JP2017022688A - Imaging element, image reading device, image forming device, and imaging method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent noise generated in a digital signal at a timing other than the timing when a signal is output to an external unit at a post stage from being mixed into an analog signal.SOLUTION: An imaging element includes: a plurality of pixel units which include a plurality of light receiving elements for performing photoelectric conversion; a plurality of analog-to-digital (A/D) converters provided to each of the pixel units to sequentially convert analog signals obtained by photoelectric conversion performed by the light receiving elements to digital signals; a plurality of first holding units provided to each of the pixel units to sequentially hold digital signals obtained by conversion performed by the A/D converters; and a plurality of second holding units which receive and hold digital signals held by the first holding units in a period when the A/D converters do not convert analog signals to digital signals.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an imaging element, an image reading apparatus, an image forming apparatus, and an imaging method.

  A CMOS image sensor is known that performs analog signal processing and digital signal processing in a chip so as to capture light, convert it into an analog signal, and further convert it into a digital signal for processing. In this case, noise generated in the digital circuit may be superimposed on the analog signal to deteriorate the image signal.

  Further, in Patent Document 1, the logic noise of the digital signal processing circuit and the output noise of the output end are 1 by outputting the signal accumulated in the line memory by the output circuit outside the noise canceling operation period in the horizontal scanning period. A signal processing apparatus that controls a line memory so as not to occur during a horizontal noise cancellation operation period is disclosed.

  However, in the conventional technology, the signal is output from the output circuit outside the noise canceling operation period, so that noise is reduced from being mixed into the signal. However, the signal is deteriorated by noise generated at other timings. There was a problem that could not be prevented.

  The present invention has been made in view of the above, and an image pickup device and an image reading device capable of preventing noise generated in a digital signal other than timing for outputting a signal to an external subsequent stage from being mixed into an analog signal. It is an object to provide an apparatus, an image forming apparatus, and an imaging method.

  In order to solve the above-described problems and achieve the object, the present invention provides a plurality of pixel units including a plurality of light receiving elements that perform photoelectric conversion, and an analog signal obtained by photoelectric conversion of the plurality of light receiving elements in turn as a digital signal. A plurality of A / D conversion units provided for each of the pixel units so as to convert into a plurality of pixels, and a plurality of A / D conversion units provided for each of the pixels so as to sequentially hold digital signals converted by the plurality of A / D conversion units. A plurality of second holding units that receive and hold the digital signals held by the first holding unit during a period when the analog signal is not converted into a digital signal by the A / D conversion unit. Have.

  According to the present invention, it is possible to prevent noise generated in a digital signal other than the timing for outputting a signal to an external subsequent stage from being mixed into an analog signal.

FIG. 1 is a diagram illustrating an outline of the image sensor according to the first embodiment. FIG. 2 is a timing chart showing an operation example as a comparative example of the image sensor. FIG. 3 is a diagram illustrating an outline of an image sensor according to the second embodiment. FIG. 4 is a timing chart showing the timing of the first operation of the image sensor shown in FIG. FIG. 5 is a timing chart showing the timing of the second operation of the image sensor shown in FIG. FIG. 6 is a diagram illustrating an outline of an image sensor according to the third embodiment. FIG. 7 is a timing chart showing an operation example as a comparative example of the image sensor. FIG. 8 is a timing chart showing the operation timing of the image sensor shown in FIG. FIG. 9 is a diagram illustrating an outline of an image forming apparatus including an image reading apparatus having an image sensor.

  Hereinafter, an image sensor according to a first embodiment will be described with reference to the accompanying drawings. FIG. 1 is a diagram illustrating an outline of an image sensor 10 according to the first embodiment. The image pickup device 10 includes, for example, N pixel units 12, an ADC unit 14, a memory unit 16, a latch unit 18, and a timing control unit (TG: control unit) 20 that controls the operation timing thereof. It is a CMOS linear sensor.

  The pixel unit 12 is a pixel group including, for example, a total of six light receiving elements (photodiodes: PD) that perform photoelectric conversion of two (two pixels) each of R, G, and B. The pixel unit 12 also includes a pixel circuit such as an analog memory that converts charges into voltage signals for each light receiving element and holds the converted voltage, and a reset circuit. In response to the signal (PIX_EN [5: 0]) input from the timing control unit 20, the pixel unit 12 generates the charges generated by the six light receiving elements according to the amount of incident light for the analog signal OUT_P [ 5: 0] are sequentially output to the ADC unit 14. In addition, the pixel unit 12 can sequentially output analog signals indicating the dark reference level of each of the six light receiving elements to the ADC unit 14 as signals for six pixels.

  In response to the signal (ADC_EN) input from the timing control unit 20, the ADC unit 14 outputs an analog signal indicating the dark reference level input from the pixel unit 12 and an analog signal indicating the signal level during light reception. It is an A / D conversion unit that sequentially converts to a digital signal. The ADC unit 14 outputs the digital signal as OUT_AD [11: 0].

  The memory unit 16 is a first holding unit capable of sequentially holding digital signals output from the ADC unit 14. Then, the memory unit 16 outputs the digital signal as OUT_MEM [11: 0].

  The latch unit 18 is a second holding unit that receives and holds a plurality of digital signals OUT_MEM [11: 0] held by the memory unit 16. As will be described later, the latch unit 18 receives and holds a plurality of digital signals held by the memory unit 16 after the ADC unit 14 finishes converting each analog signal into a digital signal for each pixel unit 12.

  Here, the imaging device 10 includes one pixel unit 12, an ADC unit 14, a memory unit 16, and a latch unit 18 as one column 100. That is, the image sensor 10 is a columnar CMOS linear sensor including N (1 to N) columns 100. The value of N is 3750, for example. In this case, the image sensor 10 is a CMOS linear color image sensor having 7500 light receiving elements for each of R, G, and B. When applied to an image reading apparatus, for example, the light receiving element for reading an A3 document size. Are arranged in the main scanning direction for each color.

  In the imaging device 10, the timing control unit 20 performs control so that the digital signal is transferred from the memory unit 16 to the latch unit 18 during a period in which the ADC unit 14 does not convert the analog signal into a digital signal. That is, in the image sensor 10, the period during which analog signal processing is performed and the period during which data is transferred do not overlap.

  FIG. 2 is a timing chart illustrating an operation example as a comparative example of the image sensor 10. FIG. 2 shows the operation timing when one column 100 receives light. That is, in the imaging device 10, all the columns 100 (for example, 3750 columns) operate as a whole.

  In the image sensor 10, the timing control unit 20 generates each drive signal with reference to the reference clock (CLK). Lsync is a line synchronization signal and indicates a period of one line of image data in the main scanning direction. Each light receiving element (PD) in the pixel unit 12 accumulates electric charge according to the intensity of the incident light quantity. Here, the pixel unit 12 holds each analog signal that has been subjected to charge-voltage conversion during the execution period of PIX_EN [5: 0] input from the timing control unit 20 in an internal analog memory.

  The ADC unit 14 sequentially reads out analog signals for two pixels of R, G, and B held by the pixel unit 12 and performs A / D conversion in the execution period of the signal (ADC_EN) input from the timing control unit 20. The ADC unit 14 sequentially outputs digital signals determined by A / D conversion, for example, one bit at a time (for example, in the case of cyclic A / D conversion).

  The memory unit 16 sequentially holds digital signals output from the ADC unit 14. When the A / D conversion is completed, a digital signal is transferred from the memory unit 16 to the latch unit 18 during the execution period of MEM_EN.

  Transfer data from the memory unit 16 to the latch unit 18 is 12-bit image data. Since the transfer data from the memory unit 16 to the latch unit 18 is simultaneously performed in each column 100 (N = 3750), the processing in the image sensor 10 is highly loaded. In other words, the load of data transfer from the ADC unit 14 to the memory unit 16 is 1 bit each, whereas the load of data transfer from the memory unit 16 to the latch unit 18 is 12 bits, so the load becomes 12 times. End up.

  For this reason, there is a case where the digital power supply that supplies power to the memory unit 16 and the latch unit 18 fluctuates with the start of data transfer (MEM_EN) from the memory unit 16 to the latch unit 18 as a trigger. When the digital power supply fluctuates, GND may be fluctuated via a decoupling capacitor for suppressing the fluctuation. When the GND changes, the voltage fluctuation goes to the analog power supply via the decoupling capacitor between the GND and the analog power supply, and as a result, the digital power supply fluctuation is transmitted to the analog power supply.

  Further, as another transmission path, it is common to divide the digital power source and the analog power source in the image sensor 10, and the fluctuation may be transmitted via the parasitic capacitance in the supply process after the division. Accordingly, when fluctuations generated by digital processing, that is, noise components are superimposed on the analog power supply, the noise components are also superimposed on the output signal of the analog processing circuit that operates with the voltage supplied from the analog power supply.

  When noise is superimposed on the output signal from the analog processing circuit, a component different from the image information read by the image sensor 10 is transmitted to the subsequent stage as image information, resulting in degradation of image quality.

  Here, a series of operations in which the analog signal held by the pixel unit 12 is read out by the ADC unit 14 and A / D converted and held by the memory unit 16 are sequentially processed as soon as the operation for one pixel is completed. That is, processing independent of the data transfer operation to the latch unit 18 is possible depending on the execution timing of the signal MEM_EN.

  If the data transfer timing to the latch unit 18 and the AD conversion timing of the ADC unit 14 performing parallel processing overlap, fluctuation noise generated in the digital signal is mixed into the analog signal as described above. become. In addition, noise (fluctuation) of a digital signal due to data transfer from the memory unit 16 to the latch unit 18 converges with a certain time width after it is generated. However, if the analog signal processing (A / D conversion) of the next pixel is started before the noise converges as in the example shown in FIG. 2, the noise is mixed into the analog signal and the signal is deteriorated. Will end up.

  FIG. 3 is a diagram illustrating an outline of the image sensor 10a according to the second embodiment. The image sensor 10a includes N (for example, N = 3750) pixel units 12, PGA units 13, ADC units 14, CDS units 15, and latch units 18, and a timing control unit that controls the operation timing ( TG: control unit) 20a. In addition, the same code | symbol is attached | subjected to the substantially same thing as the component mentioned above.

  The PGA unit 13 amplifies the analog signal OUT_P [5: 0] output from the pixel unit 12 sequentially in accordance with the dynamic range of the ADC unit 14 during the execution period of the signal PGA_EN input from the timing control unit 20a. It is. Then, the PGA unit 13 outputs the amplified analog signal as OUT_PG [5: 0] to the ADC unit 14.

  The CDS unit 15 outputs a digital signal indicating a dark reference level of each of the six light receiving elements included in the pixel unit 12 and a signal level during light reception during the execution period of the signal CDS_EN input from the timing control unit 20a. Each of the received digital signals is held, and correlated double sampling (CDS) is performed for each light receiving element. The CDS unit 15 outputs the result of the correlated double sampling to the second holding unit as a plurality of digital signals OUT_CD [11: 0].

  As described above, the CDS unit 15 also has a function as a first holding unit that sequentially holds each digital signal A / D converted by the pixel unit 12 for each pixel unit 12. In addition, data is transferred from the CDS unit 15 (first holding unit) to the latch unit 18 (second holding unit) at the execution timing of Latch_EN output by the timing control unit 20a. In the imaging element 10a, each of the pixel unit 12, the PGA unit 13, the ADC unit 14, the CDS unit 15, and the latch unit 18 is used as one column 100a.

  First, the first operation of the image sensor 10a will be described. FIG. 4 is a timing chart showing the timing of the first operation of the image sensor 10a shown in FIG. In the first operation, the imaging device 10a performs the operation from the operation of the pixel unit 12 to the operation of the ADC unit 14 for each pixel (for each light receiving element in the pixel unit 12), and from the CDS unit 15 to the latch unit 18 for each light receiving element. Data transfer processing to is performed.

  Here, since the image pickup device 10a divides the period of the analog signal processing state and the period of noise generation by data transfer for each light receiving element, analog signal processing (A / D conversion or the like) is prevented by preventing digital noise from being mixed. )It can be performed.

  More specifically, the image sensor 10a not only performs data transfer during a period in which analog signal processing is not performed, but is also provided with an A period until analog signal processing of the next light receiving element is started. During this period A, the image sensor 10a waits for the digital noise generated by the transfer processing of the previous pixel signal (digital signal) from the CDS unit 15 to the latch unit 18a to converge, and the noise due to the digital signal processing is reduced. The analog signal processing of the next light receiving element is prevented from being affected.

  Next, the second operation of the image sensor 10a will be described. FIG. 5 is a timing chart showing the timing of the second operation of the image sensor 10a shown in FIG. In the second operation, the imaging device 10a performs the operations from the operation of the pixel unit 12 to the operation of the CDS unit 15 for all the six pixels in the column 100a (all the light receiving elements in the pixel unit 12). Later, data transfer processing from the CDS unit 15 to the latch unit 18 is sequentially performed.

  Therefore, the image pickup device 10a continuously prevents the digital noise from being mixed without waiting for the convergence of the digital noise generated by the transfer process of the previous pixel signal (digital signal) from the CDS unit 15 to the latch unit 18a. Thus, analog signal processing (A / D conversion or the like) can be performed. Furthermore, since the image sensor 10a can perform data transfer processing from the CDS unit 15 to the latch unit 18 continuously and sequentially, the noise generation period can be reduced.

  Therefore, after the image sensor 10a reads an image for one line, a margin period (period B in FIG. 5) occurs before starting the image reading of the next line. It becomes possible to operate in a cycle. In other words, noise due to digital signal processing is prevented during the analog processing period in one line without sacrificing speedup.

  FIG. 6 is a diagram illustrating an outline of the image sensor 10b according to the third embodiment. The image sensor 10b includes N (for example, N = 3750) pixel units 12, a PGA unit 13, an ADC unit 14, a CDS unit 15, a latch unit 18, a DSP unit 19 that performs digital signal processing, and these It is a CMOS linear sensor which has a timing control part (TG: control part) 20b which controls the operation timing of. In the imaging device 10b, one pixel unit 12, PGA unit 13, ADC unit 14, CDS unit 15, and latch unit 18 are used as one column 100b.

  For example, the DSP unit 19 has a function of converting parallel signals output from the plurality of latch units 18 into serial signals, a function of outputting only the signals of arbitrary pixels in the image signal read by the image sensor 10b, and image signals. And a digital signal processing function such as a function of converting an image signal into an LVDS signal for output to the outside. The DSP unit (digital signal processing unit) 19 operates according to the signal DSP_CNT input from the timing control unit 20b.

  FIG. 7 is a timing chart showing an operation example as a comparative example of the image sensor 10b. As described above, the DSP unit 19 performs digital signal processing on the image data (digital signal) output from the latch unit 18 in accordance with DSP_CNT input from the timing control unit 20b.

  For example, the DSP unit 19 converts the parallel image data output from each latch unit 18 into serial image data. Further, the DSP unit 19 may perform a process of subtracting the light shielding data from the non-light shielding data in the pixel data optically shielded / non-light shielding among the pixel data of the same line. In addition, the DSP unit 19 performs amplification processing by digital gain integration on the image data, processing for applying a predetermined offset level to the amplified image data, converts the image data and the line synchronization signal into parallel data, and outputs an LVDS signal. The process etc. which output as are performed.

  In the operation of the image sensor 10b shown in FIG. 7, in the analog signal processing period and the digital signal processing after the processing performed by the CDS unit 15, the digital signal processing period of the n-th pixel is the (n + 1) -th line. It overlaps with the analog signal processing period of the pixel (C period).

  The pixels of the image sensor 10b include test pixels, optical light-shielding pixels, effective pixels, and invalid pixels. A period shown as a digital signal processing period in FIG. 7 is a digital processing period for image data of effective pixels in a document image read by the image sensor 10b, and is generally one for processing pixels other than effective pixels. Digital signal processing is performed during the line period. In FIG. 7, a period other than the digital signal processing period (period without a vertical line) is a period in which the processing necessary for the image data is completed and the digital signal processing is stopped, and no digital noise is generated. Therefore, there is no superimposition of digital noise on the analog signal during this period. Note that the digital signal processing period shown in FIG. 7 includes a period in which the latch unit 18 operates and a period in which the DSP unit 19 operates.

  FIG. 8 is a timing chart showing the operation timing of the image sensor 10b shown in FIG. The image sensor 10b starts digital signal processing after completion of all analog signal processing periods. That is, the image sensor 10b completely separates the analog signal processing period and the digital signal processing period, and prevents noise due to the digital signal from being mixed into the analog signal. As described above, the imaging device 10b completely separates the analog signal processing period from the digital signal processing period even when noise due to digital processing other than data transfer occurs, such as the DSP unit 19, and the like. This prevents noise caused by digital signals from being mixed in.

  Next, an image reading apparatus and an image forming apparatus including the image sensor 10 (or the image sensor 10a and the image sensor 10b) will be described. FIG. 9 is a diagram illustrating an outline of an image forming apparatus 50 including an image reading device 60 having the image sensor 10 (or the image sensor 10a and the image sensor 10b). The image forming apparatus 50 is, for example, a copying machine or an MFP (Multifunction Peripheral) having an image reading device 60 and an image forming unit 70.

  The image reading device 60 includes, for example, the image sensor 10, an LED driver (LED_DRV) 600, and an LED 602. The LED driver 600 drives the LED 602 in synchronization with, for example, a line synchronization signal output from the timing control unit (control unit) 20. The LED 602 irradiates the original with light. The image sensor 10 receives reflected light from the document in synchronization with a line synchronization signal or the like, and a plurality of light receiving elements generate electric charges to start accumulation. The imaging element 10 outputs image data to the image forming unit 70 after performing parallel serial conversion or the like.

  The image forming unit 70 includes a processing unit 80 and a printer engine 82, and the processing unit 80 and the printer engine 82 are connected via an interface (I / F) 84.

  The processing unit 80 includes an LVDS 800, an image processing unit 802, and a CPU 11. The CPU 11 executes a program stored in a memory or the like, and controls each part of the image forming apparatus 50 such as the image sensor 10.

  The image sensor 10 outputs, for example, image data of an image read by the image reading device 60, a line synchronization signal, a transmission clock, and the like to the LVDS 800. The LVDS 800 converts received image data, a line synchronization signal, a transmission clock, and the like into parallel 10-bit data. The image processing unit 802 performs image processing using the converted 10-bit data, and outputs image data and the like to the printer engine 82. The printer engine 82 performs printing using the received image data.

10, 10a, 10b Image sensor 11 CPU
12 pixel portion 13 PGA portion 14 ADC portion 15 CDS portion 16 memory portion 18 latch portion 19 DSP portion 20, 20a, 20b timing control portion 50 image forming device 60 image reading device 70 image forming portions 100, 100a, 100b column

Japanese Patent No. 4482758

Claims (9)

A plurality of pixel units including a plurality of light receiving elements that perform photoelectric conversion;
A plurality of A / D conversion units provided for each of the pixel units so as to sequentially convert the analog signals photoelectrically converted by the plurality of light receiving elements into digital signals;
A plurality of first holding units provided for each of the pixel units so as to sequentially hold the digital signals converted by the plurality of A / D conversion units;
A plurality of second holding units that respectively receive and hold the digital signals held by the first holding unit during a period in which the A / D conversion unit does not convert the analog signals into digital signals;
An image pickup device comprising: The plurality of second holding portions are
Each of the plurality of A / D conversion units receives and holds a plurality of digital signals held by the plurality of first holding units after completing conversion of an analog signal into a digital signal for each pixel unit. The imaging device according to claim 1. A control unit that controls the digital signal held by the first holding unit to be transferred to the plurality of second holding units after the A / D conversion unit has finished converting the analog signal into the digital signal. The imaging device according to claim 1, further comprising: Each of the first holding portions is
A digital signal indicating a dark reference level of each of the plurality of light receiving elements included in the pixel unit and a digital signal indicating a signal level during light reception are held, and correlated double sampling is performed for each light receiving element. 4. The image pickup device according to claim 1, wherein the result is output to the second holding unit as a plurality of digital signals held by the first holding unit. 5. A plurality of amplifiers for amplifying each of the digital signals converted by the plurality of A / D converters;
The plurality of first holding portions are:
5. The image pickup device according to claim 1, wherein each of the digital signals amplified by the plurality of amplification units is sequentially held for each pixel unit. A digital signal processing unit that processes a plurality of digital signals held by the plurality of second holding units;
The plurality of pixel portions are
After the digital signal processing unit finishes processing the plurality of digital signals held by the plurality of second holding units, the next photoelectric conversion is performed,
The plurality of A / D converters are
6. The following conversion is performed after the digital signal processing unit finishes processing the plurality of digital signals held by the plurality of second holding units. 6. Image sensor. An image reading apparatus comprising the image pickup device according to claim 1. An image reading apparatus according to claim 7;
An image forming apparatus comprising: an image forming unit that forms an image based on image data read by the image reading apparatus. Converting each analog signal photoelectrically converted by a plurality of light receiving elements into a digital signal sequentially for each pixel unit including a predetermined number of the light receiving elements;
Sequentially holding each of the converted digital signals in a plurality of first holding units for each of the pixel units;
After converting each analog signal into a digital signal for each pixel unit, a plurality of digital signals held by each of the plurality of first holding units are transferred to a plurality of second holding units for each of the first holding units. And a process of
An imaging method including:

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