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The relationship and difference between CCD and CMOS sensor technology
Both CCD and CMOS adopt sensitive elements as the basic means of image capture. The core of ccd/cmos sensor is a photodiode, which can generate output current after receiving light irradiation, and the intensity of current corresponds to the intensity of light.
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But in the peripheral composition, the sensitive element of CCD is different from that of CMOS. In addition to the photodiode, the former includes a storage unit for controlling the adjacent charge. The photodiode occupies the vast majority of the area - in other words, the effective sensitive area in the CCD sensor is large, and strong light signal can be received under the same conditions The corresponding output electrical signal is also more clear. The composition of CMOS sensor is more complex. Besides the key position of photodiode, it also includes amplifier and analog-to-digital conversion circuit. Each image point is composed of one photodiode and three transistors. The area occupied by the photodiode is only a small part of the whole component, which makes the opening rate of CMOS sensor far lower than that of CCD (opening rate: the ratio of effective sensitive area to the whole sensitive element); in this way, when receiving the same illumination and the same size of the element, the light signal that CMOS sensor can capture is obviously smaller than that of CCD element, and the sensitivity is lower; it is reflected in the output result that the image content captured by CMOS sensor is not as rich as that of CCD sensor, and the image details are lost The serious situation and obvious noise are also one of the reasons why the early CMOS sensors can only be used in low-end applications.
Another problem caused by low opening rate of CMOS is that the pixel density of CMOS can not be comparable to CCD. Because with the increase of density, the specific gravity area of the sensitive element will be reduced. However, the opening rate of CMOS is too low, the effective sensitive area is too small, and the loss of image details will be more serious. Therefore, under the same size of the sensor, the pixel size of CCD is always higher than that of CMOS sensor in the same period, which is one of the important reasons that CMOS has not entered the mainstream digital camera market for a long time. Each sensor corresponds to an image point in the image sensor. Because the light sensitive element can only sense the intensity of light and cannot capture color information, it is necessary to cover the color filter above the sensor. In this regard, different sensor manufacturers have different solutions. The most common method is to cover RGB red green blue three color filter. The 1:2:1 component consists of four image points to form a color pixel (i.e. red blue filter covers one image point respectively, and the remaining two image points are covered with green filter). The reason for this proportion is that human eyes are more sensitive to green. Sony's four-color CCD technology replaces one of the green filters with emerald (some media call e-channel), which forms a new R, G, B, e four color scheme. No matter which technology scheme, it takes four image points to form a color pixel, which is a big housework need to be clear in advance.
After receiving light, the sensor generates corresponding current, and the current size corresponds to the light intensity. Therefore, the electrical signal directly output by the sensor is simulated. In CCD sensor, each sensor does not further deal with this, but directly outputs it to the storage unit of the next sensitive element. Combined with the analog signal generated by the element, it is output to the third sensitive element, and so on. Only when the signal of the last sensor is combined can the unified output be formed. Because the electrical signal generated by the sensitive element is too weak to directly perform analog-to-digital conversion, the output data must be amplified uniformly. The task is the amplifier in CCD sensor, and after the amplifier processing, the intensity of the electrical signal of each image point increases by the same range; however, because CCD itself cannot directly convert the analog signal directly Therefore, a special analog-to-digital conversion chip is needed to process the digital signal. Finally, it is output to the special DSP processing chip in the form of binary digital image matrix. For CMOS sensors, the above workflow is completely unsuitable. Each sensor in CMOS directly integrates amplifier and analog-to-digital conversion logic. When the photodiode receives light and generates analog electrical signal, the electrical signal is amplified by the amplifier in the sensor first, and then directly converted into corresponding digital signal. In other words, in CMOS sensor, each sensor can produce the final digital output, and the digital signal is directly sent to DSP chip for processing after the combination of the digital signal. The problem is precisely here. The amplifier in CMOS sensor belongs to the simulator, which can not guarantee that the amplification rate of each image point is strictly consistent, which makes the amplified image data unable to represent the photographing The original appearance of the object is reflected in the final output result, which is that there are a lot of noise in the image, and the quality is obviously lower than that of CCD sensor.
Charge coupled Devoce), which is made of a semiconductor material with high sensitivity, can convert light into charge, and convert it into digital signal by ADC chip. After compression, the digital signal is saved by flash memory or internal hard disk card in camera. Therefore, it can transmit data to computer easily and light, and according to the needs and requirements of computer processing means Imagine to modify the image. CCD consists of many sensitive units, usually in megapixels. When the CCD surface is exposed to light, each sensitive unit will reflect the charge on the component. The signal generated by all the sensitive units is added together to form a complete picture.
Like CCD, complementary metal oxide semiconductor is a semiconductor that can record light changes in digital cameras. The manufacturing technology of CMOS is no different from that of general computer chips. It is mainly made of silicon and germanium, which makes them coexist with semiconductor with n (band electricity) and P (band + electric) level on CMOS. The current generated by these two complementary effects can be recorded and interpreted as images by processing chips. However, the disadvantage of CMOS is that it is easy to appear clutter, which is mainly because the early design makes CMOS overheat because of the frequent current changes when processing rapidly changing images.
The advantage of CCD is good imaging quality, but because of the complex manufacturing process, only a few manufacturers can master it, so the manufacturing cost is high, especially for large CCD, which is very expensive. At the same resolution, CMOS is cheaper than CCD, but the image quality produced by CMOS devices is lower than that of CCD. So far, CCD is used as sensor in most of the consumption levels and high-end digital cameras in the market; CMOS sensors are used as low-end products on some cameras. If any camera manufacturer uses CCD sensors for the camera manufacturer, the manufacturer will spare no effort to publicize it as a selling point, even named "digital camera". For a while, whether or not there is a CCD sensor has become one of the standards for judging the level of digital cameras.
One of the advantages of CMOS image sensor is that the power consumption is lower than CCD. In order to provide excellent image quality, CCD pays a higher power consumption. In order to make charge transmission smooth and noise reduced, the transmission effect needs to be improved by high voltage difference. But CMOS image sensor converts the charge of each pixel into voltage, and amplifies it before reading. It can be driven by 3.3V power supply, and the power consumption is lower than CCD. Another advantage of CMOS image sensor is high integration with peripheral circuits, which can integrate ADC with signal processor, and reduce the volume greatly.
Since each photodiode of CMOS needs to be equipped with an amplifier, if it is in megapixel, more than one amplifier is needed. The amplifier belongs to analog circuit, so it is difficult to keep the results of each amplifier consistent. Therefore, compared with CCD sensor with only one amplifier on the edge of chip, the noise of CMOS sensor will increase a lot, and the influence chart Like quality.
The image acquisition mode of CMOS sensor is active, and the charge generated by the photodiode will be amplified and output directly by the nearby transistor; while CCD sensor is passive acquisition, the charge in each pixel must be moved to the transmission channel by applying voltage. The external voltage usually needs 12-18v, so CCD must also have more precise power circuit design and voltage withstand strength. High driving voltage makes the power consumption of CCD much higher than CMOS. CMOS consumes only 1 / 8 to 1 / 10 of CCD.
Because CMOS sensor adopts the CMOS technology which is most commonly used in general semiconductor circuit, it can easily connect peripheral circuits (such as AGC, CDs, timing Generator or DSP) is integrated into sensor chip, so it can save the cost of peripheral chip; while CCD uses charge transfer to transmit data, as long as one pixel cannot run, it will lead to a whole row of data transmission, so it is difficult to control the finished rate of CCD sensor than CMOS sensor. Even experienced manufacturers are difficult to control the finished product rate of CCD sensor. Even experienced manufacturers are difficult to control the semi-finished rate of CCD sensor The manufacturing cost of CCD sensor will be higher than that of CMOS sensor.
The foreground of CCD and CMOS sensor is better than CMOS in image quality, while CMOS has the characteristics of low cost, low power consumption and high integrity. However, with the development of CCD and CMOS sensor technology, the difference between them will gradually decrease. The new generation of CCD sensors has been improving power consumption, while CMOS sensors have not enough in improving resolution and sensitivity. We believe that the continuous improvement of CCD and CMOS sensors will bring us a better digital image world.
Dark current is the amount of current released by an electric diode when there is no incident light. The ideal dark current of the image sensor should be zero. However, the actual situation is that the photodiode in each pixel acts as a capacitor at the same time. When the capacitance slowly releases the charge, even if there is no incident light, the voltage of the dark current will be equivalent to the output voltage of the low brightness incident light. Dark current is one of the factors that affect the quality of painting.
With the rise of digital cameras and mobile cameras, image sensors are becoming one of the most dazzling stars in semiconductor products. In image sensors, CCD sensors owned by Japanese merchants and CMOS sensors are trying to overcome their shortcomings and hope to become the mainstream technology in the market. In view of this, this paper will first introduce the difference between CCD and CMOS sensor in principle, then discuss the technical development blueprint of leading manufacturers and understand the development trend of these different image sensors in the application market.
The difference between CCD and CMOS sensor technology
CCD and CMOS sensors are two kinds of image sensors which are widely used at present. Both of them are photoelectric conversion by photodiode, which converts images into digital data, and the main difference is that the transmission of digital data is different.
The charge data of each pixel in each line of CCD sensor will be transmitted to the next pixel in turn, which is output by the bottom part, and then amplified and output through the amplifier at the edge of the sensor; in CMOS sensor, each pixel will be adjacent to an amplifier and a/d conversion circuit, and the data will be output in a memory like circuit.
The reason for this difference is that: the special process of CCD can ensure that the data can not be distorted during transmission, so the data of each pixel can be gathered to the edge and then amplified; while the data of CMOS process will generate noise when the transmission distance is long, so it is necessary to enlarge the data before integrating the data of each pixel.
Because of different data transmission modes, CCD and CMOS sensors also have many differences in performance and application, including:
1 sensitivity difference
Because each pixel of CMOS sensor is composed of four transistors and a photodiode (including amplifier and a/d conversion circuit), the sensitive area of each pixel is far smaller than the surface area of pixel itself. Therefore, the sensitivity of CMOS sensor is lower than that of CCD sensor in the case of the same pixel size.
2 cost difference
Because CMOS sensor adopts the CMOS technology which is most commonly used in general semiconductor circuit, it can easily connect peripheral circuits (such as AGC, CDs, timing Generator, DSP, etc.) is integrated into sensor chip, so it can save the cost of peripheral chip; besides, because CCD transmits data by charge transfer, as long as one of the pixels can not run, it will lead to a whole row of data transmission, so it is difficult to control the finished product rate of CCD sensor than CMOS sensor, even experienced manufacturers are difficult In the half year of the product, the cost of CCD sensor will be higher than that of CMOS sensor.
3 resolution difference
As mentioned above, each pixel of CMOS sensor is more complex than CCD sensor, and its pixel size is difficult to reach the level of CCD sensor. Therefore, when we compare CCD with CMOS sensor of the same size, the resolution of CCD sensor is usually better than that of CMOS sensor. For example, the current CMOS sensor can reach the highest level of 2.1 million pixels (omnivision's ov26102 launched in June 2002), with a size of 1/2 inch and pixel size of 4.25 μ m, but Sony launched icx452 in December 2002, which is not much different from ov2610 (1/1.8 inch), but the resolution can reach 5.13 million pixels, and the pixel size is only 2.78mm.
4 noise difference
Because each photodiode of CMOS sensor needs to be equipped with an amplifier, and the amplifier belongs to analog circuit, it is difficult to keep the results of each amplifier consistent. Therefore, compared with CCD sensor with only one amplifier on the edge of chip, the noise of CMOS sensor will increase a lot, which will affect the image quality.
5 power difference
The image acquisition mode of CMOS sensor is active, the charge generated by the photodiode will be amplified and output directly by the transistor. However, the CCD sensor is passive acquisition, and the applied voltage is required to move the charge in each pixel. The external voltage usually needs to reach 12-18v; therefore, the CCD sensor is more difficult in the design of power management circuit (power is required IC), high drive voltage makes its power consumption far higher than CMOS sensor level. For example, omnivision's recent ov7640 (1/4-inch, VGA) is at 30 FP