KR20120017709A - Image sensor - Google Patents

Abstract

PURPOSE: An image sensor is provided to differently control a capacitor combination which comprises a gain value for each low within an arbitrary low unit, thereby reducing VFPN(Vertical Fixed Pattern Noise) and an average value difference between columns. CONSTITUTION: A programmable gain amplifier(11) comprises a plurality of pixel signal input parts(110) which receives a pixel signal from a plurality of pixel arrays. The programmable gain amplifier comprises an amplification device(132) which includes a first input terminal and a second input terminal and a switch part(120) which inputs the pixel signal of the pixel array and an output signal of the amplification device to the first input terminal. The switch part comprises a plurality of capacitors(C11-C13) in which the pixel signal of the pixel array is respectively and selectively connected to the first input terminal. The amplification device is comprised of a differential amplifier.

Description

Image sensor {IMAGE SENSOR}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image sensor, and more particularly, to a structure of a programmable gain amplifier that minimizes vertical fixed pattern noise (VFPN) caused by capacitor mismatch in a column of an image sensor. will be.

Recently, the demand of digital cameras is exploding with the development of video communication using the Internet.

The camera module basically includes an image sensor. In general, an image sensor refers to a device that converts an optical image into an electrical signal. As such image sensors, CMOS (Complementary Metal-Oxide-Semiconductor) image sensors are widely used.

However, analog signals generated by light receiving elements, such as photo diodes, in CMOS image sensors have various parasitic effects caused by parasitic capacitance, resistance, dark current leakage, or mismatch of semiconductor device characteristics. Such parasitic effects are inherently generated in semiconductor devices, resulting in a decrease in the signal to noise ratio of the image data. Therefore, noise is an important factor limiting the performance of the CMOS image sensor.

Noise in the CMOS image sensor is caused by kT / C noise related to the sampling of the image data, 1 / f noise associated with the circuit used to amplify the image signal, and fixed by the mismatch of the signal processing circuit of the sensor. Patterned noise (VFPN). Dual VFPNs are not very good visually because they appear as vertical lines or strips in the image and are easily found in the human eye.

 1 is a diagram illustrating a programmable gain amplifier circuit of a conventional image sensor.

Referring to FIG. 1, the conventional programmable gain amplifier circuit includes capacitors C1, C2, and C3 having the same capacitor capacity, and the gain for each row of such an image sensor is represented by the equation shown in Table 1 below. Like this, the first row (1 ^st row) to the fourth row (4 ^st row) are configured to be the same.

 Gain Configuration 1 ^st row = (C1 + C2) / C3 2 ^st row = (C1 + C2) / C3 3 ^st row = (C1 + C2) / C3 4 ^st row = (C1 + C2) / C3

In the conventional programmable gain amplifier circuit, even if the capacitors C1, C2, and C3 in the column are different from each other due to process changes, the fixed gain in the column is applied to all rows as shown in Table 2 below. As a result, the difference between the average values of the columns is large, and thus, a fixed fixed pattern noise (VFPN) related to the mismatch of the signal processing circuit of the image sensor is generated.

C1 One 1.05 1.1 One C2 1.05 1.1 One 1.05 C3 1.1 One 1.05 1.1 row / column One 2 3 4 One 1.863636 2.15 2 1.863636 2 1.863636 2.15 2 1.863636 3 1.863636 2.15 2 1.863636 4 1.863636 2.15 2 1.863636 5 1.863636 2.15 2 1.863636 6 1.863636 2.15 2 1.863636 7 1.863636 2.15 2 1.863636 8 1.863636 2.15 2 1.863636
Average

1.863636
2.15
2
1.863636

FIG. 2 is a graph showing the average value of the gain for each column by the conventional programmable gain amplifier, and has a problem in that the variation of the average value of the gain between columns is large.

That is, when the fixed gains between the columns differ greatly from each other, there is a problem in that VFPN, which is expressed as vertical lines or strips, occurs in the image due to a mismatch of signal processing circuits on the image sensor.

An object of the present invention is to provide a programmable gain amplifier that minimizes vertical fixed pattern noise (VFPN) caused by capacitor mismatch in a column of an image sensor. .

The programmable gain amplifier according to an embodiment of the present invention includes an amplifier including a first input terminal and a second input terminal, and an output signal of the amplifier and a pixel signal of a pixel array to be input to the first input terminal. And a plurality of capacitors configured to be selectively connected, wherein at least one capacitor of the plurality of capacitors is connected to input an output signal of the amplifying element to the first input terminal, and the remaining capacitors of the plurality of capacitors are connected to the first input terminal. And connect the pixel signal to the first input terminal of the amplifier.

By adjusting the combination of capacitors constituting the gain value for each row in an arbitrary row unit, the gain value of all the rows in a specific column is not fixed, thereby reducing the difference between the average values between columns and reducing the VFPN.

1 is a circuit diagram of a programmable gain amplifier of a conventional image sensor.
Figure 2 is a graph showing the average value of the gain for each column by a conventional programmable gain amplifier.
3 is a schematic block diagram showing the overall configuration of an image sensor;
4 is a programmable gain amplifier circuit diagram according to an embodiment of the present invention.
5 is a graph showing average values of gains for each column by the programmable gain amplifier of the present invention.

Hereinafter, an embodiment of an image sensor according to the present invention will be described with reference to the accompanying drawings.

3 is a schematic block diagram showing the overall configuration of an image sensor according to the present invention.

Referring to FIG. 3, an image sensor generates and accumulates photoelectrons from incident light in which tens or millions of unit pixels are collected, and generates electric signals, that is, pixel signals through the accumulated photocharges. A plurality of pixel arrays 10 in which a row pixel and a plurality of column pixels are connected in a matrix form, and a program connected to the plurality of column pixels, respectively, to amplify and output an output of the pixel array 10, that is, a pixel signal to a desired range. A sampling unit 12 for performing a programmable gain amplifier (PGA) 11 and a correlated double sample (CDS) for outputting pure image information excluding noise components among electrical signals among the outputs of the programmable gain amplifier 11. And an analog / digital converter 13 for converting the analog signal, which is the output of the sampling unit 12, into a digital signal and outputting the digital signal. Take me the output of log / digital converter 13 is composed of DSP (Digital Signal Processor, 14) to finally output the image information by processing this signal.

Hereinafter, a programmable gain amplifier according to an embodiment of the present invention will be described in detail with reference to FIG. 4.

Referring to FIG. 4, the programmable gain amplifier 11 is connected to a plurality of pixel signal input units 110 that receive pixel signals from the plurality of pixel arrays 10, and to a plurality of column pixels, respectively, each of which includes a first input. An amplifier 132 including a terminal and a second input terminal, and a switch unit 120 for inputting an output signal of the amplifier 132 and a pixel signal of the pixel array 10 to the first input terminal. .

The first input terminal is a negative feedback input terminal, and the second input terminal is a reference voltage input terminal.

The switch unit 120 includes a plurality of capacitors C11, C12, and C13 configured to selectively connect pixel signals of the pixel array 10 to the first input terminal, respectively. Gain is determined as the plurality of capacitors C11, C12, and C13 are selectively connected in different combinations with respect to each of the plurality of row pixels.

The amplifier 132 may be configured as a differential amplifier, and the output of the amplifier 132 is transmitted to the sampling unit 12.

The switch unit 120 includes a first capacitor C11, a second capacitor C12, and a third capacitor C13, and the first capacitor C11 is connected to the first and second switches S1 and S2. The second capacitor C12 is connected to the third and fourth switches S3 and S4, and the third capacitor C13 is connected to the fifth and sixth switches S5 and S6.

The capacities of the first capacitor C11, the second capacitor C12, and the third capacitor C13 may be the same or different from each other. The first to sixth switches S1 to S6 may be configured as transistors.

The first, second and third capacitors C11, C12, and C13 are respectively selectively inputted to the first input terminal of the amplifying element 132 and the output signal of the amplifying element 132 and the pixel signal of the pixel array 10. It can be configured to be connected.

The equation for the gain for each row of the image sensor of the programmable gain amplifier 11 according to the present invention is shown in Table 3 below.

 Gain Configuration 1 ^st row = (C11 + C12) / C13 2 ^st row = (C12 + C13) / C11 3 ^st row = (C13 + C11) / C12 4 ^st row = (C11 + C12) / C13

For example, the first row (1 ^st row) the gain of the pixel is turned on by the second switch S2, the fourth switch S4 and the fifth switch S5 so that the first and second capacitors C11 and C12 convert the pixel signal to the amplifying element 132. The third capacitor C13 is obtained by connecting the output signal of the amplifying element 132 to the first input terminal of the amplifying element 132.

The gain of the second row (2 ^st row) is turned on by the first switch S1, the fourth switch S4 and the sixth switch S6, so that the second and third capacitors C12 and C13 amplify the pixel signal. It is connected to the first input terminal of the element 132, the first capacitor (C11) is obtained by connecting the output signal of the amplifier 132 to the first input terminal of the amplifier 132.

The gain of the third row (3 ^st row) is turned on by the second switch S2, the third switch S3 and the sixth switch S6, so that the first and third capacitors C11 and C13 amplify the pixel signal. It is connected to the first input terminal of the element 132, a second capacitor (C12) is obtained by connecting the output signal of the amplifier 132 to the first input terminal of the amplifier 132.

The gain of the fourth row (4 ^st row) is turned on by the second switch S2, the fourth switch S4 and the fifth switch S5 so that the first and second capacitors C11 and C12 amplify the pixel signal. It is connected to the first input terminal of the element 132, and the third capacitor (C13) is obtained by connecting the output signal of the amplifier 132 to the first input terminal of the amplifier 132.

According to the present invention, since the programmable gain amplifier 11 is configured as shown in FIG. 3, the combination of the capacitors C11, C12, and C13 constituting the gain for every row in an arbitrary row unit is performed using the switch unit 120. Different adjustments are made as described in Table 3.

Therefore, according to the present invention, different gains may be obtained for each of the plurality of row pixels.

5 is a graph showing values obtained by averaging the gains of the columns through the programmable gain amplifier 11 of the present invention.

According to the present invention, even when the capacitors C11, C12, and C13 in the columns are different from each other due to process changes, the gain of all the rows in a specific column is not fixed, thereby reducing the difference in the mean value between columns, thereby reducing the signal processing circuit of the image sensor. The fixed fixed pattern noise (VFPN) associated with the mismatch of s is reduced.

C11 One 1.05 1.1 One C12 1.05 1.1 One 1.05 C13 1.1 One 1.05 1.1 row / column One 2 3 4 One 1.863636 2.15 2 1.863636 2 2.15 2 1.863636 2.15 3 2 1.863636 2.15 2 4 1.863636 2.15 2 1.863636 5 2.15 2 1.863636 2.15 6 2 1.863636 2.15 2 7 1.863636 2.15 2 1.863636 8 2.15 2 1.863636 2.15
Average

2.005114
2.022159
1.986364
2.005114

That is, the present invention can reduce the gain error between columns due to capacitor mismatch, and can maximize the image sensor low light characteristics by minimizing the extra-pixel analog noise.

In addition, in the existing structure, a back-end calibration block is required to remove the gain error between columns due to capacitor mismatch, but the proposed structure has an advantage that such a block is not required.

Pixel array: 10, Programmable gain amplifier: 11, Sampling section: 12, Analog-to-digital converter: 13, DSP: 14, Pixel signal input section: 110, Switch section: 120, Differential amplifier: 132, Output section: 130

Claims (8)

An amplifier comprising a first input terminal and a second input terminal,
A plurality of capacitors configured to be selectively connected to input the output signal of the amplifier and the pixel signal of the pixel array to the first input terminal,
At least one capacitor of the plurality of capacitors is connected to input the output signal of the amplifying element to the first input terminal,
And a remaining capacitor of the plurality of capacitors is connected to input the pixel signal to the first input terminal of the amplifying device. The method of claim 1,
The programmable gain amplifier for an image sensor, characterized in that the plurality of capacitors are configured to be connected in parallel. The method of claim 1,
The gain of the amplifier is determined by the ratio of the capacitance of the capacitor connected to the output terminal of the amplifier and the capacitance of the remaining capacitor switched to input the pixel signal to the input terminal of the amplifier. Programmable gain amplifier for an image sensor characterized in that the loss. A pixel array in which a plurality of row pixels and a plurality of column pixels are arranged in a matrix form;
An amplifying device connected to the plurality of column pixels, each amplifying element including a first input terminal and a second input terminal;
A plurality of programmable gain amplifiers including a plurality of capacitors selectively connected to input the output signal of the amplifier and the pixel signal of the pixel array to the first input terminal,
And wherein each of the plurality of programmable gain amplifiers is selectively connected to the plurality of capacitors in different combinations with respect to each of the plurality of row pixels. The method of claim 4, wherein
And each of the plurality of programmable gain amplifiers is selectively connected to each other in different combinations to have different gains for each of the plurality of row pixels. The method of claim 4,
And the capacitances of the plurality of capacitors are the same. The method of claim 4,
And the capacitances of the plurality of capacitors are different from each other. The method of claim 4,
The plurality of low pixels includes at least a first low pixel, a second low pixel, and a third low pixel, and the plurality of capacitors output the output signal of the amplifier and the pixel signal of the pixel array to the first input terminal. At least a first capacitor, a second capacitor, and a third capacitor, each configured to be selectively connectable to input;
The gain of the programmable amplifier for the first low pixel is connected to the first capacitor so that the output signal of the amplifier is input to the first input terminal, and the second and third capacitors receive the pixel signal. Are connected and obtained so as to be configured in parallel to input to the first input terminal of the amplifying element,
The gain of the programmable amplifier device for the second low pixel is connected to the second capacitor such that an output signal of the amplifier device is input to the first input terminal, and the first and third capacitors are configured to receive the pixel signal. Are connected and obtained so as to be configured in parallel to input to the first input terminal of the amplifying element,
The gain of the programmable enable amplifier for the third low pixel is connected to the third capacitor such that an output signal of the amplifier is input to the first input terminal, and the first and second capacitors are connected to the pixel signal. And is configured to be connected in parallel so as to be input to the first input terminal of the amplifying element.

Images