CCD and CMOS sensors are currently two commonly used image sensors. Both use photodiodes to photoelectrically convert images into digital data. The main difference is the way digital data is transmitted.
Due to different data transmission methods, CCD and CMOS sensors also have many differences in performance and application. These differences include:
1. Sensitivity difference: Since each pixel of a CMOS sensor is composed of four transistors and a photosensitive diode (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, so In the case of the same pixel size, the sensitivity of the CMOS sensor is lower than that of the CCD sensor.
2. Cost difference: Because the CMOS sensor uses the most commonly used CMOS process for general semiconductor circuits, peripheral circuits (such as AGC, CDS, Timing generator, or DSP, etc.) can be easily integrated into the sensor chip, so the cost of the peripheral chip can be saved In addition, because the CCD uses charge transfer to transfer data, as long as one of the pixels cannot run, it will cause a whole row of data to not be transferred, so it is much more difficult to control the yield of the CCD sensor than the CMOS sensor, even It is also difficult for experienced manufacturers to break through the 50% level within half a year of the product's launch. Therefore, the cost of CCD sensors will be higher than that of CMOS sensors.
3. Resolution difference: As mentioned above, each pixel of the CMOS sensor is more complicated than the CCD sensor, and its pixel size is difficult to reach the level of the CCD sensor. Therefore, when we compare the CCD of the same size with the CMOS sensor, the CCD The resolution of the sensor is usually better than that of the CMOS sensor. For example, the CMOS sensor currently on the market can reach a level of 2.1 million pixels (Omnivision's OV2610, launched in June 2002), its size is 1/2 inch, the pixel size is 4.25μm, but Sony in December 2002 The ICX452 was launched with a size similar to the OV2610 (1 / 1.8 inch), but the resolution can be as high as 5.13 million pixels and the pixel size is only 2.78mm.
4. Noise difference: Since each photodiode of the CMOS sensor needs to be equipped with an amplifier, and the amplifier is an analog circuit, it is difficult to keep the results obtained by each amplifier consistent, so it is the same as a CCD sensor with only one amplifier placed on the edge of the chip In comparison, the noise of the CMOS sensor will increase a lot, affecting the image quality.
5. Difference in power consumption: The image acquisition method of the CMOS sensor is active, the charge generated by the photodiode will be directly amplified by the transistor output, but the CCD sensor is passively collected, and an external voltage is required to move the charge in each pixel, and In addition, the applied voltage usually needs to reach 12 ~ 18V; therefore, in addition to the more difficult design of the power management circuit (power IC is required), the high driving voltage makes its power consumption much higher than that of the CMOS sensor. For example, OmniVision ’s recently launched OV7640 (1 / 4-inch, VGA) runs at 30 fps and consumes only 40mW of power; Sanyo, which is dedicated to low-power CCD sensors, launched 1 / 7-inch last year The power consumption of CIF grade products is still above 90mW. Although the company will launch 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. CMOS sensors have the characteristics of 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 narrowed. For example, CCD sensors have been improved in power consumption to apply to the mobile communication market (the representative industry in this regard is Sanyo); CMOS sensors are insufficient in improving 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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