JP2006236364A - Signal processing circuit and fingerprint detector using it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image sensor signal processing circuit for obtaining an optimum signal level in compliance with a signal mean level in AD conversion of a signal from a photoelectric conversion element. <P>SOLUTION: This signal processor is constructed of a variable gain amplifier 1 amplifying a difference with a reference voltage by using a signal output from an image sensor as an input to a reverse input terminal, a peak hold circuit 2 performing sample holding of the maximum value and the minimum value for a periodically varying pixel signal as an output of the variable gain amplifier 1, an integrator 3 receiving differential voltage between the maximum value and the minimum value generated in the peak hold circuit 2 for outputting a low frequency constituent (mean voltage) through a low-pass filter having a predetermined cutoff frequency, and a reference voltage source 5 receiving the output from the integrator 3 and outputting a reference voltage matching the output from the integrator 3 to a non-reverse input terminal in the variable gain amplifier 1. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image sensor signal processing circuit suitable for application to a system that makes a determination based on a signal pattern from an image sensor, typified by a fingerprint detection device using an optical sensor.

  FIG. 8 is a block diagram of a signal processing circuit in a fingerprint detection system using a conventional optical sensor.

  FIG. 9 is a signal output of a typical optical sensor in the system. The reason why the HI / LO signal periodically changes in FIG. 9 is that the shape of the fingertip due to the fingerprint has irregularities, so that the output is low in the concave part and high in the convex part.

  Since a light source is arranged around the finger to read the unevenness of the fingerprint with the optical sensor, the optical sensor output level by the fingerprint around the finger inevitably close to the light source is high, the distance from the light source is large, Furthermore, from the point of light transmittance in the finger, the optical sensor output due to the fingerprint at the center of the finger is low, much closer to the curved shape of the finger, as shown in FIG. So-called shading that has a large period and is not related to the original optical signal output occurs.

  Due to this shading, the SN ratio of the sensor output indicating the fingerprint at the center of the finger is lowered, and there is a problem that the recognition rate in a fingerprint recognition system using a digital value obtained by AD conversion of this signal is lowered. If the light intensity of the light source placed around the finger is increased in order to increase the light intensity at the center of the finger, the sensor output of the fingerprint around the finger will become excessive, causing the so-called whiteout, again the fingerprint The recognition rate is lowered.

  The conventional signal processing method controls the gain and reference voltage of the amplifier circuit in accordance with the average level of the signal of one frame. Depending on the shape of shading of the sensor signal to be controlled, as shown in FIG. However, there is a problem that the pixel signal level in the shading valley portion is not optimally corrected (see, for example, Patent Document 1).

In this example, when the average value of the pixel signal level for one frame is higher than the reference value and some of the pixel signals are extremely low, the average value is higher than the reference, so the reference voltage of AGC (Auto Gain Control Amp.) Is set high or the gain is set low, the signal of the pixel having a low signal level is further reduced, and the SN ratio is deteriorated.
JP 2000-298464 A

  However, when the average level of the signal of the image sensor changes greatly as shown in FIG. 9 slowly compared to the pixel rate, the gain of an amplification circuit such as AGC that amplifies the signal is simply set according to the signal average level. The signal SN ratio and the dynamic range of the AD converter arranged after that cannot be used effectively only by changing the signal. This is because the reference voltage for amplifying the signal is constant.

  Accordingly, the present invention provides a signal processing circuit for obtaining an optimum signal level according to the average signal level when AD conversion is performed on a signal from a photoelectric conversion element in a system that makes a determination based on a signal pattern from an image sensor. The purpose is to do.

  In order to solve the above-described problems, the present invention provides a signal processing circuit for analog / digital conversion of a signal from a photoelectric conversion element, using a signal output from the photoelectric conversion element as one input and calculating a difference from a reference voltage. The differential amplifier circuit to amplify, the peak hold circuit that samples and holds the maximum and minimum values of the pixel signal at the output of this differential amplifier circuit, and the low-frequency component that receives the output of this peak hold circuit And a reference voltage source that receives the output of the integrator and outputs a reference voltage according to a result of comparing the output of the integrator and a reference value to the other input of the differential amplifier circuit The cutoff frequency of the integrator is adjusted and set according to the magnitude of the difference between the average value of the signal output from the photoelectric conversion element and the reference value.

  According to the present invention, when AD conversion is performed on a signal whose DC level (average value) varies greatly with respect to the amplitude (AC change) of the signal, the input dynamic range of the AD converter is effectively used. This is utilized to improve the signal S / N ratio, so that only the change in the signal can be amplified to a level suitable for the input dynamic range of the AD converter.

  In addition, by adjusting the reference voltage of the amplifier circuit, the dynamic range of the AD converter that has been conventionally used for the conversion of the DC signal portion can be used only for the signal change portion, and the AD conversion accuracy of the change portion can be increased. Can be improved.

  In the best mode described below, the reference value of the amplifier circuit is determined according to the integral value (average value) of the change in the signal voltage for several pixels to several tens of pixels before the pixel to be amplified and the polarity thereof. By controlling the voltage and gain, and setting the degree of influence on the average value that controls the reference voltage and gain for signals output later in time, the optimum for the signal level of the pixel to be amplified in the future It is characterized in that a large reference voltage and gain are obtained, and the input dynamic range of the AD converter located thereafter is utilized to the maximum extent.

  That is, the integrated value (average value) of signal voltages for several pixels to several tens of pixels before the pixel to be amplified is compared with the reference value, and the reference voltage and gain of the amplifier circuit are controlled according to the result. (See FIG. 11).

  In other words, in the frequency domain, the information required here is in a relatively high frequency domain, and therefore, lower frequency components are not necessary because they interfere with signal amplification. This low frequency component is extracted (integrated), applied to one input terminal of the differential amplifier circuit as a reference voltage based on it, and subtracted from the signal from the sensor, etc. applied to the other input terminal, and amplified. (Refer to FIG. 12).

  The initial values of the reference voltage and the gain are set to a fixed value or the average value of the amplifier output voltage of the first few pixels and the reference value (for example, 1 of the input dynamic range of the AD converter arranged at the subsequent stage of the amplifier). May be determined so that the difference is minimized.

  The number of pixels to be referred to as several pixels to several tens of pixels is determined from the change rate (differential value) of the shape of the shading and the size of the difference, and the average value (integrated value) of the difference voltage of the unevenness of the signal with the fast cycle in FIG. When the difference between the change amount (differential value) and the reference value is large, the reference pixel number is set small. In other words, in the frequency domain, the amount of high frequency components in the unnecessary low frequency component signal is greater than the low frequency component, so the cutoff frequency of the integrator is changed to a higher value than before. This is to efficiently remove unnecessary frequency components (see FIG. 13).

  Further, the degree of weighting of the reference pixel signal is similarly determined from the shading gradient (differential value), and the weighting of the most recent pixel signal is set larger as the change is larger (the differential value is larger).

  The absolute value of the pixel signal is not so significant, and it is effective to apply the present invention to a fingerprint sensor having information effective for the difference between the peak and valley of each pixel signal.

  Next, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 is a circuit block diagram showing a first embodiment of the present invention. The image sensor signal processing circuit of the present invention has a variable (or constant) gain amplifier (differential amplifier circuit) 1 that amplifies a difference from a reference voltage using a signal output from an image sensor (not shown) as an input to an inverting input terminal. A peak hold circuit 2 that samples and holds the maximum and minimum values of the periodically changing pixel signal that is the output of the variable gain amplifier 1, and the maximum and minimum values generated in the peak hold circuit 2. And an integrator 3 that outputs a low-frequency component (average voltage) with a low-pass filter having a predetermined cut-off frequency, and an output of the integrator 3 that is received by the non-inverting input terminal of the variable gain amplifier 1 The reference voltage source 5 outputs a reference voltage corresponding to the output of the integrator 3. The outputs of the integrator 3 and the reference voltage source 5 change in real time in a digital or analog manner in synchronization with a certain pulse input.

FIG. 2 shows details of the operation example. In this example, for simplification of the figure, the cutoff of the integrator is about twice as long as the input changing period (or an average value for two pixels). Further, it is assumed that the integrator captures a signal at the timing when the pixel signal of the photosensor is output and performs an integration operation. (Reset level is output between pixel signals)
In FIG. 2, the case where the result of comparing the average value of the variable gain amplifier output with respect to the pixels 1 and 2 with 1/2 of the input dynamic range of an AD converter (not shown) that AD converts the output of the variable gain amplifier is small Show. The reference voltage of the variable gain amplifier is reduced at the pixels 3 and 4 as shown in the figure according to the difference voltage of the comparison result.

  As a result, as shown in the right diagram of FIG. 2, the amplifier output for the pixels 3 and 4 is in the vicinity of ½ of the input dynamic range of the AD converter, and stable AD conversion with a good S / N ratio is possible. become. In addition, the left figure of FIG. 2 has shown the case where a reference voltage is not adjusted as a prior art example.

  Alternatively, as shown in the left diagram of FIG. 2, the difference between the amplifier outputs of the pixels 1 and 2 is compared with a certain threshold value, and when the difference is exceeded, the reference voltage of the variable gain amplifier is increased or decreased according to the difference polarity. . That is, in the left diagram of FIG. 2, the output of the second pixel is significantly lower than the output of the first pixel, and the amount of the reduction has exceeded the threshold value, so the reference voltage is reduced. Conversely, if the output of the second pixel increases, the reference voltage is increased, so that the determination and reference voltage are controlled using the differential value of the pixel output and its polarity.

  When the low frequency component corresponding to the change in the pixel signal is considered to be a change corresponding to the original optical signal, the AD conversion is performed by transmitting the information on the changed reference voltage to the AD converter. It is also possible not to deteriorate the accuracy.

  FIG. 3 is a circuit block diagram showing a second embodiment of the present invention. In this embodiment, the gain of the variable gain amplifier 1 is further controlled by using the output of the integrator 3 from the first embodiment, and is changed along with the reference voltage of the variable gain amplifier, and the input dynamic range of the AD converter that receives the output of the amplifier. On the other hand, the signal-to-noise ratio at the time of AD conversion of the signal is maximized. Similar to the first embodiment, the signal outputs of the pixels 1 and 2 of the optical sensor are sampled, the average value is obtained by the integrator 3, and the input dynamic range of the AD converter that receives the output of the variable gain amplifier is used here. If the average value is small, the reference voltage of the variable gain amplifier is decreased and the gain of the variable gain amplifier is increased, or only the gain of the variable gain amplifier is obtained, as in the first embodiment. Increase Alternatively, when the change amount of the signal output of the pixels 1 and 2 exceeds the threshold value, the gain of the amplifier is increased or decreased by a certain amount according to the polarity of the change amount. Also in this case, the conversion accuracy can be prevented from deteriorating by transmitting the changed reference voltage and gain information to the AD converter as in the first embodiment.

  As described above, in the first and second embodiments, the signal is input before the signal to be corrected by the control means including the gain amplifier 1, the peak hold circuit 2 for sampling and holding, the integrator 3, and the reference voltage source 5. Based on the received signal, fluctuations in the amplitude of the low-frequency component contained in the plurality of sequentially input signals are suppressed.

  FIG. 4 is a circuit block diagram showing a third embodiment of the present invention. In this embodiment, the weighting of the latest pixel signal voltage taken into the integrator is changed by the differentiating circuit 4 according to the magnitude (differential value) of the change in the pixel output by the differentiating circuit 4 to change the pixel signal output. I made it correspond to the size of the rate.

  FIG. 5 shows an example of the integrator 3 in FIG. 1, which is a so-called SWITCHED CAPASITOR circuit with three switches and two capacitors, and compares the output of the differentiation circuit 4 in FIG. 4 with a reference value, and according to the result. SW1 in FIG. 5 is controlled to change the ratio of the capacitors C1 and C2 in FIG. As shown in FIG. 10, when the change in the pixel signal output is large, the followability of the reference voltage and the gain is deteriorated in the first and second embodiments and exceeds the dynamic range of the worst AD converter. To increase the tracking performance of the integrator.

  As described above, in the third embodiment, before the signal to be corrected by the control means including the gain amplifier 1, the peak hold circuit 2 for sampling and holding, the integrator 3, the differentiation circuit 4, and the reference voltage source 5. Based on the input signals, fluctuations in the amplitude of the low frequency components included in the plurality of signals that are sequentially input are suppressed.

  FIG. 6 shows an example in which the present invention is applied to a signal processing circuit for analog / digital conversion of a signal of an image sensor having a two-dimensional matrix configuration. In the first, second, and third embodiments, signal processing in one line in which photoelectric conversion pixels are arranged in one column is assumed. In this embodiment, in order to handle a plurality of line signals in which a plurality of pixel columns are arranged (so-called two-dimensional matrix configuration), a plurality of preset lines before a line (row) on which signal processing such as reading is currently performed are performed. Each row has an average signal level for each line, and storage means 18 for storing an average value of the difference output between the pixel signal and the dark signal from the photoelectric conversion elements for one row. Further, a differentiator 19 is provided which receives the outputs of the plurality of storage means 18 and outputs the changes.

  Further, a signal output from a plurality of rows of photoelectric conversion elements (sensor cells 11) is used as one input, a differential amplifier circuit 14 that amplifies a difference from a reference voltage, and an output of the differential amplifier circuit 14 is a certain period. A peak hold circuit 16 that samples and holds the maximum value and minimum value of a pixel signal that changes in response to an output, and an integrator that receives the output of the peak hold circuit 16 and outputs a low-frequency component by a low-pass filter having a predetermined cutoff frequency. 17 and the other input of the differential amplifier circuit 14, the output of the integrator 17 and the output of the differentiator 19 are added, and a reference voltage generation circuit 13 that outputs a reference voltage according to the result of comparison with a certain reference value; By providing, it responds also to the change of the signal level between lines. The output of the differential amplifier circuit 14 is converted from analog to digital by the AD converter 15.

  In the example of FIG. 6, the average values of the signal levels of the two lines are stored in the storage unit 18, added to the pixel signals of the line currently being read, averaged, and the result of the reference voltage generation circuit 13 is used. Control the voltage. That is, when the average value of the pixel signal level in the first row and the second row is decreasing, when the pixel signal in the third row is processed, the signal of the third row is described in the first embodiment. By subtracting a value corresponding to the differential value of the average value of the first row and the second row from the integrated value, the reference voltage is set smaller than that when the differential value is zero.

  FIG. 7 shows a configuration of a fingerprint authentication apparatus using the image sensor signal processing circuit of the present invention. The image sensor 21 is an image sensor such as a CCD or CMOS sensor that captures a fingerprint image. The analog / digital conversion unit (A / D conversion unit) 22 includes the image sensor signal processing circuit of the above-described embodiment, and converts the analog signal from the image sensor 21 into a digital signal by optimal processing as shown in the above-described embodiment. To do. The timing generator 23 drives the image sensor 21 and the A / D converter 22. The signal processing unit 24 performs various signal processes on the fingerprint image data from the A / D conversion unit 22. The memory 25 stores fingerprint image data from the signal processing unit 4. The registered fingerprint database 26 stores in advance fingerprint characteristic data extracted from fingerprint image data.

  Here, the feature data of the fingerprint will be briefly described. A “ridge”, which is a raised portion that forms a fingerprint pattern, includes a cut portion and a branched portion. The “end point” which is a cut portion and the “branch point” which is a branched portion are collectively referred to as a “feature point”. The “position”, “direction”, and the like obtained from the feature points are handled as feature data for specifying a fingerprint.

  The fingerprint collation unit 27 detects fingerprint feature data from the fingerprint image data in the memory 25 and collates it with the fingerprint feature data registered in the registered fingerprint database 26. This fingerprint authentication device is mounted on, for example, a mobile phone that can make a call with a desired destination via a public line.

1 is a circuit block diagram showing a first embodiment of the present invention. Illustration of average value of variable gain amplifier output for pixel Circuit block diagram showing a second embodiment of the present invention Circuit block diagram showing a third embodiment of the present invention The figure which shows an example of the integrator in FIG. Signal processing circuit block diagram of an image sensor having a two-dimensional matrix configuration Configuration diagram of fingerprint authentication apparatus using image sensor signal processing circuit of the present invention Signal processing circuit block diagram of a conventional fingerprint detection system The figure which shows the signal output of the optical sensor in the conventional system Illustration of signal correction in conventional signal processing method The figure which shows the comparison of the signal waveform of the amplifier output by the past and this invention Illustration in the frequency domain of the present invention Illustration in the frequency domain of the present invention

Explanation of symbols

1 Variable Gain Amplifier 2 Peak Hold Circuit 3 Integrator 4 Differentiation Circuit 5 Reference Voltage Source

Claims (4)

In a signal processing circuit for analog / digital conversion of a signal from a photoelectric conversion element,
A differential amplifier circuit that amplifies a difference from a reference voltage using a signal output from the photoelectric conversion element as one input,
At the output of this differential amplifier circuit, a peak hold circuit that samples and holds the maximum value and minimum value of the pixel signal,
An integrator that receives the output of this peak hold circuit and outputs a low-frequency component with a low-pass filter;
A reference voltage source that receives the output of the integrator and outputs a reference voltage according to a result of comparing the output of the integrator and a reference value to the other input of the differential amplifier circuit;
The signal processing circuit, wherein a cutoff frequency of the integrator is adjusted and set according to a difference between an average value of signal outputs from the photoelectric conversion element and the reference value.   2. The signal processing circuit according to claim 1, wherein the reference value is set to ½ of an input dynamic range of the analog / digital converter. The differential amplifier circuit is a variable gain differential amplifier circuit that controls a gain using an output of the integrator,
2. One or both of a reference voltage of the reference voltage source and a gain of the variable gain differential amplifier circuit are changed according to a result of comparison between the output of the integrator and the reference value. The signal processing circuit described. A fingerprint detection device that reads and detects a fingerprint,
An image sensor that captures fingerprint images;
An analog / digital conversion unit comprising the signal processing circuit according to any one of claims 1 to 3 for converting an analog signal from the signal processing circuit into a digital signal;
A memory for storing digitally converted fingerprint image data;
A fingerprint detection apparatus comprising: a fingerprint detection unit configured to detect fingerprint feature data from the stored fingerprint image data.

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