JP3792995B2 - Imaging device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image pickup apparatus, and more particularly to an image pickup apparatus suitably used for a solid-state image pickup apparatus in which an AD (analog / digital) converter is built in a solid-state image pickup element.
[0002]
[Prior art]
As an image pickup apparatus in which an AD converter is built in each pixel, there is an image pickup apparatus disclosed in JP-A-6-205307. In this imaging apparatus, integrated pixel voltage is compared with a comparator to obtain AD data.
[0003]
As an imaging device in which an AD converter is formed for each pixel column, there is an A250 mW, 600 Frames / s 1280 × 720 pixel 9b CMOS Digital Image Sensor in ISSCC99 SESSION17 PAPERWA17.7. In this example, AD data is obtained by comparing a signal read for each pixel column with a reference voltage of a comparator.
[0004]
In the ISSCC 2000 Session 6 Image Sensors PAPER MP 6.4 A 60mW 10b CMOS Image Sensor with Column-to-Column FPN Reduction, AD data is obtained by comparing the pixel signal with the ramp signal.
[0005]
Thus, in the conventional example, the pixel signal is directly AD converted, or the pixel signal is subjected to AD conversion after amplification control by an analog gain circuit.
[0006]
[Problems to be solved by the invention]
When the pixel signal is directly AD-converted, if the image is picked up in a dark condition, the signal level is small with respect to the rated input voltage of AD, so that the AD quantization error increases with SN degradation.
[0007]
In addition, in order to increase the signal voltage, gain control is performed by an analog gain circuit. In this case, the analog gain error for each column becomes streak noise in the vertical direction on the screen, and the image quality is significantly reduced.
[0008]
In addition, when considering colorization, the dynamic range for each color changes depending on the light source at the time of photographing, and the performance of the AD converter cannot be fully exhibited. For example, when the color temperature of the light source is low, the red pixel signal becomes large, and when the color temperature is high, the blue pixel signal becomes large, and the AD input voltage is limited. In addition, the pixel size of an image pickup device has been reduced year by year, and the provision of a pixel noise removal circuit, an analog gain circuit, a comparator, and a DA converter for each column has a drawback of increasing the chip size. In addition, it is very difficult to design these circuits with the accuracy between circuits at a pitch for each column. In any case, this becomes vertical streak noise on the screen.
[0009]
(Object of invention)
An object of the present invention is to improve the performance, function, and cost of an AD converter in an image sensor.
[0010]
[Means for Solving the Problems]
The imaging apparatus of the present invention includes a plurality of pixels each having a photoelectric conversion unit, an amplification unit that receives a photoelectric conversion signal from the photoelectric conversion unit, and a reset unit that resets the input unit of the amplification unit,
The first output and the second the output to that hand stage output from said amplifying means when the photoelectric conversion signal is in a state of being input to the input portion of the amplifying means when the input unit is in a reset state And an imaging device comprising:
A comparator for detecting a differential output between the output of the amplifying means and the first output; and a signal output from the digital-analog converting means in response to a digital value output from a counter that counts the output of the comparator By controlling the output of the amplifying means by outputting to the main electrode of the amplifying means, the input level of the comparator is changed from the differential output between the first output and the second output until it becomes substantially zero, While the input level of the comparator is changing, the counter performs a counting operation and outputs the digital value changed from the start to the stop of the counter.
[0012]
【Example】
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0013]
FIG. 1 shows an overall configuration diagram of an embodiment of an imaging apparatus of the present invention. In FIG. 1, reference numeral 100 denotes an optical system including a diaphragm and an imaging lens, and 110 denotes a solid-state imaging device. The solid-state imaging device 110 includes a pixel unit 111 in which pixels including a photoelectric conversion unit, a pixel amplifier, and a control switch are arranged in an area, a vertical scanning circuit unit 112 that scans the pixel unit 111, and a pixel signal from the pixel unit 111 An analog / digital signal processing circuit unit 113 comprising a noise removal circuit, a gain control circuit, and an AD (analog / digital) conversion circuit, a horizontal scanning circuit unit 114 for controlling the analog / digital signal processing circuit 113, and a timing generator (TG) ) Part 115.
[0014]
The timing generator (TG) unit 115 controls the pixel unit and each circuit unit in the solid-state imaging device 110 by a pulse from the camera CPU 130. AD data from the solid-state image sensor 110 is input to the camera signal processing system 120.
[0015]
The camera signal processing system 120 includes a camera signal processing circuit unit 121, an AE (exposure) information detection circuit unit 122, and a white balance information detection circuit unit 123.
[0016]
The camera signal processing circuit unit 121 performs luminance signal processing and color signal processing from AD data. The AE (exposure) information detection circuit unit 122 detects the image signal level from the luminance information of the camera signal processing circuit unit 121 and generates AE (exposure) information. Based on the AE information result, the range of the ramp-shaped reference voltage of a DA (digital / analog) conversion circuit, which will be described later, of the analog-digital signal processing circuit unit 113 is switched, and the pixel signal is controlled to be the optimum value of the AD input voltage range. As will be described later, when the pixel signal is small, the amplitude of the ramp-shaped reference voltage is reduced, and when the pixel signal is large, the amplitude of the ramp-shaped reference voltage is switched to the larger one to increase the AD input voltage as much as possible. Next, the white balance (WB) information detection circuit unit 123 compares the levels of the color signals R, G, and B of the camera signal processing circuit unit 121, and controls the result of the lamp-like reference voltage for each color.
[0017]
In this way, whether the ramp-shaped reference voltage is controlled by the AE information or the WB information can be determined by the specification of the imaging apparatus. You may control individually, you may control the whole level control with AE information, and you may control the signal level for every color with WB information. This control increases the input signal level of the AD converter, so that SN does not deteriorate, and the quantization error due to the AD converter can be reduced.
[0018]
Reference numeral 130 denotes a camera CPU that controls the camera unit of the solid-state imaging device. Reference numeral 140 denotes a recording / reproducing system, and 150 denotes a display system.
[0019]
FIG. 2 shows a schematic circuit diagram of the pixel unit 111, the analog / digital signal processing circuit unit 113, and the vertical scanning circuit unit 112. In FIG. 2, 11 is a comparator, 10 is a counter, and 9 is a DA converter.
[0020]
FIG. 3 is an equivalent circuit block diagram showing one pixel of the pixel portion and a noise removal / analog / digital conversion circuit portion. DA output range switching information for switching the range of the ramp-shaped reference voltage is input to the DA converter.
[0021]
In FIG. 3, 21 is a photoelectric conversion element, 22 is a transfer transistor that transfers the signal charge of the photoelectric conversion element 21 to the gate of the amplification transistor 23, 24 is a pixel selection transistor, 25 is a reset transistor, and 26 is an output current of the amplification transistor 23. Is stored as a voltage by the switch 27 and the storage capacitor 28, and the current source transistor 31 outputs the voltage while converting the voltage into a current, and 31 is output via the output current of the current source transistor 26 and the pixel selection transistor 24. A comparator that detects a difference from the output current of the amplification transistor 23, a counter 30 that counts the output of the comparator 31, and a voltage 29 that is supplied from the digital signal output from the counter 30 to the source (main electrode) terminal of the amplification transistor 23. It is a DA converter that outputs DA output range switching information is input to the DA converter 29, and the range of the ramp-shaped reference voltage output from the DA converter is switched to control the pixel signal to the optimum value of the AD input voltage range.
[0022]
With the above configuration, a method for obtaining a signal voltage based on signal charges from the photoelectric conversion element after the amplification transistor is reset will be described as an example with reference to the timing chart of FIG. Here, the transistors 22, 23, 24, and 25 in FIG. 3 will be described as PMOS transistors, and the transistor 26 will be described as an NMOS transistor. The DA converter 29 is set to output a certain high potential (V _HD ). Assume that the counter 30 is reset and is not counting. The terminal φR is set to “L” level (pulse 201), the reset transistor 25 is turned on, and the gate terminal of the amplification transistor 23 is reset to a predetermined potential. At the same time, the terminal φX is set to the “L” level (pulse 202), the selection transistor 24 is turned ON, and the switch 27 is also turned ON. The output current when the amplification transistor 23 is reset is stored in the storage capacitor 28 in the form of a gate voltage generated when the gate and drain of the transistor 26 are short-circuited (a comparison reference voltage is stored). Thereafter, the transistors 25 and 24 and the switch 27 are turned OFF, and the signal charge corresponding to the light input to the photoelectric conversion element is set to the “L” level at the terminal φT (pulse 203), whereby the transfer transistor 22 is turned ON and the amplification transistor. 23 is transferred to the gate terminal. If the gate potential at this time is a voltage lower than that at the time of resetting, the output current of the amplifying transistor 23 is correspondingly larger than that at the time of resetting. On the other hand, the transistor 26 receives the voltage of the storage capacitor 28 and outputs a current when the amplification transistor 23 is reset. When the terminal φX is set to “L” level (pulse 204) and the transistor 24 is turned on again, the input potential of the comparator 31 rises to a certain high potential (V _H ). Thereafter, the counter 30 is operated to increase the value of the digital output, and the output voltage of the DA converter 29 receiving it is gradually decreased (the DA converter 29 generates an analog output voltage having a negative polarity with respect to the digital input signal). Assume that.) At a certain point in time, the output current of the amplification transistor 23 and the output current of the transistor 26 become equal, and the input voltage of the comparator 31 decreases rapidly. Therefore, the change is detected and the counting operation of the counter 30 is stopped.
[0023]
The digital value changed from the count start to the stop of the counter 30 becomes a value equal to the difference between the gate potential of the amplification transistor 23 and the potential when the signal charge is transferred. In this way, AD conversion corresponding to the difference is performed.
[0024]
FIG. 2 is an example in which the configuration example of FIG. 3 is applied when the photoelectric conversion units are arranged two-dimensionally in three rows and three columns. Photoelectric conversion element 1 (1-1-1, 1-1-2,...), Transfer transistor 2 (2-1-1, 2-1-2,...), Amplification transistor 3 (3-1-1, 3) , 1-2,..., Pixel selection transistor 4 (4-1-1, 4-1-2,...), Reset transistor 5 (5-1-1, 5-1-2,...), Constant current transistor 6 (6-1, 6-2,...), A switch 7 (7-1, 7-2,...) For taking in and storing a voltage generated between the gate and source of the transistor 6 by the output current of the amplification transistor 3. ) And capacitors 8 (8-1, 8-2,...) And vertical signal lines 12 (12-1, 12-2,...) Comparators 11 (11-1, 11-2,. ), A counter 10 (10-1, 10-) for supplying a digital signal to the DA converter 9 (9-1, 9-2,...). , ...) such configuration is the same as that of FIG.
[0025]
In order to obtain the output of the pixels in the first row, the drive line 15-1 from the vertical shift register 15 is set to the “L” level, and the reset transistor 5 (5-1-1, 5-1-2, 5-1-3. ), And then the drive line 14-1 is set to “L” level to turn on the pixel selection transistor 4 (4-1-1, 4-1-2, 4-1-3) to amplify the reset state. The output current of the transistor 3 (3-1-1, 3-1-2, 3-1-3) is output to the vertical signal line 12 (12-1, 12-2, 12-3), and the drive line 13 is output. The switch 7 (7-1, 7-2, 7-3) is turned on by setting it to the “H” level, and the output current in the reset state is supplied to the transistor 6 in the capacitor 8 (8-1, 8-2, 8-3). Holds the voltage generated between the gate and source when supplied.
[0026]
Thereafter, the counting operation of the counter 10 (10-1, 10-2, 10-3) is started in the same manner as the operation of FIG. 3, and the output of the DA converter 9 (9-1, 9-2, 9-3) is started. The voltage is decreased, the potential fluctuation of the vertical signal line 12 is detected by the comparator 11, the operation of the counter 10 is stopped, and the AD conversion is completed with the digital value change from the count start to the stop as a digital output. FIG. 9 is a circuit diagram showing a case where AE information is used as DA output range switching information in the DA converter 9 (9-1, 9-2) . Reference numeral 40 denotes one pixel. In FIG. 9, the comparator is omitted (the comparator is also omitted in FIGS. 11 and 12).
[0027]
FIG. 5 shows a circuit diagram of a configuration example of the DA converter shown in FIGS.
[0028]
This configuration example is composed of 2n resistors and n switches in the R-2R ladder system and n bits AD. The resistance value of the ladder is changed by switching the reference voltage, and the ratio between the resistance value and the resistance value _{Rf of the} amplifier becomes the DA output voltage. The method of the DA converter is not limited to the R-2R ladder method, but may be a capacitor array method.
[0029]
As a method of switching the DA output range, FIG. 6 shows a method of switching to a plurality of reference voltages (E1, E2, E3) with a switch, and FIG. 7 shows switching of a feedback resistance _{Rf of the} amplifier and a ladder resistance value and a resistance value _Rf. The method of changing the ratio with is shown.
[0030]
FIG. 8 is a characteristic diagram showing the DA output when the DA output range is switched by the method of FIG. 6 or FIG. The comparison reference voltage of the DA converter may have KNEE characteristics or γ characteristics. By doing so, the imaging apparatus can be simplified. In the case of a color imaging device, it is preferable to share the white balance and γ and Knee functions described later.
[0031]
FIG. 14 is a characteristic diagram of another embodiment of the present invention. Depending on the video information of the subject, the conversion range of AD sensor output signal level, the conversion weight, or the digitized output bit number can be changed depending on the application.
[0032]
In FIG. 14, in the case of (A), AD conversion is linearly performed between the sensor output signal DA output voltage range V1 and V2.
[0033]
In (B), AD conversion is similarly performed between the DA output voltage ranges V3 and V4. However, the DA output voltage range is small, and this is a case where the reflected light amount range of the subject is narrow, for example, document information and barcode information. Suitable for detection.
[0034]
(C) shows a case where the DA output voltage is converted non-linearly. For example, when the DA output voltage is in the vicinity of V5, the voltage range is reduced and increased in the vicinity of V6, whereby AD conversion can be performed by performing γ conversion processing on the sensor output signal. Since the sensor signal is directly digitally converted, there is no increase in quantization error or noise, and high quality image quality can be obtained.
[0035]
(D) is a case where the number of bits of the DA output voltage in (A) is reduced, and can correspond to an application in which document information or AD conversion only requires a low number of bits.
[0036]
FIG. 10 is a circuit diagram showing another embodiment of the solid-state imaging device of the present invention. Although the embodiment described with reference to FIGS. 2 to 4 described above shows an embodiment in which the variable range of the output level of the pixel signal is switched based on the DA output range switching information, this embodiment has a variable comparison reference voltage. The Example which switched the range based on DA output range switching information is shown.
[0037]
As shown in FIG. 10 , the pixel signal from the pixel 40 is compared with the DA output voltage from the DA converter 43 for each column by the comparator 42, and converted into AD data by the AD converter 44. The DA converter 43 receives DA output range switching information (AE information) for switching the range of the ramp-shaped reference voltage. Reference numeral 41 denotes a load which forms a source follower circuit with the transistor of the pixel 40.
[0038]
FIG. 11 is a circuit diagram showing an example of colorization. This embodiment shows a case where it is applied to the embodiment described with reference to FIGS. The variable range of the output level of the pixel signal is switched based on white balance (WB) information.
[0039]
In the pixel 40, R, G, and B color filters are arranged in a mosaic pattern. For each pixel row (... (N + 1) row, n row), a WB information switching signal is input to the DA converter 9 (9-1, 9-2, 9-3) corresponding to each color, and corresponding to each color. By switching the DA output range, the AD conversion range of each color signal is switched. This WB information is based on the WB detection signal of the camera signal processing system 120 in FIG. The AD conversion range of the B signal and the G signal is sequentially switched in the nth row, and the AD conversion range of the G signal and the R signal is sequentially switched in the (n + 1) th row.
[0040]
FIG. 12 shows an embodiment in which the present invention is applied to color sequential output.
[0041]
Similarly to FIG. 11, R, G, B color filters are arranged in a mosaic pattern in the pixel 40. In the nth row, first, the pixel signal of the B signal column is AD-converted by the column switching control pulse Codd and output as AD data. Next, the pixel signal of the G signal column is AD converted by the column switching control pulse Ceven and output as AD data. At this time, the WB information switching signal is input to the DA converter 9 (9-1, 9-2, 9-3) corresponding to each color, and the DA output range is switched corresponding to each color, whereby each color signal. The AD conversion range is switched.
[0042]
In this way, analog / digital circuits can be reduced by performing AD conversion for each arbitrary column. Thus, reducing the circuit scale can improve the circuit characteristics or reduce the chip area.
[0043]
Note that the pixel portion may be provided with one common amplifier for a plurality of photoelectric conversion portions. FIG. 13 is a diagram illustrating an example of a common amplifier pixel. As shown in FIG. 13, a11, a12, a21, and a22 are photodiodes that serve as photoelectric conversion units of the respective pixels, MSF is an amplifying transistor that serves as a common amplifier, and MTX1 to MTX4 share signal charges accumulated in the photodiodes. A transfer transistor for transferring to the input section of the amplifier, MRES is a reset transistor for resetting the input section of the common amplifier, and MSEL is a selection transistor for selecting a common amplifier pixel. The transistors MSF and MSEL constitute a source follower circuit. In such a common amplifier pixel, signals from four photodiodes are output via the common amplifier, and one pixel is constituted by four pixels. One pixel includes a photodiode and a transfer transistor, and includes a part of a common circuit including a common amplifier, a reset transistor, and a selection transistor. A G filter is arranged for the photodiodes a11 and a22, a B filter is arranged for the photodiode a21, and an R filter is arranged for the photodiode a12.
[0044]
【The invention's effect】
As described above, according to the present invention, the AD input voltage range can be set according to the image signal level, so that the performance of the AD conversion means can be fully extracted.
[0045]
In addition, since the analog-digital conversion circuit scale can be reduced, the yield can be improved and the cost can be reduced. By adopting a feedback configuration of the pixel and the AD conversion means, it is possible to achieve a high AD accuracy, and in particular, a design that does not require an analog gain circuit.
[0046]
In addition, the signal processing apparatus can be simplified by sharing the white balance and the γ and KNEE functions.
[Brief description of the drawings]
FIG. 1 is an overall configuration diagram of an embodiment of an imaging apparatus according to the present invention.
2 is a schematic circuit diagram of a pixel unit, a noise removal / analog / digital conversion circuit unit, and a vertical scanning circuit unit of the imaging apparatus of FIG. 1;
FIG. 3 is an equivalent circuit block diagram showing one pixel of a pixel unit and a noise removal / analog / digital conversion circuit unit.
4 is a timing chart for explaining the operation of the circuit of FIG. 3;
5 is a circuit diagram of a configuration example of the DA converter shown in FIGS. 2 and 3. FIG.
FIG. 6 is a diagram showing a method of switching to a plurality of reference voltages (E1, E2, E3) with a switch.
7 is a diagram showing a method of switching the Kikan resistance R _f of the amplifier switches the ratio of the resistance value of the ladder and the resistance value R _f.
FIG. 8 is a characteristic diagram showing DA output when the DA output range is switched by the method of FIG. 6 or FIG.
FIG. 9 is a circuit diagram showing a case where AE information is used as DA output range switching information in a DA converter.
FIG. 10 is a circuit diagram showing another embodiment of the solid-state imaging device of the present invention.
FIG. 11 is a circuit diagram showing an example of colorization.
FIG. 12 is a circuit diagram showing an embodiment in which the present invention is applied to color sequential output.
FIG. 13 is a circuit diagram illustrating an example of a common amplifier pixel.
FIG. 14 is a characteristic diagram of another embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 100 Optical system 110 Solid-state image sensor 111 Pixel part 112 Vertical scanning circuit part 113 Analog digital signal processing circuit part 114 Horizontal scanning circuit part 115 Timing generator (TG) part 120 Camera signal processing system 121 Camera signal processing circuit part 122 AE (exposure) Information detection circuit unit 123 White balance information detection circuit unit 130 Camera CPU
140 Recording / playback system 150 Display system

Claims (5)

A plurality of pixels each having a photoelectric conversion unit, an amplification unit to which a photoelectric conversion signal from the photoelectric conversion unit is input to the input unit, and a reset unit to reset the input unit of the amplification unit;
The first output and the second the output to that hand stage output from said amplifying means when the photoelectric conversion signal is in a state of being input to the input portion of the amplifying means when the input unit is in a reset state And an imaging device comprising:
A comparator for detecting a differential output between the output of the amplifying means and the first output; and a signal output from the digital-analog converting means in response to a digital value output from a counter that counts the output of the comparator By controlling the output of the amplifying means by outputting to the main electrode of the amplifying means, the input level of the comparator is changed from the differential output between the first output and the second output until it becomes substantially zero, While the input level of the comparator is changing, the counter performs a counting operation and outputs the digital value changed from the start to the stop of the counter. The imaging apparatus according to claim 1, wherein a digital analog output range of the digital-analog conversion unit is changed based on AE information. The imaging apparatus according to claim 1, wherein a digital-analog output range of the digital-analog conversion unit is changed for each color signal of the second output based on white balance information. The imaging device according to any one of claims 1 to 3 , wherein the first and second outputs are output for each column from the plurality of pixels.
An image pickup apparatus comprising a set of the comparator, the counter, and the digital-analog converting means for each of a plurality of columns. 5. The imaging apparatus according to claim 4 , wherein one set of the comparator, the counter, and the digital / analog converting means is used for each color signal.

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