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CMOS Technology

CMOS and CCD Technology Compared

The two most common sensor technologies used today in industrial digital cameras are Charged Coupled Device (CCD) sensors and Complementary Metal-Oxide Semiconductor (CMOS) sensors. Both CCD and CMOS sensors are manufactured in a silicon foundry and the manufacturing equipment used is similar. But different manufacturing processes make these sensors very different in capability and performance.

CCDs were developed in the 1970s and 1980s and were specifically optimized for the best possible optical qualities and image quality. A CCD typically includes photosites (pixels) arranged in an X-Y matrix of rows and columns. Each pixel contains a photodiode and an adjacent charge holding region. The photodiode collects light photons and converts them into charge (electrons). The number of electrons collected is proportional to the light intensity. Typically, light is collected over the entire surface of the sensor for a period of time and then the charge from each pixel is transferred to the adjacent charge holding region. Next, the accumulated charges are read out of the sensor. Each row of charge data is first moved to a horizontal charge transfer register. The charge packets for each row are then read out serially, are changed from a charge to a voltage, and are amplified.

This architecture produces a low-noise, high-performance imager, however, it makes integrating other electronics onto the sensor impractical. It also means that operating the CCD requires application of several clock signals, clock levels, and bias voltages. This complexity increases the difficulty of camera design and increases power consumption, system size, and cost.

A CMOS sensor, on the other hand, is made with standard silicon processes in high-volume chip foundries. Peripheral electronic devices such as digital logic, clock drivers, amplifiers, and analog-to-digital converters can be readily integrated into the sensor using standard manufacturing processes.

This integration can be achieved because a CMOS sensor's architecture is arranged more like a flat-panel display. Each pixel contains a photodiode that converts light to electrons, a charge-to-voltage converter, a reset and select transistor, and an amplifier. A metal grid overlays the entire sensor to apply timing and read out signals and an array of column output signal interconnects. The column lines connect to multiplexing electronics that are arranged by column outside of the pixel array. This architecture allows signals from the entire array, a portion of the array, or a single pixel in the array to be read out by simple X-Y addressing.

CMOS Advantages

So given this difference in architecture, what are the advantages of CMOS sensor technology?

Lower Power Consumption - Since many of the supporting external electronic components required by a CCD sensor can be fabricated directly onto a CMOS sensor, the overall power consumption of the system can be reduced. This results in lower power requirements for your camera and lower operating temperatures.

Increased Speed - Since charge conversion and analog-to-digital conversion are accomplished at the pixel level, complex shifting schemes are not required and frame read out can be accomplished much more quickly.

Access Flexibility - The simple X-Y pixel addressing method used in CMOS sensors allows direct access to a single pixel or to a group of pixels. This results in extremely high frame rates when working with smaller "areas of interest" on the sensor.

Low Smear and Blooming - The charge holding regions used in CCD sensors are subject to leakage between pixels. This can result in a condition called "blooming". Also, the nature of the charge holding regions and of the shift registers used in CCD sensors can cause unwanted streaks in the acquired images. Because CMOS sensors convert charge to voltage directly in the pixels and because they don't use shift registers, CMOS sensors have inherent anti-blooming and anti-smearing characteristics.

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