The main difference between CCD and CMOS in manufacturing is that CCD is integrated on a semiconductor single crystal material, while CMOS is integrated on a semiconductor material called metal oxide, and there is no essential difference in working principle. Only a few CCD manufacturers such as Sony and Panasonic have mastered this technology. Moreover, the CCD manufacturing process is more complicated, and the price of the camera using CCD will be relatively expensive. In fact, after technological transformation, the gap between the actual effects of CCD and CMOS has been reduced a lot. Moreover, the manufacturing cost and power consumption of CMOS are much lower than that of CCD, so many camera manufacturers use CMOS photosensitive elements. Imaging aspect: CCD imaging permeability and sharpness are very good under the same pixel, color reproduction and exposure can be guaranteed to be basically accurate. However, CMOS products tend to have average permeability, weak color reproduction capabilities for real objects, and poor exposure. Due to their own physical characteristics, there is still a certain distance between the image quality of CMOS and CCD. However, due to the low price and high integration, it has been widely used in the camera field.
CCD is a relatively mature imaging device, and CMOS is regarded as the imaging device of the future. Because the CMOS structure is relatively simple and the same as the existing large-scale integrated circuit production process, the production cost can be reduced. In principle, the CMOS signal is a charge signal in points, while a CCD is a current signal in behavior units. The former is more sensitive, faster, and more power-efficient. The current advanced CMOS is not worse than the general CCD, but the CMOS process is not very mature. The general SMOS generally has low resolution and poor imaging.
CCD or CMOS, basically both use silicon photodiodes to convert light to electricity. The principle of this conversion is similar to the “solar battery” effect of computers with “solar power” in your hands. The stronger the light, the stronger the electricity; conversely, the weaker the light, the weaker the electricity. The light image is converted into Electronic digital signal.
Comparing the structure of CCD and CMOS, the location and number of ADCs are the biggest difference. Simply put, according to what we mentioned in the previous lecture “The working principle of CCD photosensitive element (Part 1)”. Each time the CCD is exposed, the pixel transfer process is performed after the shutter is closed. The charge signal of each pixel (pixel) in each row is sequentially transferred to the “buffer”, which is guided by the line at the bottom to output to the amplifier next to the CCD for amplification , And then series the ADC output; relatively, each pixel in the CMOS design is directly connected to the ADC (amplification and analog digital signal converter), the signal is directly amplified and converted into a digital signal.
Comparison of the advantages and disadvantages of the two
CCD versus CMOS
Due to the basic differences in structure, we can list the differences in performance between the two. The characteristic of CCD is to fully keep the signal without distortion during transmission (dedicated channel design). Through the collection of each pixel to a single amplifier for unified processing, the integrity of the data can be maintained; the CMOS process is relatively simple, and there is no dedicated channel. Design, so you must first zoom in and then integrate the data of each pixel.
On the whole, the application of CCD and CMOS designs reflects the imaging effect, resulting in different types of differences including ISO sensitivity, manufacturing cost, resolution, noise, and power consumption:
ISO sensitivity difference: Since each pixel of CMOS includes an amplifier and A/D conversion circuit, too many additional devices compress the surface area of the photosensitive area of a single pixel, so under the same pixel, the same size of the sensor, the CMOS sensitivity Will be lower than CCD.
Cost difference: CMOS application of the MOS process commonly used in the semiconductor industry can integrate all peripheral facilities in a single chip at one time, saving the cost and yield loss of processing the chip; relatively CCD uses charge transfer to output information, which must be developed separately Transmission channel, if there is a pixel failure (Fail) in the channel, it will cause a whole row of signals to be congested and cannot be transmitted. Therefore, the yield rate of CCD is lower than that of CMOS. In addition, the additional transmission channel and the addition of ADC and other peripherals, CCD The manufacturing cost is relatively higher than CMOS.
Resolution difference: In the first point of “sensitivity difference”, because the structure of each pixel of CMOS is more complex than CCD, its photosensitive aperture is not as large as CCD. When comparing CCD and CMOS photoreceptors of the same size, the analysis of CCD photoreceptor The degree is usually better than CMOS. However, if the size limitation is exceeded, the current CMOS photosensitive originals in the industry can reach 14 million pixels/full-frame design. The advantage of CMOS technology in volume rate can overcome the difficulties in the manufacture of large-size photosensitive originals, especially full-frame The width is 24mm-by-36mm.
Noise difference: Since CMOS is equipped with an ADC amplifier next to each photosensitive diode, if it is measured in megapixels, then more than one million ADC amplifiers are required. Although it is a uniformly manufactured product, each amplifier has more At least there are slight differences, and it is difficult to achieve the effect of amplification and synchronization. Compared with the CCD of a single amplifier, the CMOS finally calculates more noise.
Power consumption difference: CMOS image charge driving method is active, the charge generated by the photodiode will be directly amplified and output by the transistor next to it; but CCD is passive, and a voltage must be applied to make the charge in each pixel Move to the transmission channel. The applied voltage usually needs to be above 12 volts (V). Therefore, CCDs must have more precise power supply circuit design and withstand voltage. The high driving voltage makes the power of CCD far higher than CMOS.
