JP2008182637A - Imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem that, in a conventional imaging apparatus, when high-luminance light is made incident, a feed-through voltage of a CCD becomes high and an image signal of lightness different from that of actually photo-detected charge may be outputted. <P>SOLUTION: An imaging apparatus includes a correlative double sampling means for subtracting a feed-through voltage of relevant one of pixels from signal voltages of pixels of an imaging element constituted of a valid pixel region and an invalid pixel region. The imaging apparatus comprises a voltage holding means for holding a predetermined voltage, a switching means for switching the voltage to be subtracted by the correlative double sampling means to either the feed-through voltage of the relevant pixel or the predetermined voltage held by the voltage holding means, and a comparison means for comparatively determining whether a differential between the feed-through voltage of a pixel in the valid pixel region and the predetermined voltage held by the voltage holding means is greater than a threshold, wherein if it is determined based on a result of the comparison by the comparison means that the differential between the feed-through voltage of the pixel in the valid pixel region and the predetermined voltage held by the voltage holding means is greater than the threshold, the correlative double sampling means causes the switching means to switch to the predetermined voltage held by the voltage holding means and then subtracts it from the signal voltage. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an imaging apparatus having a correlated double sampling circuit.

In general, an imaging apparatus such as a CCD has a circuit for removing noise. For example, when outputting an image signal from a CCD, the feed-through voltage and the signal voltage are sampled for each pixel, and the noise component for each pixel is removed by subtracting the feed-through voltage from the signal voltage. A double sampling circuit is used (see, for example, Patent Document 1).
JP 2005-167790 A

  However, when high-luminance light such as sunlight is incident on the imaging device, an abnormality in the horizontal transfer signal of the CCD may occur, and the feedthrough voltage of the CCD may increase. In this case, if sampling is performed for each pixel by the correlated double sampling circuit, an image signal that is smaller than the charge actually received in the abnormal portion is generated. On the other hand, when the feedthrough voltage is lowered, the image signal becomes larger than the actually received charge.

  An object of the present invention is to provide an imaging apparatus that can extract an appropriate image signal even when high-luminance light such as sunlight is incident.

  An imaging apparatus according to the present invention includes a correlated double sampling unit that subtracts a feedthrough voltage of a pixel from a signal voltage of each pixel of an imaging element including an effective pixel area and an ineffective pixel area. Holding means, switching means for switching the voltage subtracted by the correlated double sampling means to either the feedthrough voltage of the pixel or a predetermined voltage held by the voltage holding means, and the effective pixel Comparing means for comparing whether or not the difference between the feedthrough voltage of the pixel in the region and the predetermined voltage held by the voltage holding means is larger than a threshold value, and the correlated double sampling means is a comparison result of the comparing means When the difference is larger than the threshold value, the switching means is switched to a predetermined voltage held by the voltage holding means. Characterized by subtracting from the signal voltage Te.

In particular, the voltage holding unit holds a feedthrough voltage of a pixel in the ineffective pixel region as a predetermined voltage.
Alternatively, the voltage holding unit holds an average value of feedthrough voltages of a plurality of pixels in the ineffective pixel region as a predetermined voltage.
Further, the comparison means compares the feedthrough voltages of consecutive pixels read out in the effective pixel region.

  In the image pickup apparatus of the present invention, when high-luminance light such as sunlight is incident, the correlated double sampling means uses a predetermined voltage previously held instead of the feedthrough voltage of the abnormal pixel from the signal voltage of the pixel. Since the subtraction is performed, an image signal with appropriate brightness can be always taken out.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
(First embodiment)
FIG. 1 is a block diagram of a camera 101 using an imaging apparatus 101 according to the first embodiment. In FIG. 1, the camera 100 includes an imaging device 101, a lens 102, an image processing unit 105, a monitor 106, and a CPU 107. The CPU 107 operates according to a program stored in advance and controls the entire camera 100.

  The imaging apparatus 101 includes an imaging element 103 such as a CCD image sensor and an AFE (analog front end) 104. The AFE 104 is a CDS (correlated double sampling circuit) 108 that removes noise caused by dark current of a pixel signal read from the image sensor 103, an analog gain 109 that adjusts the gain of an image signal, and an analog image signal as a digital signal. An A / D conversion 110 for conversion and a timing generation circuit 111 for controlling the timing of each part of the image sensor 103 and the AFE 104 are configured.

  Next, before describing the CDS circuit 108, the general CDS circuit 208 shown in FIG. 2 will be described first so that the features of the circuit can be easily understood. When a signal for each pixel is input from the image sensor 103, the CDS circuit 208 first holds a feedthrough voltage as a reference voltage of the pixel in the capacitor 152 by an S / H (sample hold switch) 151, and then the pixel. Is held in the capacitor 154 by S / H153. Note that SHP indicates a timing pulse for sampling the feedthrough voltage, and SHD indicates a timing pulse for sampling the signal voltage, and is given by the timing generation circuit 111 in FIG.

  Here, the signal 201 input from the image sensor 103 will be described with reference to FIG. In the signal 201, the voltage when the charge stored in the photoelectric conversion unit included in the pixel is reset is the reset voltage 202, and the photoelectrically converted charge is stored in the photoelectric conversion unit when light enters the image sensor 103. The voltage sometimes appearing at the output is the feedthrough voltage 203, and the voltage when the charge stored in the photoelectric conversion unit is read is the signal voltage 204. The feedthrough voltage is a voltage indicating how much a signal appears in the output of the pixel when the photoelectric conversion unit of the pixel holds the signal, and the signal voltage 204 for each pixel with reference to the feedthrough voltage 203. Read. The feedthrough voltage is sampled and held by the timing pulse SHP, and the signal voltage is sampled and held by the timing pulse SHD. Since the feedthrough voltage varies depending on each pixel, it appears as fixed pattern noise in the output image. Therefore, the feedthrough voltage for each pixel is held in the capacitor 152 and held in the capacitor 154 by the CDS circuit 208 as shown in FIG. The signal voltage is subtracted by the subtractor 155 and output to the analog gain 109. Thereby, fixed pattern noise can be removed from the signal of the pixel.

  However, when high-intensity incidence occurs on the image sensor 103, smear as shown in FIG. 4 occurs. FIG. 4 is a diagram showing the state of smear on the image sensor 103, where 301 is an effective pixel region, 302 is an optical black (OB) region, 303 is a high luminance portion, and 304 is smear. The high-intensity portion 303 incident on the effective pixel region 301 causes smear 304 including the OB region 302.

  Here, the pixel configuration of the image sensor 103 will be described with reference to FIG. The same reference numerals as those in FIG. 4 denote the same components. In the effective pixel region 301 of the image sensor 103, a plurality of unit pixels including a photodiode that converts light from an object into an electric signal, a transistor that reads out a photoelectrically converted signal, and the like are arranged. Unit pixels are also arranged in the region 302. For example, a signal read out from the unit pixel 401 in the effective pixel region 301 to the vertical signal line 403 and a signal read out from the unit pixel 402 in the OB region 302 to the vertical signal line 405 are horizontal signal output units 404 for each row. Is output from. Note that the OB region 302 is around the image sensor 103, and the unit pixels 402 in the OB region 302 may have a plurality of columns of unit pixels as shown in FIG.

  Next, signals for one row in the horizontal direction read by the horizontal output unit 404 will be described with reference to FIG. FIG. 6 is a diagram showing a part of the waveform of a signal for one row in the horizontal direction. The left side of the alternate long and short dash line 501 shows the readout signal of each pixel in the OB region 302 and the right side shows the readout signal of each pixel in the effective pixel region 301. ing. Reference numeral 502 denotes the position of the feedthrough voltage. Although normal feedthrough voltages are obtained in the normal waveform portions 503 and 505, the feedthrough voltages of the two pixels 506 and 507 in the abnormal waveform portion 504 are greatly reduced. That is, since smear 304 as shown in FIG. 4 is generated, signal leakage occurs and the feedthrough voltage of pixels 506 and 507 is increased. For this reason, the signal output of the pixel when this feedthrough voltage is used as a reference is reduced. Conversely, when the feedthrough voltage is decreased, the signal output of the pixel is increased.

As described above, in a general CDS circuit 208, when a high-luminance incidence occurs, the image becomes dark or conversely bright, which greatly affects AE determination and the like. In the CDS circuit 108 of FIG. 1 of the present embodiment, the above problem is solved by using the feedthrough voltage of the pixels in the OB region when the feedthrough voltage becomes abnormal.
Next, the CDS circuit 108 in this embodiment will be described in detail with reference to FIG. The same reference numerals as those in FIG. 2 denote the same elements. The CDS circuit 108 in FIG. 7 further includes an S / H 251, a capacitor 252, a comparison unit 253, and a switch 254 in addition to the CDS circuit 208 in FIG. 2. For example, when a signal for one line in the horizontal direction is read out, first, a timing pulse SHP2 is applied to S / H 251 and the feedthrough voltage of the pixel in the OB region 302 is held in the capacitor 252. Next, in the effective pixel region 301, the feedthrough voltage of each pixel is held in the capacitor 152, and the signal voltage is held in the capacitor 154. The timing pulse SHP2 is given by the timing generation circuit 111 in FIG.

  Next, the operation of the CDS circuit 108 will be described using the timing chart of FIG. FIG. 8 shows a part of a horizontal output waveform for one line output from the image sensor 103. The horizontal output waveform for several pixels extending from the OB area to the effective pixel area, and each of the horizontal output waveforms shown in FIG. The timing relationship between the timing pulses SHP2, SHP, and SHD and the output of the comparison unit 253 is shown. Reference numerals 801 to 808 each denote a waveform range of one pixel, and hereinafter, the expression of the waveform of the pixel 801 indicates the waveform of one pixel.

  First, the feedthrough voltage of the waveform of the pixel 801 in the OB region is sampled by the timing pulses SHP and SHP2 and held in the capacitors 152 and 252 in FIG. Subsequently, the feedthrough voltage of the waveform of the pixel 802 in the effective pixel region is sampled by the timing pulse SHP and held in the capacitor 152 of FIG. The comparison unit 253 compares whether or not the difference between the voltage held in the capacitor 252 and the voltage held in the capacitor 152 is greater than or equal to a predetermined value. If the difference is greater than or equal to a predetermined value, the switch 254 is switched to the S / H 251 side and the voltage held in the capacitor 252 is output to the subtractor 155. If the difference is smaller than the predetermined value, the switch 254 is switched to the S / H 151 side. The voltage held at 152 is output to the subtractor 155. Note that the signal voltage of the waveform of the pixel 802 is sampled by the timing pulse SHD, and the voltage held in the capacitor 154 is also output to the subtractor 155.

  When the comparison of the feedthrough voltage of the waveform of the pixel 802 is completed, the feedthrough voltage of the waveform of the next pixel 803 is sampled by the timing pulse SHP and held in the capacitor 152. As in the case of the waveform of the pixel 802, the comparison unit 253 compares whether or not the difference between the waveform of the pixel 801 in the OB region held in the capacitor 252 and the feedthrough voltage is equal to or greater than a predetermined value, and the switch 254 To control. Thereafter, similarly, each time the feedthrough voltage of the waveform from the pixel 804 to the pixel 808 is sampled by the timing pulse SHP in order and held in the capacitor 152, the comparison unit 253 sets the feedthrough voltage of the waveform of the pixel 801 in the OB region. The switch 254 is controlled by comparing whether or not the difference is equal to or greater than a predetermined value.

In FIG. 8, since the feedthrough voltage of the waveform from the pixel 802 to the pixel 804 is stable and the difference from the feedthrough voltage of the waveform of the pixel 801 in the OB region is small, the comparison unit 253 sets the switch 254 to S in the period T1. A signal is output so as to switch to the / H151 side.
However, when there is a high luminance incident, the feedthrough voltage becomes unstable like the waveform of the pixel 805. In such a state, the difference between the feedthrough voltage of the waveform of the pixel 801 in the OB region and the feedthrough voltage of the waveform of the pixel 805 increases and exceeds a predetermined value. A signal is output so as to switch to the H251 side. Similarly, since the feedthrough voltage of the waveform of the pixel 806 is also unstable, the comparison unit 253 outputs a signal so that the switch 254 is switched to the S / H 251 side. That is, in the period T2, the comparison unit 253 outputs a signal so that the switch 254 is switched to the S / H 251 side. Since the waveforms of the next pixel 807 and pixel 808 are stable, the difference from the feedthrough voltage of the waveform of the pixel 801 in the OB region is reduced, and the comparison unit 253 switches the switch 254 to the S / H 151 side in the period T3. The signal is output as follows. In the output waveform of the comparison unit 253 in FIG. 8, the switch 254 is switched to the S / H 151 side at a low level, and the switch 254 is switched to the S / H 251 side at a high level.

  As described above, the comparison unit 253 compares the difference between the feedthrough voltage of the waveform of the pixel 801 in the OB region and the feedthrough voltage of the pixel in order one pixel at a time. Instead of the feedthrough voltage, the switch 254 is controlled so that the subtractor 155 subtracts the feedthrough voltage of the waveform of the pixel 801 in the OB region from the signal voltage of the pixel. In the above description, the operation for one line has been described, but the same operation is repeated for one screen of the image sensor 103.

  As described above, in the imaging apparatus 101 according to the present embodiment, when the fluctuation of the feedthrough voltage of the pixel waveform is less than the predetermined value, the switch 254 is switched to the S / H 151 side and held by the capacitor 152. 2 is output to the subtractor 155, the same operation as the conventional CDS circuit 208 described in FIG. 2 is performed. However, when the fluctuation of the feedthrough voltage of the pixel waveform is equal to or greater than a predetermined value, the switch 254 is switched to S / H251 switching to the voltage held by the capacitor 252, that is, the feedthrough voltage of the waveform of the pixel in the OB area is output to the subtractor 155, so that the pixel signal output when the feedthrough voltage is unstable such as a smear portion Fluctuation can be reduced, and an image signal with appropriate brightness can be extracted.

(Second Embodiment)
Next, an imaging apparatus according to the second embodiment will be described. The imaging apparatus of the present embodiment has the same configuration as the imaging apparatus 101 except for the CDS circuit 108 in the first embodiment shown in FIG. Here, the CDS circuit 308 different from the first embodiment will be described with reference to FIG. The same reference numerals as those in FIG. 7 denote the same elements.

  The CDS circuit 308 of FIG. 9 includes eight S / H circuits S / H 251 to S / H 351 of the CDS circuit 108 of the first embodiment, and the outputs of the respective S / H circuits pass through resistors RA1 to RA8. Are added to each other and output to the comparison unit 253 via the amplifier 353. Eight timing pulses SHP2 to SHP9 are input to the eight S / H circuits, respectively. The eight timing pulses SHP2 to SHP9 are output from the timing generation circuit 111 at a timing shifted by one pixel, for example, and hold the feedthrough voltages of the pixels in the eight consecutive OB regions. In FIG. 9, a part of the eight S / H circuits is omitted, and the same circuit including the S / H 351, the capacitor 352, and the resistor RA8 is provided. The gain of the amplifier 353 is determined by the ratio of the feedback resistor RA0 and the combined resistor RA1 to RA8. In this embodiment, the gain is set to 1/8. That is, the output of the amplifier 353 is an average value of the feedthrough voltages in the OB region held in the capacitors of the eight S / H circuits.

  On the other hand, since the comparison unit 253 operates in the same manner as in the first embodiment, the difference between the average value of the feedthrough voltage of the waveform of the plurality of pixels in the OB region and the feedthrough voltage of the pixel is sequentially compared pixel by pixel. When the difference is equal to or larger than a predetermined value, the switch 254 is used to subtract the average value of the feedthrough voltages of the waveforms of the plurality of pixels in the OB region from the signal voltage of the pixel by the subtracter 155 instead of the feedthrough voltage of the pixel. To control.

  As described above, the imaging device 101 according to the present embodiment uses the average value of the feedthrough voltages of the waveforms of the plurality of pixels in the OB region when the fluctuation of the feedthrough voltage of the pixel waveform is equal to or greater than a predetermined value. Compared to the first embodiment using only one pixel in the OB region, the operation can be performed more stably, and an image signal with appropriate brightness can be extracted even when smear or the like occurs.

  In the present embodiment, the reference value of the feedthrough voltage is an average value for 8 pixels, but the same effect can be obtained if there are a plurality of pixels. In this embodiment, a simple averaging circuit is configured using resistors and amplifiers. However, any circuit that takes in and averages the feedthrough voltages of a plurality of pixels in the OB region can be realized in the same manner. . Furthermore, in this embodiment, the feedthrough voltage of the continuous pixels in the OB area is used. However, the pixels in the OB area to be averaged may not be continuous, such as every other pixel.

1 is a block diagram of a digital camera 101 according to a first embodiment. 3 is a block diagram showing a configuration of a CDS circuit 208. FIG. 4 is a timing chart for explaining the operation of the CDS circuit 208. It is an auxiliary figure for explaining smear. 3 is an auxiliary diagram illustrating a configuration of a pixel of the imaging device 103. FIG. It is an auxiliary | assistant figure for demonstrating the output waveform at the time of smear generation | occurrence | production. 1 is a block diagram illustrating a configuration of a CDS circuit 108 of an imaging apparatus 101 according to a first embodiment. 3 is a timing chart for explaining the operation of the CDS circuit 108. It is a block diagram which shows the structure of the CDS circuit 308 of the imaging device 101 which concerns on 2nd Embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 101 ... Imaging device 102 ... Lens 103 ... Imaging element 104 ... AFE
105: Image processing unit 107: CPU
108, 208, 308 ... CDS circuit 109 ... analog gain 111 ... timing generation circuit 151,153, 251,351 ... S / H (sample hold circuit)
152,154,252,352 ... capacitor 155 ... subtractor 253 ... comparator 254 ... switch 353 ... amplifier

Claims (4)

In an imaging apparatus having correlated double sampling means for subtracting the feedthrough voltage of the pixel from the signal voltage of each pixel of the imaging device composed of an effective pixel area and an ineffective pixel area,
Voltage holding means for holding a predetermined voltage;
Switching means for switching a voltage to be subtracted by the correlated double sampling means to either a feedthrough voltage of the pixel or a predetermined voltage held by the voltage holding means;
Comparing means for comparing whether or not a difference between a feedthrough voltage of a pixel in the effective pixel region and a predetermined voltage held by the voltage holding means is larger than a threshold value,
The correlated double sampling means switches the switching means to a predetermined voltage held by the voltage holding means and subtracts it from the signal voltage when the difference is larger than the threshold based on the comparison result of the comparison means. An imaging apparatus characterized by the above. The imaging apparatus according to claim 1,
The imaging apparatus, wherein the voltage holding unit holds a feedthrough voltage of a pixel in the ineffective pixel region as a predetermined voltage. The imaging apparatus according to claim 1,
The voltage holding unit holds an average value of feedthrough voltages of a plurality of pixels in the ineffective pixel region as a predetermined voltage. In the imaging device according to any one of claims 1 to 3,
The imaging device characterized in that the comparison means compares the feedthrough voltages of consecutive pixels read out in the effective pixel region.

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