CCD and CMOS sensors are two kinds of image sensors that are currently widely used. Both use photoelectric diodes for photoelectric conversion and convert images into digital data. The main difference is that digital data transmission is different.
As shown in the figure below, the charge data of each pixel in each line of the CCD sensor is sequentially transmitted to the next pixel, and is output from the bottommost part, and then is amplified and output via the amplifier at the sensor edge; in the CMOS sensor, Each pixel is adjacent to an amplifier and A/D conversion circuit, and the data is output in a manner similar to a memory circuit.
The reason for this difference is that the special process of the CCD ensures that the data will not be distorted during transmission. Therefore, the data of each pixel can be converged to the edge and amplified. The data of the CMOS process is generated when the transmission distance is long. Noise, therefore, must be amplified before integrating the data for each pixel.
Because of the different data transmission methods, there are many differences in the performance and application of CCD and CMOS sensors. These differences include:
1. Sensitivity difference: Since each pixel of the CMOS sensor is composed of four transistors and one photodiode (including an amplifier and an A/D conversion circuit), the photosensitive area of ​​each pixel is much smaller than the surface area of ​​the pixel itself. With the same element size, the CMOS sensor has lower sensitivity than the CCD sensor.
2. Cost difference: Since the CMOS sensor adopts the most common CMOS technology for general semiconductor circuits, peripheral circuits (such as AGC, CDS, Timing generator, or DSP, etc.) can be easily integrated into the sensor chip, thus saving the cost of peripheral chips; In addition, since the CCD uses charge transfer to transmit data, as long as one pixel cannot be operated, an entire row of data cannot be transmitted, so controlling the yield of the CCD sensor is much more difficult than the CMOS sensor, even if there is experience Manufacturers also have a hard time breaking the 50% level within half a year of the product's introduction. As a result, CCD sensors cost more than CMOS sensors.
3. Resolution difference: As described above, each pixel of the CMOS sensor is more complex than the CCD sensor, and its pixel size is difficult to reach the level of the CCD sensor. Therefore, when we compare CCD and CMOS sensors of the same size, the CCD sensor The resolution is usually better than the CMOS sensor level. For example, currently available CMOS sensors can reach a level of up to 2.1 million pixels (OmniVision's OV2610, introduced in June 2002) with a size of 1/2 inch and a pixel size of 4.25 μm, but Sony in December 2002. The ICX452 was introduced, which is about the same size as the OV2610 (1/1.8 inch), but its resolution is as high as 5.13 million pixels and its pixel size is only 2.78mm.
4. Noise difference: Since each photodiode of the CMOS sensor needs to be matched with an amplifier, and the amplifier is an analog circuit, it is difficult to make the result obtained by each amplifier consistent. Therefore, it is compared with a CCD sensor having only one amplifier placed on the edge of the chip. CMOS sensor noise will increase a lot, affecting image quality.
5. Power consumption difference: The CMOS sensor's image acquisition mode is active. The charge generated by the photodiode is amplified and output directly by the transistor. However, the CCD sensor is a passive acquisition. The applied voltage is needed to move the charge in each pixel. The voltage usually needs to reach 12~18V; therefore, in addition to the difficulty in designing the power management circuit of the CCD sensor (the power IC needs to be applied), the high driving voltage makes the power consumption much higher than that of the CMOS sensor. For example, OmniVision's recently launched OV7640 (1/4-inch, VGA) operates at 30 fps and consumes only 40mW; Sanyo, which is dedicated to low-power CCD sensors, launched 1/7 inch last year. The CIF-class products still have power consumption of over 90mW. Although the company will introduce 35mW new products in the near future, it still has a gap with CMOS sensors and is still in the sample stage.
In summary, CCD sensors are superior to CMOS sensors in terms of sensitivity, resolution, and noise control, while CMOS sensors are characterized by low cost, low power consumption, and high integration. However, with the advancement of CCD and CMOS sensor technology, the difference between the two has gradually decreased. For example, CCD sensors have been improved in power consumption for use in the mobile communications market (representatives in this regard are Sanyo); CMOS sensors are used to improve the resolution and sensitivity. To apply to higher-end image products, we can see some clues from the product plans of the following major manufacturers.
As shown in the figure below, the charge data of each pixel in each line of the CCD sensor is sequentially transmitted to the next pixel, and is output from the bottommost part, and then is amplified and output via the amplifier at the sensor edge; in the CMOS sensor, Each pixel is adjacent to an amplifier and A/D conversion circuit, and the data is output in a manner similar to a memory circuit.
The reason for this difference is that the special process of the CCD ensures that the data will not be distorted during transmission. Therefore, the data of each pixel can be converged to the edge and amplified. The data of the CMOS process is generated when the transmission distance is long. Noise, therefore, must be amplified before integrating the data for each pixel.
Because of the different data transmission methods, there are many differences in the performance and application of CCD and CMOS sensors. These differences include:
1. Sensitivity difference: Since each pixel of the CMOS sensor is composed of four transistors and one photodiode (including an amplifier and an A/D conversion circuit), the photosensitive area of ​​each pixel is much smaller than the surface area of ​​the pixel itself. With the same element size, the CMOS sensor has lower sensitivity than the CCD sensor.
2. Cost difference: Since the CMOS sensor adopts the most common CMOS technology for general semiconductor circuits, peripheral circuits (such as AGC, CDS, Timing generator, or DSP, etc.) can be easily integrated into the sensor chip, thus saving the cost of peripheral chips; In addition, since the CCD uses charge transfer to transmit data, as long as one pixel cannot be operated, an entire row of data cannot be transmitted, so controlling the yield of the CCD sensor is much more difficult than the CMOS sensor, even if there is experience Manufacturers also have a hard time breaking the 50% level within half a year of the product's introduction. As a result, CCD sensors cost more than CMOS sensors.
3. Resolution difference: As described above, each pixel of the CMOS sensor is more complex than the CCD sensor, and its pixel size is difficult to reach the level of the CCD sensor. Therefore, when we compare CCD and CMOS sensors of the same size, the CCD sensor The resolution is usually better than the CMOS sensor level. For example, currently available CMOS sensors can reach a level of up to 2.1 million pixels (OmniVision's OV2610, introduced in June 2002) with a size of 1/2 inch and a pixel size of 4.25 μm, but Sony in December 2002. The ICX452 was introduced, which is about the same size as the OV2610 (1/1.8 inch), but its resolution is as high as 5.13 million pixels and its pixel size is only 2.78mm.
4. Noise difference: Since each photodiode of the CMOS sensor needs to be matched with an amplifier, and the amplifier is an analog circuit, it is difficult to make the result obtained by each amplifier consistent. Therefore, it is compared with a CCD sensor having only one amplifier placed on the edge of the chip. CMOS sensor noise will increase a lot, affecting image quality.
5. Power consumption difference: The CMOS sensor's image acquisition mode is active. The charge generated by the photodiode is amplified and output directly by the transistor. However, the CCD sensor is a passive acquisition. The applied voltage is needed to move the charge in each pixel. The voltage usually needs to reach 12~18V; therefore, in addition to the difficulty in designing the power management circuit of the CCD sensor (the power IC needs to be applied), the high driving voltage makes the power consumption much higher than that of the CMOS sensor. For example, OmniVision's recently launched OV7640 (1/4-inch, VGA) operates at 30 fps and consumes only 40mW; Sanyo, which is dedicated to low-power CCD sensors, launched 1/7 inch last year. The CIF-class products still have power consumption of over 90mW. Although the company will introduce 35mW new products in the near future, it still has a gap with CMOS sensors and is still in the sample stage.
In summary, CCD sensors are superior to CMOS sensors in terms of sensitivity, resolution, and noise control, while CMOS sensors are characterized by low cost, low power consumption, and high integration. However, with the advancement of CCD and CMOS sensor technology, the difference between the two has gradually decreased. For example, CCD sensors have been improved in power consumption for use in the mobile communications market (representatives in this regard are Sanyo); CMOS sensors are used to improve the resolution and sensitivity. To apply to higher-end image products, we can see some clues from the product plans of the following major manufacturers.
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