The difference between CCD and CMOS

Pixel structure is different

The first difference between CCD and CMOS is reflected in the different photosensitive unit or pixel structure. In addition to the photodiode, the photosensitive element of the former includes a storage unit for controlling adjacent charges. The photosensitive diode occupies most of the area. The composition of the CMOS photosensitive element is more complicated. In addition to the photosensitive diode at the core, it also includes an amplifier and an analog-to-digital conversion circuit. Each pixel is composed of a photosensitive diode and three transistors, and the area occupied by the photosensitive diode is only A small part of the entire component.

Compared with a CMOS sensor, the effective photosensitive area of ​​the CCD photosensitive element is larger, it can receive a stronger optical signal under the same conditions, and the corresponding output electrical signal is also stronger; reflected in the output result is the image captured by the CMOS sensor The content is not as rich as the CCD sensor, the sharpness is poor, the loss of image details is serious, and the noise is obvious, which is why the early CMOS sensors are mostly used in low-end occasions.

ADC location and number are different

The second difference between CCD and CMOS is reflected in the position and number of ADC (Analog-to-Digital Converter). Each pixel of the CCD corresponds to only one ADC, and each time the photosensitive element is exposed, the pixel transfer processing is performed after the shutter is closed, and the control circuit sequentially enters the "buffer" in a serial manner, and the output is guided by the bottom line. To the amplifier next to the CCD for amplification, and then the ADC output in series; in contrast, in the CMOS design, each pixel is connected to an ADC, the signal is directly amplified and converted into a digital signal.

It is precisely because of this difference that the signal of each photosensitive element in the CCD sensor can form a uniform output. After these output data are uniformly processed by the amplifier, the electrical signal strength of each pixel is increased by the same amplitude; Each photosensitive element carries an ADC, which cannot guarantee that the magnification of each image point is strictly consistent, so that the enlarged image data cannot represent the original appearance of the photographed object. It is reflected in the final output result that the noise inevitably appears in the image, and the image quality is lower than the CCD sensor.

Comparison between CCD and CMOS

After reading the above introduction, you may think that CCD imaging is clear and less noisy, so it has obvious advantages over CMOS. In fact, it is not true. Although a large number of ADCs bring CMOS defects of low noise, they also show great benefits in other aspects.

Comparing CCD and CMOS, they have advantages and disadvantages in the following five aspects:

Sensitivity

Because each pixel of CMOS contains a photodiode, a charge / voltage conversion unit, a transistor, and an amplifier, the area occupied by the photodiode is only a small part of the entire device. Too many extra devices compress the surface area of ​​the effective photosensitive area of ​​a single pixel, so under the same pixel size, the sensitivity of the CMOS sensor is lower than that of the CCD sensor. The direct consequence is that under low illumination, CMOS cannot be as sensitive as CCD, and the image definition is greatly reduced.

cost

The charge information stored by the CCD charge coupler needs to be read bit by bit under the control of the synchronization signal. The charge information transfer and reading output require a clock control circuit and three different power supplies to cooperate. The whole circuit is relatively complex. If there is a pixel failure in the dedicated channel, it will cause a whole row of signal congestion and cannot be transmitted. Therefore, the yield of CCD is lower than that of CMOS. The MOS process commonly used in the CMOS application semiconductor industry can integrate all peripheral facilities into a single wafer at a time, saving the cost of processing wafers and the loss of yield, and the cost is greatly reduced.

Noise

Since each photosensitive diode of CMOS is equipped with an ADC amplifier, if it is measured in megapixels, then more than one million ADC amplifiers are required. Since the amplifier is an analog device, it cannot guarantee that the magnification of each image point is strictly consistent, so that the enlarged image cannot represent the original appearance of the photographed object. Therefore, comparing the CCD with only a single amplifier per row, the CMOS finally calculates more noise.

speed

The CCD charge coupler needs to output information bit by bit under the control of a synchronous clock, and the speed is slow; while the CMOS photoelectric sensor can take out the electrical signal while collecting the optical signal, and can also process the image of each unit Information, the speed is much faster than CCD charge coupler.

Power consumption

In addition to the higher difficulty in the design of power management circuits for CCD sensors, most charge-coupled devices require three sets of power supplies for power consumption, while CMOS photoelectric sensors only need to use one power supply, which consumes very little power and is 1/8 to 1/10 of CCD charge coupler, CMOS photoelectric sensor has great advantages in energy saving.