KR20020044367A - Cmos image sensor - Google Patents

Abstract

PURPOSE: A CMOS image sensor is provided to obtain a video signal from which an offset value has been removed when the CMOS image sensor operates normally, thereby improving picture quality of the CMOS image sensor. CONSTITUTION: A CMOS image sensor includes the first, second and third switches(24,29,33), a CDS(25), a column offset measuring unit(27), a column offset generator(31) and a column offset storage(32). The first switch selects a video signal output from a pixel array(23) or a predetermined reference offset value. The CDS double-samples output data of the first switch to remove fixed pattern noise of a pixel. The third switch receives an analog video signal including a column offset from the CDS to select one of the video signal and a signal for generating a column offset. The column offset measuring unit removes the column offset from the video signal switched by the third switch. The second switch selectively outputs the analog video signal from which the column offset was removed or an analog column off signal. The column offset generator generates a column offset signal using a digital video signal. The column offset storage stores the column offset value.

Description

CMOS Image Sensor {CMOS IMAGE SENSOR}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a CMOS image sensor, and more particularly, to a CMOS image sensor capable of improving the image quality of an image sensor by removing an offset of a correlated double sampling (CDS) circuit.

In general, an image sensor refers to an apparatus for capturing an image by using a property of a semiconductor that reacts to light.

Each part of each subject in the natural world has different electrical values in each pixel of the sensing device due to different light brightness and wavelength, which makes the image sensor a level capable of signal processing. Is what it does.

Many image electronic devices have recently been applied to such image sensors.

Currently, research and production of CMOS image sensors have been made to implement an image pickup device in a CMOS process capable of low voltage operation, low power consumption, and low process cost.

Current CMOS image sensors require more research and development due to problems in image quality, although they can be applied to most of the manufacturing processes of CMOS transistors that can be processed very finely.

In general, an image sensor has as many correlated double sampling (CDS) circuits as there are columns of a pixel array of the image sensor. These CDS circuits have inconsistent circuit characteristics due to electrical characteristics and process reasons.

Inconsistencies in the circuit characteristics lead to deterioration in image quality when the video signal is actually processed.

Hereinafter, a CMOS image sensor according to the related art will be described with reference to the accompanying drawings.

1 is a block diagram of a conventional CMOS image sensor.

As shown in FIG. 1, the CMOS image sensor includes a timing generator 1 that generates a control signal and an address signal, and a row decoder that decodes the address signal in a row direction. 2) an image sensor pixel array 3 in which a plurality of photo cells receiving a light source are regularly arranged, and a CDS for removing fixed pattern noise of a pixel; (Correlated Double Sampling) (4), a column decoder (5) for decoding an address signal in the column direction, and an analog video signal including column-specific offset values from the CDS (4). Offset Calibrator (6) for removing offset by column, Analog Signal Processor (ASP) 7 for processing analog video signals, and ADC for converting analog video signals into digital video signals ( Analog to Digital Converter) (8).

In this case, the column offset value is externally input to the offset measurer 6 of the column to control the offset measurement operation.

The offset cancellation operation of the CMOS image sensor will be described below.

First, the timing generator 1 generates a control signal for driving the pixel of the image sensor.

With the control signal as input, the row decoder 2 selects one column of the pixel array 3 at regular time intervals.

In addition, the column decoder 5 generates an address signal for selecting a specific column of a column already selected by the row decoder 2 with the signal generated by the timing generator 1 as an input.

Thereafter, the selected first image signal is input to the CDS 4 to remove the existing noise of the pixel array 3.

The CDS 4 obtains the voltage corresponding to the reset level, and then obtains the voltage of the data level of the first video signal.

Subsequently, subtracting the voltage of the data level from the voltage of the reset level obtains a second image signal from which noise is removed.

Here, the second image signal has an offset due to the electrical characteristics and process mismatches of the pixel array 3 and the CDS 4.

The second image signal including the offset is input to an offset meter 6.

The offset meter 6 obtains a difference between the second image signal and an offset value set to a constant value to obtain a third image signal.

Here, the offset value set to a constant value is uniformly applied to all the pixels of the pixel array 3.

Thereafter, the third image signal is input to the analog signal processor 7 to adjust a white balance and a dynamic range.

The third video signal output from the analog signal processor 7 is input to the ADC 8 to be converted from an analog signal to a digital video signal.

The conventional CMOS image sensor has the following problems.

The offset meter applies an offset value set to a constant value to remove the offset, so it is difficult to accurately remove the offset of all columns.

That is, the image quality of the image sensor is degraded.

The present invention has been made to solve such a problem of the conventional CMOS image sensor, by finding the offset value of each column before operation, the CMOS that can accurately obtain the image signal from which the offset by column is removed in actual operation The purpose is to provide an image sensor.

1 is a block diagram of a conventional CMOS image sensor

2 is a configuration diagram of a CMOS image sensor according to a first embodiment of the present invention;

3 is a flow chart showing the operation of the column offset generator of the CMOS image sensor according to the first embodiment of the present invention

4 is a configuration diagram of a CMOS image sensor according to a second embodiment of the present invention.

5 is a flowchart illustrating the operation of a column offset generator of a CMOS image sensor according to a first embodiment of the present invention.

Explanation of symbols for the main parts of the drawings

21: timing generator 22: low decoder

23 pixel array 24 first switching unit

25: CDS 26: column decoder

27: column offset meter 28: analog signal processor

29: second switching unit 30: ADC

31: column offset generator 32: column offset value storage unit

33: third switching unit

According to the present invention, a CMOS image sensor includes a pixel array including a plurality of photo cells, and a row / column decoder for decoding an address signal in a row / column direction to select a specific photo cell. An image sensor comprising: a first switching unit to select an image signal output from the pixel array or a set reference offset value; a CDS which double-samples output data of the first switching unit to remove fixed pattern noise of a pixel; A third switching unit configured to receive an analog image signal including the offset for each column output from the CDS and to process an image signal or a signal for generating an offset for each column; switched by the third switching unit Column offset meter to remove column offset from video signal; A second switching unit for selectively outputting the removed analog image signal or the offset signal for each analog column; a column offset generator for generating an offset signal for each column using the digitally changed image signal and a column offset value for storing offset values for each column Characterized in that it comprises a storage unit.

Hereinafter, a CMOS image sensor and its operation according to the present invention will be described with reference to the accompanying drawings.

According to the present invention, the offset value of each column of the CDS circuit is detected before the normal operation so that it can be considered in the normal operation, thereby preventing the degradation of the image quality due to the offset.

2 is a configuration diagram of a CMOS image sensor according to a first embodiment of the present invention, and FIG. 3 is a flowchart illustrating an operation of a column offset generator of a CMOS image sensor according to a first embodiment of the present invention.

The CMOS image sensor according to the first exemplary embodiment of the present invention selects a pixel array 23 including a plurality of photocells that accepts a light source and outputs an electrical image signal, and selects each photocell of the pixel array 23. A timing generator 21 for generating a control signal and an address signal for reading image data, a low decoder 22 for decoding the address signal in the row direction by the control signal and the address signal of the timing generator 21, The first switching unit 24 selects an image signal output from the pixel array 23 or a set reference offset value by a first switching control signal, and double-samples the output data of the first switching unit 24. The address signal in the row direction by the CDS 25 for removing the fixed pattern noise of the pixel and the control signal and the address signal of the timing generator 21. Receives an analog video signal including a column decoder 26 for decoding the address signal in the column direction to select a specific column of the column selected by the column, and a column-specific offset output from the CDS 25 by a third switching control signal. A third switching unit 33 for selecting whether to process an image signal or a signal for generating offset for each column, and switching by the third switching unit 33 by using an offset signal for each column output. A column offset measurer 27 for removing the column offset from the extracted image signal, an analog signal processor 28 for processing and outputting the analog image signal from which the column offset is removed, and an analog signal processor 28 by the second switching control signal. And a second switching unit 29 for selectively outputting the offset signal for each analog column output by the third switching unit 33 or the output of Column offset that generates the offset signal for each column by determining the offset value for each column using the ADC 30 for digitally converting and outputting the analog image signal output by the second switching unit 29 and the digitally changed image signal. And a generator 31 and a column-specific offset value storage unit 32 for storing the column-specific offset values.

The offset value storage unit 32 for each column uses an internal memory or an external memory.

The CMOS image sensor according to the first exemplary embodiment of the present invention automatically finds the offset for each column in the image sensor and removes the offset.

First, the first switching unit 24 is set to select a reference offset value instead of an image signal generated in the pixel array 23 by the first switching control signal.

Therefore, the reference offset signal is stored in the CDS 25 and the reference offset signal stored in the CDS 25 is read by sequentially increasing the address of the column decoder 26.

The column-specific signals read in this way are bypassed because they do not require signal processing steps in the column offset measuring device 27 and the analog signal processor 28.

The bypass of each column signal is performed by a selective switching operation of the second and third switching units 29 and 33.

Since the bypassed signal is an analog signal, it is digitally converted by the ADC 30.

The signal digitally converted by the ADC 30 is input to the column offset generator 31, and the operation of the column offset generator 31 is performed as shown in FIG.

That is, the level of the reference offset signal has an added offset value through a circuit such as the CDS 25. The difference between these levels is obtained, and this value is generated as the offset value for each column.

The offset value for each column is stored in the offset value storage unit 32 for each column.

Here, a constant weight k is added if necessary due to the characteristics of the image sensor.

The mathematical calculation for obtaining the offset value for each column is performed as follows, and the calculation process is as shown in FIG.

That is, the offset value (V _OFFSET ) = k * [reference offset value (V _REF ) _{minus the} added offset value (V _CDS )] for each column.

Subsequently, in order to perform a normal operation, the first switching unit 24 is set such that an image signal of the pixel array 23 is input to the CDS 25.

In addition, the second switching unit 29 and the third switching unit 33 are also set to perform normal operations, respectively.

The image signal generated in the pixel array 23 through the normal operation is input to the column offset measurer 27 by the third switching unit 33 via the CDS 25.

Thereafter, the column offset measuring unit 27 removes the offset from the image signal by using the offset value for each column stored in the offset value storage unit 32 for each column.

After the offset is removed, the video signal is processed through the analog signal processor 28, and then input to the ADC 30 to be converted into a digital signal and transmitted to the outside.

Here, the analog signal processor 28 adjusts the white balance and gain of the video signal.

By repeating the above driving step for all columns, more accurate column offset can be found.

Next, an image sensor according to a second embodiment of the present invention will be described.

4 is a configuration diagram of a CMOS image sensor according to a second embodiment of the present invention, and FIG. 5 is a flowchart illustrating an operation of a column offset generator of a CMOS image sensor according to a first embodiment of the present invention.

The CMOS image sensor according to the second embodiment of the present invention is for a user to directly set an offset for each column by using image data from the outside.

The configuration includes a pixel array 43 composed of a plurality of photocells that accepts a light source and outputs an electrical image signal, and a control signal and address for selecting each photocell of the pixel array 43 to read image data. A timing generator 41 for generating a signal, a row decoder 42 for decoding an address signal in a row direction by the control signal and address signal of the timing generator 41, and an image output from the pixel array 43 In order to select a specific column of a column selected by the row signal in the row direction by the control signal and the address signal of the timing generator 41 and the CDS 44 which double-samples the signal to remove the fixed pattern noise of the pixel. Column-by-column using a column decoder 45 for decoding the address signal in the column direction and an offset signal for each column input from the outside A column offset generator 50 generating an offset signal, a column offset value storage unit 49 for storing offset values for each column, and a column offset value output from the column offset value storage unit 49 are used. A column offset measurer 46 for removing the column offset from the analog image signal including the offset for each column, an analog signal processor 47 for processing and outputting the analog image signal from which the column offset is removed, and an analog signal processor 47 And an ADC 48 for digitally converting and outputting an analog video signal output by the < RTI ID = 0.0 >

The offset cancellation operation of the CMOS image sensor according to the second embodiment of the present invention will be described below.

First, the offset value for each column is set by the user. First, the image sensor is operated in a test mode by using a light source having a constant illuminance.

The column offset generator 50 receives an offset signal for each column of all pixels input from the outside and then obtains an average value of the entire screen.

In the next screen, the image sensor is operated again with the same illumination source to find the average value of all columns of pixels in the same column.

Then, the difference between the average value of the entire screen and the average value of all pixels for each column is obtained to obtain the offset for each column.

Repeat this for all columns.

Here, a constant weight g is added if necessary due to the characteristics of the image sensor.

The formal calculation to get the offset by column is done as follows.

That is, the offset value V _{OFFSET for each} column = g * [average value V _MEAN of the entire screen-the average value V _CMEAM of all the pixels per column].

Here, the average value V _CMEAN of all the pixels for each column may be represented by (V _ROW1 + V _ROW2 + ..... + V _ROWN ) / N.

V _ROW is the pixel value of all columns of the same column and N is the number of columns.

The column-specific offsets thus obtained are stored in a memory inside and outside the image sensor.

Subsequently, the image signal generated in the pixel array 43 by performing a normal operation passes through the CDS 44 and is then input to the column offset measurer 46.

The offset meter 46 removes the column-by-column offset from the image signal by using the column-by-column offset stored in the memory.

The video signal from which the offset for each column is removed is processed through the analog signal processor 47, and then input to the ADC 48 to be converted into a digital signal and transmitted to the outside.

By repeating the above driving step for all columns, more accurate column offset can be found.

Such a CMOS image sensor of the present invention has the following effects.

By obtaining the offset value of each column of the CDS circuit using a column offset generator, an image signal obtained by removing the offset value in a normal operation can be obtained.

This has the effect of improving the image quality of the CMOS image sensor.

Claims (9)

An image sensor comprising a pixel array including a plurality of photo cells and a row / column decoder for decoding an address signal in a row / column direction to select a specific photo cell, respectively. A first switching unit which selects an image signal output from the pixel array or a set reference offset value; A CDS which double-samples the output data of the first switching unit to remove fixed pattern noise of the pixel; A third switching unit configured to receive an analog image signal including the offset for each column output from the CDS and to process the image signal or a signal for generating the offset for each column; A column offset measurer which removes a column offset from the image signal switched by the third switch; A second switching unit configured to selectively output an analog image signal from which the column offset is removed or an offset signal for each analog column; And a column offset generator for generating an offset signal for each column by using the digitally changed image signal, and a column-specific offset value storing unit for storing offset values for each column. The CMOS image sensor according to claim 1, further comprising an ADC for digitally converting an analog image signal output by the second switching unit and outputting the converted digital image signal to a column offset generator. The CMOS image sensor according to claim 1, further comprising an analog signal processor which adjusts the white balance / gain of the analog video signal from which the column offset is removed and outputs it to the second switching unit. The CMOS image sensor of claim 1, wherein the column offset generator obtains a level difference between a reference offset value and an offset value added from the CDS and generates the value as an offset value for each column. The method of claim 4, wherein the column offset generator adds a weight k to each column offset value according to the image sensor. A column image offset value (V _OFFSET ) = k * [a reference offset value (V _REF ) _-a value including an added offset value (V _CDS )]. An image sensor comprising a pixel array including a plurality of photo cells and a row / column decoder for decoding an address signal in a row / column direction for selecting a specific photo cell, respectively. A CDS which double-samples the image signal output from the pixel array to remove fixed pattern noise of the pixel; A column offset generator for generating an offset signal for each column by using an offset signal for each column input from the outside; An offset value storage unit for storing the offset value for each column; A column offset measurer for removing a column offset from an analog image signal including offsets by columns by using offset values by columns output from the offset values storage unit by columns; An analog signal processor for processing and outputting the analog image signal from which the column offset is removed; And a ADC for digitally converting and outputting the analog video signal. 7. The column offset generator of claim 6, wherein the column offset generator operates an image sensor in a test mode using a light source having a constant illuminance to directly set a column offset value, and inputs a column offset signal of all pixels input from the outside. Received, averaged over the entire screen, On the next screen, operate the image sensor again with the same illumination source to find the average value of all columns of pixels in the same column And generating a column offset value by obtaining a difference between the average value of the entire screen and all pixel average values of each column. The method of claim 6, wherein the column offset generator adds the weight k to the offset value of each column according to the image sensor. A column offset value (V _OFFSET ) = g * [average value of the entire screen (V _MEAN )-the average value of all pixels per column (V _CMEAM ). The method of claim 8, wherein the average value of all pixels per column V _CMEAN is (V _ROW1 + V _ROW2 + ..... + V _ROWN ) / N, V _ROW is a pixel value of all columns of the same column, and N is CMOS image sensor, characterized in that the number of columns.