CMOS image sensor is a typical solid-state imaging sensor, which shares a common historical origin with CCD. It is the key part in the CMOS camera module. The CMOS is usually composed of image sensitive unit array, row driver, column driver, timing control logic, AD converter, data bus output interface, control interface, etc. These parts are usually integrated on the same silicon chip. The working process can generally be divided into reset, photoelectric conversion, integration, and readout.
Other digital signal processing circuits can also be integrated on the CMOS image sensor chip. Such as AD converter, automatic exposure control, non-uniform compensation, white balance processing, black level control, gamma correction, etc. In order to perform fast calculations and even DSP devices with programmable functions are integrated with CMOS devices to form a single-chip digital camera and image processing system.
Camera module types: 1: CMOS, 2: CCD; CMOS is generally used in ordinary digital equipment, CCD is generally used in high-end digital equipment, all are optical imaging. CCD is better than CMOS unit imaging. CCD lens has better color reproduction than CMOS and higher resolution.
CMOS photoelectric sensor directly generates current (or voltage) signal after photoelectric conversion, and the signal reading is very simple. CCD image sensor can only output analog electrical signals. And the output electrical signals need to be decoded by subsequent address and analog-to-digital conversion image signal processor For processing, 3 groups of different power supplies and synchronous clock control circuits are required, and the integration level is very low.
Due to the application of CMOS image sensors, the development of a new generation of image systems has been rapidly developed. With the formation of an economic scale, its production costs have also been reduced. Highly integrated single-chip CMOS image sensors make the application of related images easier Realization.
Many functions have been added and improved. Such as automatic gain control, automatic exposure control, gamma correction, background compensation, and automatic black point correction. All color matrix processing functions are integrated on the chip. CMOS integration is high and will be Competitive advantages in power consumption, size, system and cost. Mobile phones with CMOS image sensors are inexpensive and easy to use. They can be edited in combination with PCs. With the popularity of high-performance printers and computers, the market demand will be a fast increase.
Due to the basic difference in structure, we can list the difference 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 and then perform unified processing, the integrity of the data can be maintained; the CMOS process is relatively simple, and there is no dedicated channel. Design, it is necessary to enlarge 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:
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. Therefore, under the same pixel, the sensitivity of CMOS will be lower than that of CCD with the same size of the photoreceptor.
CMOS applies the MOS process commonly used in the semiconductor industry. It can integrate all peripheral facilities in a single chip at one time, saving the cost and yield loss required for processing the chip; relatively. CCD uses charge transfer to output information, and additional transmission channels must be established. 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. With the addition of additional transmission channels and the addition of ADC and other peripherals, the manufacturing cost of CCD is relatively Higher than CMOS.
In the first point of "sensitivity difference", because the structure of each pixel of CMOS is more complex than that of CCD, its photosensitive aperture is not as large as CCD. When comparing CCD and CMOS photoreceptors of the same size, the resolution of CCD photoreceptor is usually better. In CMOS. However, if you go beyond the size limit, the industry's CMOS photosensitive original can reach 14 million pixels/full frame design. The advantage of CMOS technology in volume rate can overcome the difficulties in manufacturing large-size photosensitive originals, especially full frame Such a size of 24mm-by-36mm.
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 needed. Although it is a uniformly manufactured product, each amplifier is more or less There are some slight differences, and it is difficult to achieve the effect of amplification and synchronization. Compared with a CCD with a single amplifier, CMOS finally calculates more noise.
The CMOS image charge driving method is active. The charge generated by the photodiode is directly amplified and output by the transistor next to it. However, the CCD is passive, and a voltage must be applied to allow the charge in each pixel to move to the transmission channel. The applied voltage usually requires a level of 12 volts (V) or more. Therefore, the CCD must have a more sophisticated power supply circuit design and withstand voltage. The high driving voltage makes the CCD's power much higher than CMOS.
Although CCD is superior to CMOS in image quality and other aspects, it is undeniable that CMOS has the characteristics of low cost, low power consumption and high integration. The low-cost and stable supply of CMOS has become the favorite of manufacturers. Therefore its manufacturing technology is continuously improved and updated, making the difference between CCD and CMOS gradually narrowing. The new generation of CCDs is aimed at reducing power consumption as an improvement goal.
The CMOS series has begun to integrate large-size areas and high-speed image processing chips. The subsequent image processing is used to correct noise and image quality performance, especially Canon series EOSD30 and EOS300D The success of CMOS shows that high-speed image processing chips are already capable of shortening the image processing time and capabilities generated by high-pixel CMOS. CMOS will enter the high-end imaging market in the future, and the prospects are promising.
The digital cameras sold on the market are mainly CMOS photosensitive devices. In products that use CMOS as photosensitive components, through the use of image light source automatic gain enhancement technology, automatic brightness, white balance control technology, color saturation, contrast, edge enhancement and gamma correction and other advanced image control technologies, it is completely possible Achieve an effect comparable to that of a CCD camera. Limited by market conditions and market development, there are few manufacturers that use CCD image sensors for cameras, mainly because of the high cost of using CCD image sensors.