JP2001346106A - Image pickup device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image pickup device that can switch a dynamic range of an input signal to an analog/digital conversion means. SOLUTION: The image pickup device provided with pixels and an analog/ digital conversion means 113 that outputs a digital signal on the basis of a result of comparison between a level of a pixel signal from the pixels and a comparison reference voltage, relatively varies the pixel signal from the pixels and the comparison reference voltage and switches at least either of a variable range of the signal level of the pixel signal and a variable range of the comparison reference voltage to switch the dynamic range of the input signal given to the analog/digital conversion means.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an imaging device,
In particular, the present invention relates to an imaging device suitably used for a solid-state imaging device in which an A / D (analog-digital) converter is built in a solid-state imaging device.

[0002]

2. Description of the Related Art As an image pickup apparatus having an A / D converter for each pixel, there is an image pickup apparatus disclosed in Japanese Patent Application Laid-Open No. 6-205307. In this imaging device, the integrated pixel voltage is compared with a comparator to obtain AD data.

[0003] Further, as an imaging device in which an AD converter is formed for each pixel column, ISSCC99 SESSION17 PAPERWA17.7 has A25.
0mW, 600Frames / s 1280 × 720pixel 9b CMOS Digital Ima
There is a ge Sensor. In this example, AD data is obtained by comparing a signal read for each pixel column with a reference voltage of a comparator.

[0004] ISSCC 2000 Session 6 IMAGE Sensors
PAPER MP 6.4 A 60mW 10b CMOS Image Sensor with Co
In the lumen-to-column FPN Reduction, AD data is obtained by comparing a pixel signal with a ramp signal.

As described above, in the conventional example, the pixel signal is directly AD-converted, or the analog signal is subjected to A / D conversion after the amplification control of the pixel signal.

[0006]

In the case of direct AD conversion of a pixel signal, if the image is picked up in a dark condition, the signal level is small with respect to the rated input voltage of the AD, so that the AD deteriorates and the AD decreases.
Will have a large quantization error.

Further, in order to increase the signal voltage, gain control is performed by an analog gain circuit. In this case, an analog gain error for each column is displayed as vertical streak noise on a screen, and image quality is significantly reduced.

[0008] When colorization is considered, the dynamic range of 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 increases, and when the color temperature is high, the blue pixel signal increases, thereby limiting the AD input voltage. Further, the pixel size of the image sensor is reduced year by year, and providing a pixel noise removing circuit, an analog gain circuit, a comparator, and a DA converter for each column has a disadvantage that the chip size becomes large. Also, it is very difficult to design those circuits at the pitch of each column with the precision between the circuits being aligned. In any case, these become vertical streak noises on the screen.

(Object of the Invention) It is an object of the present invention to improve the performance, function, and cost of an AD converter in an image sensor.

[0010]

An image pickup apparatus according to the present invention comprises: a plurality of pixels; an analog-to-digital converter for outputting a digital signal based on a comparison result between a pixel signal level from the pixel and a comparison reference level; Wherein the analog-to-digital conversion means relatively varies a level of a pixel signal from the pixel to be compared and the comparison reference level, and changes a variable range in which the variation is performed. An imaging apparatus characterized by the following.

In the image pickup apparatus according to the present invention, a common analog digital signal processing means is provided for each of a plurality of columns in an image pickup apparatus which performs analog-to-digital signal processing on signals output from a plurality of pixels for each column and outputs the processed signals. An imaging apparatus characterized in that:

[0012]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 shows an overall configuration diagram of an embodiment of an imaging apparatus according to the present invention. In FIG. 1, reference numeral 100 denotes an optical system including a stop 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, and a vertical scanning circuit unit 11 that scans the pixel unit 111.
2. An analog-to-digital signal processing circuit unit 113 including a noise removal circuit, a gain control circuit, and an AD (analog-to-digital) conversion circuit for correcting variations in pixel signals from the pixel unit 111, and a horizontal circuit for controlling the analog-to-digital signal processing circuit unit 113. It comprises a scanning circuit section 114 and a timing generator (TG) section 115.

Timing generator (TG) section 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 imaging device 110 is input to the camera signal processing system 120.

The camera signal processing system 120 comprises a camera signal processing circuit 121, an AE (exposure) information detecting circuit 122, and a white balance information detecting circuit 123.

In the camera signal processing circuit section 121, luminance signal processing and color signal processing are performed from the AD data. Also, A
The E (exposure) information detection circuit section 122 detects an image signal level from the luminance information of the camera signal processing circuit section 121 and performs AE.
(Exposure) information is generated. Based on the AE information result, a DA (to be described later) of the analog / digital signal processing circuit 113 is used.
The range of the ramp reference voltage of the (digital / analog) conversion circuit is switched so that the pixel signal is controlled so as to have an optimum value of the AD input voltage range. As 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, and the AD input voltage is increased 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 above-described result for the above-described ramp-shaped reference voltage for each color.

As described above, whether the ramp reference voltage is controlled by the AE information or the WB information can be determined according to the specifications of the imaging apparatus. It may be controlled individually, or A
The overall level may be controlled by E information, and the signal level of each color may be controlled by WB information. This control increases the input signal level of the AD converter, so that SN does not deteriorate and A
The quantization error caused by the D converter can be reduced.

Reference numeral 130 denotes a camera CPU for controlling the camera section of the solid-state imaging device. 140 is a recording / reproducing system, 1
50 is a display system.

FIG. 2 is a schematic circuit diagram of the pixel section 111, the analog / digital signal processing circuit section 113, and the vertical scanning circuit section 112. In FIG. 2, reference numeral 11 denotes a comparator, 10 denotes a counter, and 9 denotes a DA converter.

FIG. 3 is an equivalent circuit block diagram showing one pixel of the pixel section and a noise removal / analog-to-digital conversion circuit section. DA output range switching information for switching the range of the ramp reference voltage is input to the DA converter.

In FIG. 3, reference numeral 21 denotes a photoelectric conversion element;
Represents the signal charge of the photoelectric conversion element 21 and the amplification transistor 23
A transfer transistor for transferring the current to the gate of the transistor, 24 is a pixel selection transistor, 25 is a reset transistor, and 26 is a switch 27 and a storage capacitor 28, which store the output current of the amplification transistor 23 as a voltage, and output the voltage while converting the voltage into a current. Current source transistor,
A comparator 31 detects a difference between the output current of the current source transistor 26 and the output current of the amplification transistor 23 output via the pixel selection transistor 24.
Is a counter for counting the output of the comparator 31, and 29 is a source (main electrode) of the amplifying transistor 23 which amplifies a voltage by a digital signal output from the counter 30.
This is a DA converter that outputs to a terminal. DA output range switching information is input to the DA converter 29, and the range of the ramp-like reference voltage output from the DA converter is switched so that the pixel signal is controlled so as to have an optimum value of the AD input voltage range.

A method of obtaining a signal voltage based on the signal charges from the photoelectric conversion element after the amplification transistor is reset in the above configuration will be described with reference to a timing chart of FIG. Here, each of the transistors 22 and 2 in FIG.
3, 24 and 25 are described as PMOS transistors, and the transistor 26 is described as an NMOS transistor. The DA converter 29 is set to output a certain high potential (V _HD ). It is assumed that the counter 30 has been reset and no counting operation has been performed. The terminal φR is set to the “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 this time, the terminal φX is set to “L” at the same time.
The selection transistor 24 is turned on at the level (pulse 202), and the switch 27 is also turned on. The output current of the amplifying transistor 23 at the time of reset is determined by the transistor 2
6 is stored in the storage capacitor 28 in the form of a gate voltage generated when the gate-drain is short-circuited (the comparison reference voltage is stored). After that, the transistors 25,
24, the switch 27 is turned off, and a signal charge corresponding to the light input to the photoelectric conversion element is turned on by setting the terminal φT to the “L” level (pulse 203), thereby turning on the transfer transistor 22.
Then, the signal is transferred to the gate terminal of the amplification transistor 23. Assuming that the gate potential at this time is lower than that at the time of reset, the output current of the amplification transistor 23 has a value corresponding to that at the time of reset. 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 .phi.X 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 ( _VH ). Thereafter, the counter 30 is operated to amplify the digital output, and the output voltage of the DA converter 29 receiving the output gradually decreases (assuming that the DA converter 29 generates a negative analog output voltage with respect to the digital input signal). Yes.) At some point, the output current of the amplification transistor 23 becomes equal to the output current of the transistor 26, and the input voltage of the comparator 31 decreases rapidly.
The count operation of 0 is stopped.

The digital value changed from the start to the stop of the counter 30 becomes a value equal to the difference between the reset potential and the potential when the signal charge is transferred, of the gate potential of the amplification transistor 23. Thus, the AD conversion corresponding to the difference has been performed.

FIG. 2 shows an example in which the configuration example of FIG. 3 is applied to a case where 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, 1-2-2,...),
Amplifying transistor 3 (3-1-1, 3-1-2,...),
The 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,
…), The voltage generated between the gate and the source of the transistor 6 by the output current of the amplification transistor 3 is taken in,
The switch 7 (7-1, 7-2,...) For storage, the capacitor 8 (8-1, 8-2,...), The vertical signal line 12 (12-
, 12-2,...), And a DA converter 9 (9-1, 9-2,...)
-1, 9-2,...) Are the same as those in FIG.

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 to set the reset transistors 5 (5-1-1, 5-1).
2, 5-1-3) is turned on, and then the drive line 14-1 is set to the "L" level to set the pixel selection transistor 4 (4-1-4-1).
1, 4-1-2, 4-1-3) are turned on to reset the amplification transistor 3 (3-1-1, 3-1-2, 3-1-3).
3-1-3) is output to the vertical signal line 12 (12-1,.
12-2, 12-3), the drive line 13 is set to the "H" level, and the switches 7 (7-1, 7-2, 7-3) are turned off.
N, and the voltage generated between the gate and the source when the output current in the reset state is supplied to the transistor 6 to the capacitor 8 (8-1, 8-2, 8-3) is held.

Thereafter, as in the operation of FIG.
0 (10-1, 10-2, 10-3) counting operation is started, the output voltage of the DA converter 9 (9-1, 9-2, 9-3) is reduced, and the vertical signal line 12 The potential change is detected by the comparator 11, the operation of the counter 10 is stopped, and the change in the digital value from the start to the stop of the count is used as a digital output to complete the AD conversion. FIG. 9 shows DA
FIG. 9 is a circuit diagram showing a case where AE information is used as DA output range switching information in the converter 9 (9-1, 9-2, 9-3). Reference numeral 40 denotes one pixel.

FIG. 5 is a circuit diagram showing an example of the configuration of the DA converter shown in FIGS. 2 and 3.

This configuration example is composed of 2n resistors and n switches in the case of n-bit AD in the R-2R ladder system. The resistance value of the ladder is changed by switch control of the reference voltage, and the ratio of the resistance value to the resistance value _{Rf of the} amplifier becomes the DA output voltage. The method of DA converter is R-2R
The present invention is not limited to the ladder method, but may be a capacitance array method.

As a method of switching the DA output range, FIG.
FIG. 7 shows a method of switching to a plurality of reference voltages (E1, E2, E3) by a switch, and FIG. 7 shows a method of switching the return value _{Rf of the} amplifier to switch the ratio between the resistance value of the ladder and the resistance value _Rf .

FIG. 8 shows the DA output range of FIG. 6 or FIG.
FIG. 9 is a characteristic diagram showing DA output when switching is performed by the method of FIG. The comparison reference voltage of the DA converter may have a KNEE characteristic or a γ characteristic. By doing so, the imaging device can be simplified. In the case of a color image pickup apparatus, it is preferable to share the white balance described later and the γ and Knee functions.

FIG. 14 is a characteristic diagram of another embodiment of the present invention. Depending on the image information of the subject, the conversion range of the sensor output signal level of AD conversion, the weight of conversion, and the number of digitized output bits can be changed depending on the application.

In FIG. 14, in the case of (A), the AD signal is linearly adjusted between the sensor output signal DA output voltage range V1 and V2.
Convert.

In (B), AD conversion is similarly performed between the DA output voltage ranges V3 and V4, but the DA output voltage range is small. This is because when the reflected light amount range of the subject is narrow, for example, document information, Suitable for bar code information detection.

(C) is a case where the DA output voltage is converted non-linearly. For example, by reducing the voltage range when the DA output voltage is near V5 and increasing the voltage range near V6, it is possible to perform AD conversion in which the sensor output signal is subjected to γ conversion processing. Since the sensor signal is directly converted into a digital signal, a high quality image can be obtained without an increase in quantization error or noise.

(D) is a case where the number of bits of the DA output voltage in (A) is reduced, and can be applied to a use in which document information or AD conversion requires a low number of bits.

FIG. 10 is a circuit diagram showing another embodiment of the solid-state imaging device of the present invention. In the embodiment described with reference to FIGS. 2 to 4, the variable range of the output level of the pixel signal is set to DA.
Although the embodiment in which the switching is performed based on the output range switching information has been described, the present embodiment illustrates an embodiment in which the variable range of the comparison reference voltage is switched based on the DA output range switching information.

As shown in FIG. 11, the pixel signal from the pixel 40 is compared with the DA output voltage from the DA converter 43 on a column basis by the comparator 42, and is converted into AD data by the AD converter 44. DA output range switching information (AE information) for switching the range of the ramp reference voltage is provided in the DA converter 43.
Is entered. A load 41 constitutes a source follower circuit with the transistor of the pixel 40.

FIG. 11 is a circuit diagram showing an embodiment of colorization. This embodiment shows a case where the present embodiment 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.

In the pixel 40, R, G, B color filters are arranged in a mosaic pattern. Pixel rows (... (n + 1) rows,
DA converter 9 (9-1, 9-2, 9-3) for every n rows)
Then, a WB information switching signal is input for each color, and the DA output range is switched for each color, thereby switching the AD conversion range of each color signal. This WB information is based on the WB detection signal of the camera signal processing system 120 in FIG. In the n-th row, the AD conversion range of the B signal and the G signal is sequentially switched, and in the (n + 1) -th row, the AD conversion range of the G signal and the R signal is sequentially switched.

FIG. 12 shows an embodiment in which the present invention is applied to color sequential output.

As in FIG. 11, R, G, B
Are arranged in a mosaic pattern. In the n-th row, first, the pixel signal of the B signal column is AD-converted by the column switching control pulse Codd, and is output as AD data. Next, the pixel signals of the G signal sequence are AD-converted by the column switching control pulse Ceven and output as AD data. At this time, DA
The WB information switching signal is input to the converter 9 (9-1, 9-2, 9-3) corresponding to each color, and the DA output range is switched corresponding to each color, so that the AD conversion range of each color signal is obtained. Switch.

As described above, it is possible to reduce the number of analog digital circuits by performing the AD conversion for each arbitrary column. Reducing the circuit scale in this way can improve the circuit characteristics or reduce the chip area.

The pixel section may be provided with one common amplifier for a plurality of photoelectric conversion sections. 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 serving as photoelectric conversion units of each pixel, MSF is an amplifying transistor serving 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 form a source follower circuit. In such a common amplifier pixel, signals from four photodiodes are output via a common amplifier, and the four pixels constitute one unit cell. 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. G filters for photodiodes a11 and a22,
A B filter is disposed on the photodiode a21, and an R filter is disposed on the photodiode a12.

[0044]

As described above, according to the present invention,
Since the AD input voltage range can be set according to the image signal level, the performance of the AD converter can be fully utilized.

Further, since the scale of the analog-to-digital conversion circuit can be reduced, the yield can be improved and the cost can be reduced.
By providing the pixels and the A / D conversion means in a feedback configuration, the A / D accuracy is improved, and in particular, a design that does not require an analog gain circuit has become possible.

The signal processing device 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.

FIG. 2 is a schematic circuit diagram of a pixel unit, a noise removal / analog-to-digital conversion circuit unit, and a vertical scanning circuit unit of the imaging apparatus of FIG.

FIG. 3 is an equivalent circuit block diagram illustrating one pixel of a pixel unit and a noise removal / analog-to-digital conversion circuit unit.

FIG. 4 is a timing chart for explaining the operation of the circuit of FIG. 3;

FIG. 5 is a circuit diagram of a configuration example of the DA converter shown in FIGS. 2 and 3;

FIG. 6 is a diagram showing a method of switching to a plurality of reference voltages (E1, E2, E3) by 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 embodiment 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]

 REFERENCE SIGNS LIST 100 optical system 110 solid-state imaging device 111 pixel unit 112 vertical scanning circuit unit 113 analog digital signal processing circuit unit 114 horizontal scanning circuit unit 115 timing generator (TG) unit 120 camera signal processing system 121 camera signal processing circuit unit 122 AE (exposure) Information detection circuit section 123 White balance information detection circuit section 130 Camera CPU 140 Recording / reproduction system 150 Display system

Claims (9)

[Claims] 1. An image pickup apparatus comprising: a plurality of pixels; and an analog-to-digital converter that outputs a digital signal based on a comparison result between a pixel signal level from the pixel and a comparison reference level, wherein the analog An image pickup apparatus, wherein the digital conversion means relatively varies a level of a pixel signal from the pixel to be compared and the comparison reference level, and changes a variable range in which the variation is performed. 2. The image pickup apparatus according to claim 1, wherein the analog-to-digital conversion means changes the comparison reference level and changes a variable range of the comparison reference level, and the comparison reference level is a digital-to-analog conversion means. An image pickup apparatus characterized by being controlled by an output of the image pickup apparatus. 3. The image pickup apparatus according to claim 1, wherein the pixel amplifies a signal from the photoelectric conversion unit and outputs the amplified signal as a pixel signal, and varies an output level of the pixel signal. And an amplifying unit having a control terminal, wherein the variable of the pixel signal level is performed by changing a control voltage applied to the control terminal. 4. The imaging device according to claim 3, wherein the control voltage is output by digital-to-analog conversion means. 5. The imaging device according to claim 1, wherein the change of the variable range is performed based on AE information. 6. The imaging apparatus according to claim 1, wherein the change of the variable range is performed for each color signal of the pixel signal based on light source information. 7. An imaging apparatus for performing analog-to-digital signal processing on signals output from a plurality of pixels for each column and outputting the same, wherein a common analog-digital signal processing unit is provided for each of a plurality of columns. . 8. The imaging apparatus according to claim 7, wherein said analog-to-digital signal processing means includes a pixel noise elimination circuit, a gain control circuit, or an analog-to-digital conversion circuit. 9. The imaging apparatus according to claim 7, wherein a common analog digital signal processing unit is used for each color signal.