SENSOR TECHNOLOGY: Which is better - CCD or CMOS?

Which is better – CCD or CMOS? Before you start choosing and debating with your partner, it is wiser for you to understand some of the technical advantages and disadvantages of the imaging-sensors.

IMAGER BASICS

Both image sensors are pixilated metal-oxide semiconductors. Electron charge is accumulated in each pixel proportional to the local illumination intensity, serving a spatial sampling function.

CCD Imager-sensor:
After exposure, a CCD transfers each pixel’s charge packet sequentially to a common structure, which converts the charge to a voltage, buffers it and sends it off-chip.

CMOS Imager-sensor:
In CMOS, the charge to voltage conversion takes place in each pixel. This difference in readout technologies has significant implications for sensor architecture, capabilities and limitations.

Characteristics of image-sensor performance:

1. Responsivity - The amount of signal the sensor delivers per unit of input optical energy.

CMOS imagers are marginally superior to CCD because gain elements are easier to place on a CMOS image sensor. Their complementary transistors allow low power high-gain amplifiers.
CCD amplification usually comes at a significant power penalty. Some CCD manufacturers are challenging this conception with new readout amplifier technique.

2. Dynamic Range - The ratio of a pixel’s saturation level to its signal threshold.

CCD has an advantage by about a factor of two to CMOS under similar circumstances. CCD enjoy significant noise advantages over CMOS imagers because of quieter sensor substrates (less on-chip circuitry), inherent tolerance to bus capacitance variations and common output amplifiers with transistor geometries that can be easily adapted for minimal noise. Externally coddling the image sensor through cooling, better optics, more resolution or adapted off-chip electronics cannot make CMOS sensors equivalent to CCD.

3. Uniformity - The consistency of response for different pixels under identical illumination conditions.

Ideally, behavior would be uniform, but spatial water processing variations, particulate defects and amplifier variations create non-uniformities.

It is important to make a distinction between uniformity under illumination and uniformity at or near dark. CMOS imagers were traditionally much worse under both regimes. Each pixel had an open-loop output amplifier and the offset and gain each amplifier varied considerably because of water processing variations, making both dark and illuminated non-uniformities worse than those in CCD.
Some people predicted that this would defeat CMOS imagers as device geometries shrank and variances increased. However, feedback-based amplifier structures can trade off gain for greater uniformity under illumination. The amplifiers have made the illumination uniformity of some CMOS imagers closer to that of CCD.

Still lacking, though is offset variation of CMOS amplifiers, which manifests itself non-uniformity in darkness. While CMOS imager manufacturers have invested considerable effort in suppressing dark non-uniformity, it is still generally worse than that of CCD. This is a significant issue in high speed applications, where limited signal levels mean that dark non-uniformities contribute significantly to overall image degradation.

4. Shuttering - The ability to start and stop exposure arbitrarily.

This is a standard feature for all consumers and most industrial CCD, especially interlines transfer devices, and is particularly important in machine vision application. CCD can deliver superior electronic shuttering with little fill factor compromise, even in small pixel image sensors.

Implementing uniform electronic shuttering in CMOS imagers requires a number of transistors in each pixel. In line-scan CMOS imagers, electronic shuttering in CMOS imagers requires a number of transistors in each pixel. Electronic shuttering does not compromise fill factor because shutter transistors can be placed adjacent to the active area of each pixel. In area-scan imagers, uniform electronics shuttering comes at the expense of fill factor because the opaque shutter transistors must be place in what would otherwise be optically sensitive area of each pixel. CMOS matrix sensor designers have dealt with this challenge in 2 ways:

- A non-uniform shutter, called a rolling shutter, exposes different lines of an array at different times. It reduces the number of in-pixel transistors, improving fill factor. This is sometimes acceptable for consumer imaging, but in higher performance application, object motion manifests as a distorted image.

- A non-uniform synchronous shutter, sometimes called global shutter, exposes all pixels of the array at the same time. Object motion stops with no distortion, but this approach consumes pixel area because it requires extra transistors in each pixel. Users must choose between low fill factor and small pixels on a small, less-expensive image sensor or large pixels with much higher fill factor on a larger, more costly image sensor.

5. Speed

This is an area in which CMOS has the advantage over CCD because all camera functions can be placed on the image sensor. With one die, signal and power trace distances can be shorter, with less inductance, capacitance and propagation delays.

6. Windowing - The ability to read out a portion of the image sensor.

CMOS has an advantage in this area. It allows elevated frame or line rates for small regions of interest. This is an enabling capability for CMOS imagers in some applications, such as high temporal precision object tracking in a sub region of an image. CCD generally have limited abilities in windowing.

7. Antiblooming - The ability to gracefully drain localized overexposure without compromising the rest of the image in the sensor.

CMOS generally has natural blooming immunity. CCDs on the other hand, require specific engineering to achieve this capability. Many CCDs that have been developed for consumer application do, but those developed for scientific applications generally do not.

8. Biasing and Clocking

CMOS imagers have a clear edge in this area. It operates with a single bias voltage and clock level. Non-standard biases are generated on-chip with charge pump circuitry isolated from the user unless there is some noise leakage. CCD typically requires a few higher voltage biases, but clocking has been simplified in modern devices that operate with low voltage clocks.

RELIABILITY

Both image chip types are equally reliable in most consumer and industrial application. In ultra rugged environments, CMOS imagers have an advantage because all circuit functions can be placed on single integrated circuit chip, minimizing leads and soldier joints, which are the leading causes of circuit failures in extremely harsh environments.

CMOS image sensors also can be more highly integrated than CCD devices. Timing generation, signal processing, analog to digital conversion, interlace and other functions can all be put on the imager chip. This means that a CMOS based camera can be significantly smaller than comparable CCD camera.

The user needs to consider, however, the cost of this integration, CMOS imagers are manufactured in water fabrication process that must be tailored for imaging performance. These process adaptations, compared with a non-imaging mixed signal process, come with some penalties in device scaling and power dissipation. Although the pixel portion on CMOS imager almost invariably has lower power dissipation than a CCD, the power dissipation of other circuits on the device can be higher than that of CCD using companion chip from optimized analog, digital and mixed signal process. At a system level, this calls into questions the notion that CMOS based cameras have lower power dissipation than CCD based cameras. Often CMOS is better, but it is not un-equivocally the case, especially at high speed (above 25 MHz readout).

The other significant considerations in system integration are adapt-ability, flexibility and speed of change. Most CMOS image sensors are designed for a large, consumer or near consumer application. They are highly integrated and tailored for one or a few applications. A system designer should be careful not to invest fruitlessly in attempting to adapt a highly application specific device for a use which it is not suited.

CCD image sensors, on the other hand, are more general purpose. The pixel size and resolution are fixed in the device, but the user can easily tailor other aspects such as readout speed, dynamic range, binning, digitalizing dept, nonlinear analog processing and other customized models of operation.

Even when it makes economic sense to pay for sensor customization to suit an application, time to market can be an issue. Because CMOS imagers are systems on a chip, development time average 18 months, depending on how many circuits functions the designer can reuse from previous design in the same wafer fabrication process. And this amount of time is growing because circuit complexity is out pacing design productivity. This compares with about eight months for new CCD designs in established manufacturing process, CCD systems can also be adapted with printed circuit board modifications, whereas fully integrated CMOS imaging systems require new wafer runs.

WHICH COST LESS?

One of the biggest misunderstandings about image sensors is cost.

CMOS may be less expensive at the system level than CCD, when considering the cost of related circuit functions as timing generation, biasing, analog signal processing, and digitalization, interface and feedback circuitry. But it is not cheaper at a component level for the pure image sensor function itself.

CMOS and CCD image sensor do not have significantly different costs when produced in similar volume and with comparable cosmetic and silicon area. Both technologies offer appreciable volumes, but neither has such commanding dominance over the other to establish untouchable economies of scale.

CONCLUSION

CMOS
CMOS imager offer superior integration, power dissipation and system size at the expense of image quality (particular in low light) and flexibility. They are the technology of choice for high volume, space constrained applications where image quality requirements are low. This makes them a natural fit for security cameras, PC videoconferencing, wireless handheld device videoconferencing, bar-code scanners, fax-machines, consumer scanners, toys, biometrics and some automotive in-vehicle uses.

CCD
CCDs offer superior image quality and flexibility at the expense of system size. They remain the most suitable technology for high end imaging applications. Such as digital photography, broadcast television, high-performance industrial imaging, and most scientific and medical applications. Furthermore, flexibility means users can achieve greater system differentiation with CCDs than with CMOS imagers.

Sustainable cost between the 2 technologies is approximately equal. This is the major contradiction to the traditional marketing pitch or virtually all the solely CMOS imager companies.

References:
CCD vs CMOS: Facts and Fiction (Author: Dave Litwiller)

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