Photosensitive element: what's the difference between CMOS CCD?

CCD or CMOS basically use silicon photodiode for photoelectric conversion. The principle of this conversion is similar to the "solar cell" effect of a computer with "solar energy" in your hand. The stronger the light, the greater the power. On the contrary, the weaker the light, the weaker the power, and the light and shadow images are converted into electronic digital signals. Comparing the structures of CCD and CMOS, the position and number of ADC are the biggest differences. Simply speaking, it is based on what we mentioned in our last class "The working principle of CCD photosensitive element (I)". Every time the CCD is exposed and the shutter is closed, the pixel transfer process is carried out, and the charge signal of each pixel in each row is transferred to the "buffer" in turn, which is guided by the bottom line and output to the amplifier next to the CCD for amplification, and then output in series with ADC; In contrast, in CMOS design, ADC (amplification and analog-digital signal converter) is directly connected to each pixel, and the signal is directly amplified and converted into a digital signal. The advantages and disadvantages of two CCD CMOS designs are compared. Under the same area, the sensitivity of a single photoreceptor connected to the amplifier is high. Small photosensitive opening, low sensitivity, high cost, great influence on line quality, high cost, CMOS integration, low cost, low resolution, low connection complexity, high resolution, new technology, higher noise than single amplification, lower noise than one million amplification, high power consumption ratio, external voltage, high power consumption, direct amplification and low power consumption. Because of the basic differences in structure, we can list the performance differences between them. The characteristic of CCD is to fully keep the signal undistorted during transmission (exclusive channel design). By collecting each pixel on a single amplifier and then processing it uniformly, the data integrity can be maintained. CMOS technology is relatively simple, and there is no special channel design, so the data of each pixel should be amplified before integration. Generally speaking, the application of CCD and CMOS design is reflected in the imaging effect, including ISO sensitivity, manufacturing cost, resolution, noise and power consumption. Different types of differences: ISO sensitivity difference: Because each pixel of CMOS contains an amplifier and an A/D conversion circuit, too many additional devices compress the surface area of the photosensitive area of a single pixel, so the sensitivity of CMOS will be lower than that of CCD under the same pixel and the same size sensor. Cost difference: CMOS can integrate all peripheral facilities into a single crystal wafer at one time by using MOS technology commonly used in semiconductor industry, saving wafer processing cost and yield loss; In contrast, CCD uses charge transfer to output information, so it needs to find another transmission channel. If there is a pixel fault in the channel, a whole line of signals will be blocked and cannot be transmitted, so the yield of CCD is lower than that of CMOS, and the manufacturing cost of CCD is relatively higher than that of CMOS. Resolution difference: In the first point "sensitivity difference", because each pixel structure of CMOS is more complicated than CCD, its photosensitive opening is not as large as CCD. When comparing CCD and CMOS photoreceptors with the same size, the resolution of CCD photoreceptors is usually better than that of CMOS. However, if we leave the size limit, the CMOS photosensitive elements in the industry can reach the design of 6.5438+0.4 million pixels/full-width. The advantages of CMOS technology in yield can overcome the difficulties in manufacturing large-size photosensitive elements, especially the size of 24mm×36mm, and the noise difference: because CMOS is equipped with an ADC amplifier next to each photodiode, if it is counted as one million pixels, it needs more than one million ADC amplifiers. Although it is a unified product, each amplifier has some subtle differences, so it is difficult to achieve the effect of amplification synchronization. Compared with the CCD of a single amplifier, CMOS finally calculates more noise. The difference of power consumption: CMOS image charge driving mode is active, and the charge generated by photodiode will be directly amplified and output by the transistor next to it; But CCD is passive, and voltage must be applied to make the charge in each pixel move to the transmission channel. This applied voltage usually needs to be above 12 volts (V), so CCD must also have more precise power supply circuit design and withstand voltage strength. The high driving voltage makes the charge of CCD much higher than that of CMOS. Although CCD is superior to CMOS in image quality, it is undeniable that CMOS has the characteristics of low cost, low power consumption and high integration. Due to the enthusiastic demand for digital images, CMOS has become the favorite of manufacturers because of its low cost and stable supply. Therefore, its manufacturing process is constantly improving and updating, gradually narrowing the difference between CCD and CMOS. The new generation CCD aims to reduce power consumption in order to enter the mobile communication market of camera phones; On the other hand, CMOS series began to integrate large-size and high-speed image processing chips to correct noise and image quality performance through subsequent image processing.