偏置信号；放大器（230），用于根据所述感应信号产生模拟输出信号；以及模数转换器（240），用于将所述模拟输出信号转换成数字输出信号，其中，在噪声检测模式中，所述放大器（230）对所述感应信号和所述偏置信号的叠加信号进行放大以产生第一模拟输出信号，并且根据所述第一模拟输出信号调节所述放大器（230）的工作频率，在图像采集模式中，所述放大器（230）对所述感应信号进行放大以产生第二模拟输出信号，并且根据所述第二模拟输出信号生成指纹图像。

Description

用于指纹传感器的信号处理电路及方法
本申请要求了2016年2月23日提交的、申请号为201610099601.1、发明名称为“用于指纹传感器的信号处理电路及方法”的中国发明专利申请的优先权，其全部内容通过引用结合在本申请中。
技术领域
本发明涉及信号处理技术，更具体地，涉及用于指纹传感器的信号处理电路及方法。
背景技术
指纹传感器是用于对指纹成像的传感器装置，已经广泛地用于门禁***、考勤***、诸如手机和平板电脑之类的移动终端中，用于验证用户的身份。指纹传感器包含光学传感器和电容传感器。光学指纹传感器包括光学扫描元件，用于获取指纹的光学图像。电容式指纹传感器获取手指有效区域的各个局部位置的电容特性，从而利用电容的变化量形成指纹图像。光学指纹传感器的信号采集和处理电路与手指隔开，因而抗静电能力强，使用寿命长，但容易受到环境中的灰尘和手指的洁净程度的影响。电容式指纹传感器的尺寸紧凑且分辨率高，但抗静电能力差。因而，电容式指纹传感在移动终端中的使用更为普遍。
图1示出电容式指纹传感器的工作原理示意图。电容式指纹传感器110包括多个感应电极组成的阵列。在手指120触摸电容式指纹传感器110，手指和感应电极之间形成第一电容Cf。由于手指120的指纹脊和指纹谷形成的第一电容Cf不相同，因此，信号处理电路通过将第一电容Cf的电容转化为电信号，从而量化指纹脊和指纹谷之间的差异。利用多个感应电极获得接触区域的电容，即可以形成指纹图像。
当手指接触指纹传感器时，也会将***噪声引入信号处理电路，导致无法正常采集到指纹图像，从而引起识别率降低甚至无法正常使用。如图2所示，当手指接触到指纹识别传感器表面时，手指的人体地电位VGND1和传感器的参考地电位VGND2并不相同。手指的触摸相当于将在传感器的参考地电位VGND2中引入波动，从而由感应电极接收并传送至信号处理电路。
因此，期望在用于电容式指纹传感器的信号处理电路中抑制手指触摸引入的噪声。
发明内容
有鉴于此，本发明的目的在于提供一种用于指纹传感器的信号处理电路及方法，其中采用偏置电路调节放大器的工作频率，采用选择的工作频率以抑制手指触摸引入的共模噪声。
根据本发明的一方面，提供一种用于指纹传感器的信号处理电路，所述指纹传感器包括响应手指触摸产生感应信号的感应元件，所述信号处理电路包括：偏置电路，用于产生偏置信号；放大器，用于根据所述感应信号产生模拟输出信号；以及模数转换器，用于将所述模拟输出信号转换成数字输出信号，其中，所述信号处理电路工作于噪声检测模式和图像采集模式，在所述噪声检测模式中，所述放大器对所述感应信号和所述偏置信号的叠加信号进行放大以产生第一模拟输出信号，并且根据所述第一模拟输出信号调节所述放大器的工作频率，在所述图像采集模式中，所述放大器对所述感应信号进行放大以产生第二模拟输出信号，并且根据所述第二模拟输出信号生成指纹图像。
优选地，所述感应元件为感应电极。
优选地，所述信号处理电路还包括第一交流信号源，其中，所述第一交流信号源与所述感应电极相连接，从而向所述感应电极提供第一交流信号作为驱动信号。
优选地，所述偏置电路包括在所述放大器的输入端和地之间依次串联连接的第一开关、第二电容和第二交流信号源，所述第二交流信号源向所述第二电容提供第二交流信号，在所述噪声检测模式中，所述第一开关闭合，在所述图像采集模式中，所述第一开关断开。
优选地，所述偏置电路包括在所述放大器的输入端和地之间依次串联连接的第二电容、第一开关和第二交流信号源、以及在所述第二电容和所述第一开关的中间节点和地之间连接的第二开关，在所述噪声检测模式中，所述第一开关闭合且所述第二开关断开，所述第二交流信号源向所述第二电容提供第二交流信号，在所述图像采集模式中，所述第一开关断开且所述第二开关闭合，使得所述第二电容经由所述第二开关接地。
优选地，所述信号处理电路还工作于初始化模式，其中，在所述初始化模式中，所述放大器对所述偏置信号进行放大以产生第三模拟输出信号，并且根据所述第三模拟输出信号调节所述偏置电路，使得所述第三模拟输出信号具有初始值。
优选地，在所述初始化模式中，通过在手指未触摸时检测所述模数转换器的输出，设置所述第二交流信号的幅值，使得所述第三模拟输出信号具有所述初始值。
优选地，所述指纹传感器包括多个感应元件，针对所述多个感应元件分别设置所述第二交流信号的幅值。
优选地，针对所述多个感应元件中的第一组感应元件和第二组感应元件，所述第三模拟输出信号分别具有第一初始值和第二初始值。
优选地，在所述噪声检测模式中，通过在手指触摸时检测所述模数转换器的输出，调节所述放大器的工作频率，从而设置所述图像采集模式中的工作频率。
优选地，所述放大器具有反相输入端、同相输入端和输出端，所述感应元件和所述偏置电路连接至所述反相输入端，所述同相输入端接收参考电压。
根据本发明的另一方面，提供一种用于指纹传感器的信号处理方法，所述指纹传感器包括响应手指触摸产生感应信号的感应元件，所述方法包括：对所述感应信号和偏置信号的叠加信号进行放大以产生第一模拟输出信号；根据所述第一模拟输出信号调节工作频率；对所述感应信号进行放大以产生第二模拟输出信号；以及根据所述第二模拟输出信号生成指纹图像，其中，在调节工作频率时选择所述工作频率以抑制手指触摸引入的共模噪声。
优选地，所述方法还包括对所述偏置信号进行放大以产生第三模拟输出信号，以及将所述第三模拟输出信号转换成数字输出信号，其中，通过在手指未触摸时检测所述数字输出信号，设置所述叠加信号的大小，使得所述第三模拟输出信号具有初始值。
优选地，所述指纹传感器包括多个感应元件，针对所述多个感应元件分别设置所述偏置信号的大小。
优选地，针对所述多个感应元件中的第一组感应元件和第二组感应元件，所述第三模拟输出信号分别具有第一初始值和第二初始值。
优选地，所述方法还包括将所述第一模拟输出信号转换成数字输出信号，其中，在调节工作频率时，通过在手指触摸时检测所述数字输出信号，以设置所述图像采集模式中的工作频率。
根据本发明实施例的信号处理电路，采用偏置电路在感应信号上叠加偏置信号，因而可以调节放大器的工作频率，使得经过信号处理电路的模拟输出以及模数转换器量化之后输出的数字值在合理的范围内波动。所述信号处理电路通过选择所述放大器的工作频率，，以减小共模噪声对感应信号的影响，从而抑制手指触摸引入的共模噪 声。
附图说明
通过以下参照附图对本发明实施例的描述，本发明的上述以及其他目的、特征和优点将更为清楚，在附图中：
图1示出电容式指纹传感器的原理示意图；
图2示出人体地电位和传感器参考地电位的波形图；
图3示出根据现有技术的信号处理电路的示意性电路图；
图4示出根据本发明第一实施例的信号处理电路的示意性电路图；
图5示出根据本发明第二实施例的信号处理电路的示意性电路图；
图6示出根据本发明第三实施例的信号处理方法的流程图；
图7示出在信号处理方法中多个感应元件的初始化信号的分布图。
具体实施方式
以下将参照附图更详细地描述本发明。在各个附图中，相同的元件采用类似的附图标记来表示。为了清楚起见，附图中的各个部分没有按比例绘制。此外，可能未示出某些公知的部分。
在下文中描述了本发明的许多特定的细节，例如器件的结构、材料、尺寸、处理工艺和技术，以便更清楚地理解本发明。但正如本领域的技术人员能够理解的那样，可以不按照这些特定的细节来实现本发明。
本发明可以各种形式呈现，以下将描述其中一些示例。
图3示出根据现有技术的信号处理电路的示意性电路图。该信号处理电路200包括与感应元件相关的第一电容Cf、第一交流信号源210、参考电压源220、放大器230、模数转换器240。第一电容Cf的第一端连接放大器230的反相输入端，第二端连接至第一交流信号源210。参考电压源220连接至放大器230的同相输入端。放大器230的输出端连接至模数转换器240的输入端。
在手指接触传感器时，手指与传感器的感应电极之间形成第一电容Cf。手指的指纹脊和指纹谷形成的第一电容Cf不相同。该第一电容Cf的感应电极可以利用集成电路工艺使用金属层或者多晶硅层形成。第一交流信号源210产生交流信号。例如为正弦波、方波和三角波中的一种或组合。采用第一交流信号源210驱动感应元件。放大 器230的反相输入端和同相输入端分别接收感应信号和参考电压Vref，并且将二者相比较，在输出端提供放大信号。放大器230将第一电容Cf的变化转化为感应信号，以量化指纹脊和指纹谷之间的差异。模数转换器240将放大信号转变为数字信号。
在手指接触传感器时，不仅产生上述第一电容Cf的变化，还引入***噪声。手指触摸传感器表面相当于一个噪声源通过Cf电容接入信号检测电路，从而对信号检测及后级处理电路的输出产生影响。
***噪声可以分为差模噪声和共模噪声两种。差模噪声可以通过提高***的电源抑制比等方法进行抑制。共模噪声是指***的电源和参考地上同时存在相同的噪声。这种噪声在人体和***不共地时对***性能的影响更为严重。共模噪声一般为具有一定频谱特性的窄带宽信号。由于其随手指触摸引入***，故而无法和正常接收的感应信号进行区分。
指纹识别传感器可以通过数字/模拟滤波等方式对一定幅度和频率的噪声进行抑制，但是当出现超过传感器处理能力的特定频率或者高幅值的干扰信号或噪声时，指纹识别传感器无法正确捕获指纹图像。
图4示出根据本发明第一实施例的信号处理电路的示意性电路图。该信号处理电路300包括与感应元件相关的第一电容Cf、第一交流信号源210、参考电压源220、放大器230、模数转换器240。第一电容Cf的第一端连接放大器230的反相输入端，第二端连接至第一交流信号源210。参考电压源220连接至放大器230的同相输入端。放大器230的输出端连接至模数转换器240的输入端。
与图3所示的现有技术的信号处理电路200不同，根据该实施例的信号处理电路300还包括偏置电路310。该偏置电路310包括依次串联连接在放大器230的反相输入端和地之间的第一开关S1、第二电容Cb、和第二交流信号源250。
第二交流信号源250产生交流信号Vdrv。例如为正弦波、方波和三角波中的一种。在该实施例中，第二交流信号源250产生预定幅值的交流信号Vdrv，第二电容Cb具有预定的电容值，因而在第二电容Cb上产生预定的电荷量作为偏置信号。
在手指接触传感器时，手指与传感器的感应电极之间形成第一电容Cf。放大器230将第一电容Cf的变化转化为感应信号，以量化指纹脊和指纹谷之间的差异。
信号处理电路300可以工作于噪声检测模式和图像采集模式，
在噪声检测模式中，第一开关S1闭合，使得放大器230对感应信号和偏置信号的叠加信号进行放大以产生第一模拟输出信号，并且根据所述第一模拟输出信号调节 放大器230的工作频率。
在图像采集模式中，第一开关S1断开，使得放大器230对所述感应信号进行放大以产生第二模拟输出信号，并且根据所述第二模拟输出信号生成指纹图像。
在该实施例中，偏置电路310用于在噪声检测模式中调节放大器230的工作频率，从而在图像采集模式中抑制噪声对指纹图像的影响。
在该实施例中，放大器230作为离散时间域的电荷转移放大器，用于获得感应信号。在替代的实施例中，可以采用连续时间域的各种信号处理电路获得感应信号。
在优选的实施例中，信号处理电路300还可以工作于初始化模式。在所述初始化模式中，放大器230对所述偏置信号进行放大以产生第三模拟输出信号，并且根据所述第三模拟输出信号调节偏置电路310，使得第三模拟输出信号具有初始值。
例如，在所述初始化模式中，第一开关S1闭合，通过在手指未触摸时检测所述模数转换器的输出，可以设置交流信号Vdrv的幅值，使得所述第三模拟输出信号具有所述初始值。
图5示出根据本发明第二实施例的信号处理电路的示意性电路图。该信号处理电路400包括与感应元件相关的第一电容Cf、第一交流信号源210、参考电压源220、放大器230、模数转换器240。第一电容Cf的第一端连接放大器230的反相输入端，第二端连接至第一交流信号源210。参考电压源220连接至放大器230的同相输入端。放大器230的输出端连接至模数转换器240的输入端。
与图4所示的根据本发明第一实施例的信号处理电路300不同，根据本发明第二实施例的信号处理电路400包括偏置电路410。该偏置电路410包括依次串联连接在放大器230的反相输入端和地之间的第二电容Cb、第一开关S1和第二交流信号源250，以及在第二电容Cb和第一开关S1的中间节点和地之间连接的第二开关S2。
信号处理电路400可以工作于噪声检测模式和图像采集模式，
在噪声检测模式中，第一开关S1闭合且第二开关S2断开，第二电容Cb经由第一开关S1与第二交流信号源250连接，使得放大器230对感应信号和偏置信号的叠加信号进行放大以产生第一模拟输出信号，并且根据所述第一模拟输出信号调节放大器230的工作频率。
在图像采集模式中，第一开关S1闭合且第二开关S2闭合，第二电容Cb经由第二开关S2接地，使得放大器230对感应信号进行放大以产生第二模拟输出信号，并且根据所述第二模拟输出信号生成指纹图像。由于第二电容Cb上的电荷量保持恒定， 因此在采集指纹图像时不会对指纹图像产生影响。
在该实施例中，偏置电路410用于在噪声检测模式中调节放大器230的工作频率，从而在图像采集模式中抑制噪声对指纹图像的影响。
在优选的实施例中，信号处理电路400还可以工作于初始化模式，其中，在所述初始化模式中，放大器230对所述偏置信号进行放大以产生第三模拟输出信号，并且根据所述第三模拟输出信号调节偏置电路410，使得第三模拟输出信号具有初始值。
例如，在所述初始化模式中，第一开关S1闭合且第二开关S2断开，通过在手指未触摸时检测所述模数转换器的输出，可以设置交流信号Vdrv的幅值，使得所述第三模拟输出信号具有所述初始值。
图6示出根据本发明第三实施例的信号处理方法的流程图。以图4所示的信号处理电路300为例说明该方法的各个流程。
在步骤S01中，设置初始值。在该步骤中，由于手指未按压在指纹传感器上，因此第一电容Cf与放大器230之间断开。第一开关S1闭合，使得偏置电路310向放大器230的反相输入端提供偏置信号。
该步骤通过设置偏置电路310中的电路元件的参数，例如设置第二电容Cb的电容值以及第二交流信号源250的交流信号Vdrv的幅值，放大器230的放大信号为预定电压，使得模数转换器240的数字输出大致为满量程数字值的二分之一。以8位模数转换器为例，其输出数字代码范围为(0-255)。在步骤S01中，在仅有第二电容Cb连接至放大器230的反相输入端的情况下，模数转换器240的输出数字值在128附近。由于输出数字值取决于传感器内部集成电路元件之间的比值，基于集成电路制造工艺，该输出数字值可以控制在比较精准的范围内波动。
在步骤S02中，手指触摸在指纹传感器上，第一电容Cf与放大器230之间连接，放大器230的反相输入端的信号为第一电容Cf的感应信号和偏置电路310产生的偏置信号的叠加信号，如公式(1)所示：
Figure PCTCN2016113375-appb-000001
其中，Vout表示放大器的模拟输出电压，A表示放大器的放大倍数，Cf和Cb分别表示第一电容和第二电容的电容值，Vn和Vdrv分别表示第一交流信号源和第二交流信号源的幅值，f为放大器的工作频率，f0为第二交流信号源的频率。
该信号处理电路可以是连续时间工作方式，也可以是开关电容形式的离散时间工作方式。一般情况下，Vdrv信号的工作频率与检测电路的控制信号频率相同或者具有 强相关性(互为整数倍)。
在步骤S03至S05中，根据噪声检测调节放大器的工作频率，以减弱甚至消除共模噪声的影响。在图4所示的实施例中，该工作频率例如是放大器230的工作频率。
当共模噪声的频率与上述工作频率相同或者两者之间存在整数倍关系时，噪声会直接对检测结果产生影响，从而导致模数转换器量化后的输出数字值偏离理想区间。更甚者，过大的共模噪声直接导致放大器230和模数转换器240饱和，从而使得电路重新建立工作状态的时间变长，影响后续的检测结果。
当共模噪声的频率及其谐波频率与工作频率相差较远时，噪声对检测结果影响减小。而且，较小的噪声的影响也可以利用数字滤波等方法将其滤除。
上述调节工作频率的过程包括：在步骤S03中设置工作频率，在步骤S04中检测模数转换器240的输出数字值，在步骤S05中判断模数转换器240的输出数字值是否偏离理想范围。
如果模数转换器240的输出数字值偏离理想范围，甚至导致放大器230和模数转换器240的至少一个饱和，则重新设置不同的工作频率，重复步骤S03至S05。
相反，如果模数转换器240的输出数字值在理想范围内，则表明设置的工作频率可以减弱甚至消除共模噪声的影响，从而可以作为所述图像采集模式中的工作频率。
共模噪声的能量一般集中在以特定频率为中心的较窄带宽内，所以***通过以上噪声检测的方法通过频率搜索的方法找到共模噪声较小的频率，并以此频率作为电路的工作频率，从而减弱甚至消除共模噪声的影响。
因此，本发明提出的信号处理电路可以通过检测模数转换器240的输出数字值来检测当前频率设置下是否存在影响图像质量的干扰。进一步地，通过检测模数转换器的输出数字值，调节放大器的工作频率，使得所述图像采集模式中的工作频率为理想值。
在该信号处理方法中，采用偏置电路调节放大器230的工作频率，以及采用选择的工作频率采集指纹图像，从而抑制手指触摸引入的共模噪声。
在该实施例中，指纹传感器可以包括多个感应元件，针对所述多个感应元件可以设置交流信号Vdrv的幅值相同。作为替代的实施例中，针对指纹传感器的所述多个感应元件，可以单独设置交流信号Vdrv的幅值，以更好的进行检测。
例如，在上述的步骤S01中，可以将指纹传感器中的所有感应元件检测电路中的交流信号Vdrv设置成两种幅值，使得多个感应元件的数字输出值为相应的两种数字 值。图7示出在信号处理方法中多个感应元件的初始化信号的分布图。以8位模数转换器为例，多个感应元件的数字输出值的均值在128左右。所述多个感应元件排列阵列，并且可以分成棋盘图案的第一组感应元件和第二组感应元件。由于交流信号Vdrv的幅值不同，第一组感应元件的数字输出值的均值为198，第二组感应元件的数字输出值的均值为58。基于上述激励方式，通过统计指纹输出图像所有感应元件的均值、和/或第一组感应元素和第二组感应元件的均值，设置合理的初始值来进行噪声强度的判断。
在上述的实施例中，为了节省调节工作频率所需的时间，在手指触摸指纹传感器之后且在采集指纹图像之前进行调节放大器的工作频率的过程。
对于面阵式的指纹识别传感器，可以选取传感器的一部分区域(中心区域)，通过检测模数转换器的输出数字值，调节放大器的工作频率。由于仅针对指纹传感器的一部分感应电极进行驱动和检测，因此节省调节工作频率所需的时间。
优选地，为了进一步节省指纹传感器的噪声检测时间，***可以将噪声检测区域的传感器分为N个区域。每个区域内检测电路中施加在第一电容Cf一端的交流信号Vn的频率互不相同。因此，单次噪声检测过程可以检测N个频率的噪声情况。通过检测多个区域的模数转换器的输出数字值，可以判断出信号处理电路的理想工作频率。
应当说明的是，在本文中，诸如第一和第二等之类的关系术语仅仅用来将一个实体或者操作与另一个实体或操作区分开来，而不一定要求或者暗示这些实体或操作之间存在任何这种实际的关系或者顺序。而且，术语“包括”、“包含”或者其任何其他变体意在涵盖非排他性的包含，从而使得包括一系列要素的过程、方法、物品或者设备不仅包括那些要素，而且还包括没有明确列出的其他要素，或者是还包括为这种过程、方法、物品或者设备所固有的要素。在没有更多限制的情况下，由语句“包括一个……”限定的要素，并不排除在包括所述要素的过程、方法、物品或者设备中还存在另外的相同要素。
依照本发明的实施例如上文所述，这些实施例并没有详尽叙述所有的细节，也不限制该发明仅为所述的具体实施例。显然，根据以上描述，可作很多的修改和变化。本说明书选取并具体描述这些实施例，是为了更好地解释本发明的原理和实际应用，从而使所属技术领域技术人员能很好地利用本发明以及在本发明基础上的修改使用。本发明仅受权利要求书及其全部范围和等效物的限制。

Claims (16)

  1. 一种用于指纹传感器的信号处理电路，所述指纹传感器包括响应手指触摸产生感应信号的感应元件，所述信号处理电路包括：
    偏置电路，用于产生偏置信号；
    放大器，用于根据所述感应信号产生模拟输出信号；以及
    模数转换器，用于将所述模拟输出信号转换成数字输出信号，
    其中，所述信号处理电路工作于噪声检测模式和图像采集模式，
    在所述噪声检测模式中，所述放大器对所述感应信号和所述偏置信号的叠加信号进行放大以产生第一模拟输出信号，并且根据所述第一模拟输出信号调节所述放大器的工作频率，
    在所述图像采集模式中，所述放大器对所述感应信号进行放大以产生第二模拟输出信号，并且根据所述第二模拟输出信号生成指纹图像。
  2. 根据权利要求1所述的信号处理电路，其中，所述感应元件为感应电极。
  3. 根据权利要求2所述的信号处理电路，还包括第一交流信号源，其中，所述第一交流信号源与所述感应电极相连接，从而向所述感应电极提供第一交流信号作为驱动信号。
  4. 根据权利要求3所述的信号处理电路，其中，所述偏置电路包括在所述放大器的输入端和地之间依次串联连接的第一开关、第二电容和第二交流信号源，所述第二交流信号源向所述第二电容提供第二交流信号，
    在所述噪声检测模式中，所述第一开关闭合，
    在所述图像采集模式中，所述第一开关断开。
  5. 根据权利要求3所述的信号处理电路，其中，所述偏置电路包括在所述放大器的输入端和地之间依次串联连接的第二电容、第一开关和第二交流信号源、以及在所述第二电容和所述第一开关的中间节点和地之间连接的第二开关，
    在所述噪声检测模式中，所述第一开关闭合且所述第二开关断开，所述第二交流信号源向所述第二电容提供第二交流信号，
    在所述图像采集模式中，所述第一开关断开且所述第二开关闭合，使得所述第二电容经由所述第二开关接地。
  6. 根据权利要求4或5所述的信号处理电路，所述信号处理电路还工作于初始化模式，其中，在所述初始化模式中，所述放大器对所述偏置信号进行放大以产生第 三模拟输出信号，并且根据所述第三模拟输出信号调节所述偏置电路，使得所述第三模拟输出信号具有初始值。
  7. 根据权利要求6所述的信号处理电路，其中，在所述初始化模式中，通过在手指未触摸时检测所述模数转换器的输出，设置所述第二交流信号的幅值，使得所述第三模拟输出信号具有所述初始值。
  8. 根据权利要求7所述的信号处理电路，其中，所述指纹传感器包括多个感应元件，针对所述多个感应元件分别设置所述第二交流信号的幅值。
  9. 根据权利要求8所述的信号处理电路，其中，针对所述多个感应元件中的第一组感应元件和第二组感应元件，所述第三模拟输出信号分别具有第一初始值和第二初始值。
  10. 根据权利要求6所述的信号处理电路，其中，在所述噪声检测模式中，通过在手指触摸时检测所述模数转换器的输出，调节所述放大器的工作频率，从而设置所述图像采集模式中的工作频率。
  11. 根据权利要求1所述的信号处理电路，其中，所述放大器具有反相输入端、同相输入端和输出端，所述感应元件和所述偏置电路连接至所述反相输入端，所述同相输入端接收参考电压。
  12. 一种用于指纹传感器的信号处理方法，所述指纹传感器包括响应手指触摸产生感应信号的感应元件，所述方法包括：
    对所述感应信号和偏置信号的叠加信号进行放大以产生第一模拟输出信号；
    根据所述第一模拟输出信号调节工作频率；
    对所述感应信号进行放大以产生第二模拟输出信号；以及
    根据所述第二模拟输出信号生成指纹图像，
    其中，在调节工作频率时选择所述工作频率以抑制手指触摸引入的共模噪声。
  13. 根据权利要求12所述的方法，还包括对所述偏置信号进行放大以产生第三模拟输出信号，以及将所述第三模拟输出信号转换成数字输出信号，其中，通过在手指未触摸时检测所述数字输出信号，设置所述叠加信号的大小，使得所述第三模拟输出信号具有初始值。
  14. 根据权利要求13所述的方法，其中，所述指纹传感器包括多个感应元件，针对所述多个感应元件分别设置所述偏置信号的大小。
  15. 根据权利要求14所述的方法，其中，针对所述多个感应元件中的第一组感 应元件和第二组感应元件，所述第三模拟输出信号分别具有第一初始值和第二初始值。
  16. 根据权利要求12所述的方法，还包括将所述第一模拟输出信号转换成数字输出信号，其中，在调节工作频率时，通过在手指触摸时检测所述数字输出信号，以设置所述图像采集模式中的工作频率。
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