CMOS Digital Image Sensors

CMOS Digital Image Sensors

Adding vision to your projects needs not be a difficult task. Whether its machine vision for robot control or the sampling and storage of images for security, CMOS images sensors can offer many advantages over traditional CCD sensors. Just some of the technical advantages of CMOS sensors are,

No Blooming Low power consumption. Ideal for battery operated devices Direct digital output (Incorporates ADC and associated circuitry) Small size and little support circuitry. Often just a crystal and some decoupling is all that is needed. Simple to design with.

There are many manufacturers making CMOS Image Sensors. Just some of the more notable ones are Micron who acquired Photobit, OmniVision, ST who acquired VLSI Vision, Mitsubishi and Kodak.

There are two different categories of CMOS Sensors based on their output. One type will have a analog signal out encoded in a video format such as PAL, NTSC, S-Video etc which are designed for camera on a chip applications. With these devices you simply supply power and feed the output straight into you AV Equipment. Others will have a digital out, typically a 4/8 or 16 bit data bus. These 'digital' sensors simplify designs, where once a traditional 'analog' camera was feed into a video capture card for conversion to digital. Today, digital data can be pulled straight from the sensor.

The main components to a Digital Video Camera design are

• CMOS Image Sensor. The heart of the camera. It produces a digital/analog output representing each pixel. It's support circuitry will normally include a Crystal Oscillator and power supply decoupling. Some sensors may need a resistive bias network of some type. All of these components are normally surface mounted on the back of the PCB and occupies very little real estate.
• The lens Holder. This will be either a plastic or metal mount which attaches to your PCB and allows a standard size lens to be screwed in. The screw thread facilitates focusing for fixed lens systems. The base of the lens mount may also have a IR (Infra Red) filter.
• The Lens. This will determine your Field of view among other things. Lenses range from fish-eye to telescopic and need to be purchased to fit the parameters of your sensor and lens holder.

Key differences between CCD and CMOS imaging sensors

Digital cameras have become extremely common as the prices have come down. One of the drivers behind the falling prices has been the introduction of CMOS image sensors. CMOS sensors are much less expensive to manufacture than CCD sensors.

Both CCD (charge-coupled device) and CMOS (complimentary metal-oxide semiconductor) image sensors start at the same point -- they have to convert light into electrons. If you have read the article How Solar Cells Work, you understand one technology that is used to perform the conversion. One simplified way to think about the sensor used in a digital camera (or camcorder) is to think of it as having a 2-D array of thousands or millions of tiny solar cells, each of which transforms the light from one small portion of the image into electrons. Both CCD and CMOS devices perform this task using a variety of technologies.

The next step is to read the value (accumulated charge) of each cell in the image. In a CCD device, the charge is actually transported across the chip and read at one corner of the array. An analog-to-digital converter turns each pixel's value into a digital value. In most CMOS devices, there are several transistors at each pixel that amplify and move the charge using more traditional wires. The CMOS approach is more flexible because each pixel can be read individually.

CCDs use a special manufacturing process to create the ability to transport charge across the chip without distortion. This process leads to very high-quality sensors in terms of fidelity and light sensitivity. CMOS chips, on the other hand, use traditional manufacturing processes to create the chip -- the same processes used to make most microprocessors. Because of the manufacturing differences, there have been some noticeable differences between CCD and CMOS sensors.

• CCD sensors, as mentioned above, create high-quality, low-noise images. CMOS sensors, traditionally, are more susceptible to noise.
• Because each pixel on a CMOS sensor has several transistors located next to it, the light sensitivity of a CMOS chip tends to be lower. Many of the photons hitting the chip hit the transistors instead of the photodiode.
• CMOS traditionally consumes little power. Implementing a sensor in CMOS yields a low-power sensor.
• CCDs use a process that consumes lots of power. CCDs consume as much as 100 times more power than an equivalent CMOS sensor.
• CMOS chips can be fabricated on just about any standard silicon production line, so they tend to be extremely inexpensive compared to CCD sensors.
• CCD sensors have been mass produced for a longer period of time, so they are more mature. They tend to have higher quality and more pixels.

Based on these differences, you can see that CCDs tend to be used in cameras that focus on high-quality images with lots of pixels and excellent light sensitivity. CMOS sensors traditionally have lower quality, lower resolution and lower sensitivity. CMOS sensors are just now improving to the point where they reach near parity with CCD devices in some applications. CMOS cameras are usually less expensive and have great battery life.

Making CMOS works

Some companies are specialising in CMOS. Sara Sowah discovers their reasons

Conventional wisdom says that CMOS is not good for analogue design. It is limited by too many factors, such as noise and the difficulty of maintaining accurate values for circuit elements without using a more exotic process.

Realising that CMOS has one big advantage - it is cheap - some companies have started to specialise in using conventional CMOS processes to build mixed-signal chips. Silicon Laboratories has developed a series of modem and wireless phone chips built on standard foundry processes.

Tyson Tuttle, product manager at Silicon Laboratories said: "In CMOS, the noise is higher than in bipolar. You can't just take [a circuit based on] bipolar transistors out and put it in CMOS. You have to understand the limitations of CMOS and design accordingly."

Some people believe they can make CMOS work, while others argue that splitting the analogue portion into other chips makes more sense, especially where accuracy, power or linearity are major concerns. The combination of bipolar with CMOS, for example, can result in lower power consumption and lower costs because the digital portion of the chip does not have to suffer the added cost of process changes made for the analogue section.

Dr Rudy Eschauzier, principal designer engineer for National Semiconductor Europe, said that he believed that many small companies were tied into using CMOS technology because they have difficulties finding foundries to manufacture bipolar designs. "It's OK for big companies like us, we have our own factories."

Dr Eschauzier also says CMOS is still suitable for low-power operations, such as wireless LANs, but that it is inappropriate for high-power applications.

New substrates for chip-scale packages make it easier to implement active and larger passive components in one package so that vendors can still build integrated mixed-signal devices without worrying about using different processes on one chip.

Bill Hunt, design engineering manager for Analog Devices' mixed-signal group in Ireland, said: "Packaging technology has progressed rapidly. It is coming up more and more as an option. You can optimise the technology for different areas and put three or four dice in a package. Some of the packages are very, very good."

Hunt said Analog Devices can integrate passive components such as inductors into the chip-scale packages that the company uses.

By pushing more into the digital domain, some of the pressure on the analogue section is easing up. As for low-voltage designs, the shift towards CMOS is seeing designers use digital techniques to attack noise and other problems in

analogue CMOS.

Dr Theresa Meng, founder and CTO of Atheros Communications and engineering professor at Stanford University said the company was able to build an all-CMOS wireless LAN chipset. Atheros overcame the possible deficiency of CMOS by applying digital signal processing techniques to cancel and compensate for the impairments that were introduced in analogue circuits.

"Using other processes may give better noise immunity in some analogue components, but better performance at the component level does not necessarily mean better performance in the overall system," said Meng

INTEGRATION IS THE KEY

"I often think that 30 years of silicon development can be summed up in one word: integration. Just because some people haven't learned how to deal with analogue circuits in CMOS doesn't mean that the history of Silicon Valley is going to change, from integration to dis- integration."

Atheros chose to use CMOS in its wireless LANs because "CMOS design gives us the most cost-effective and integrated solution, so our customers do not have to deal with multiple analogue chips and components. Highly integrated chips make the board-level design much easier, do not expose internal signals to external disturbances, and give tighter control on the overall system parameters."

Dr Meng believes that CMOS technology has been considered inferior for analogue circuits to other more exotic processes because the CMOS process was primarily developed for high-speed digital circuits such as in microprocessor design.

"Since CMOS wasn't designed for small-signal, analogue-like operation, most people tend to think that CMOS is bad for analogue design," said Meng.

"The fact remains that CMOS is the dominant silicon technology, and will continue to be so for the foreseeable future. It seems to me that it is wiser to learn to deal with it, rather than denying its advantages just because it may seem more difficult at the first glance."

Tuttle said: "We try to integrate as much as possible into CMOS. Not everything gets integrated on-chip, however. We keep phase-locked loops off-chip because there is a lot of digital noise there."

But Silicon Laboratories still tries to keep designs in CMOS. "If we step back to 1962 and started designing radio circuits where only CMOS, no bipolar, is available, it would have happened a lot earlier. People would have figured it out."

CMOS takes off

Technology Advances

From digital cameras to new SUVs, imaging sensors are everywhere.

Complementary metal oxide semiconductor (CMOS), the seeing eye dog of the digital age, may be set to revolutionize automobile safety. That's the view of Nicole Wagner, a research analyst with marting consultant Frost & Sullivan.

"The overall development of CMOS image sensors in the automotive industry," she wrote in a study, "can increase the safety of all motor vehides and especially the sport utility vehicle."

CMOS sensors are arrays of pixels that detect light and convert it to electrical voltages that vary according to the intensity of the light. The pixels have x, y addresses, and images of what the sensor "sees" are prepared by polling the pixels and measuring the voltage at each location. The sensor then compares the image with reference images and sends the result of the comparison to a logic circuit. The technology had been under development for several decades, then took a giant leap forward in the mid-'90s, when a group of engineers at NASA's Jet Propulsion Laboratory developed methods for using CMOS to garner near-scientific-quality images. Because CMOS sensors are less expensive and easier to produce than their predecessors, charge-coupled device (CCD) sensors, they're suddenly showing up everywhere: from consumer electronics such as the growing market for digital cameras to bigticket items such as automobiles to production lines.

The digital cameras of five years ago, for instance, relied on CCD technology and typically cost $1,000 or more. Those cameras have great resolution, terrific sensitivity, and short battery life. With the introduction of inexpensive CMOS sensors, digital cameras are accessible to the masses. They aren't as sensitive as the first generation of digital cameras and don't have the same resolution, but they can be purchased for $50, the batteries are long lived, and they're fine for shooting baby pictures that are going to be posted on the family Web page.

CMOS is good for more than building really cool toys, though. Used in automobiles, it might solve a lot of nagging safety problems. Already, automobile companies are using it to design smart airbags that do the following:

Recognize the difference between an event that truly requires deployment (e.g., a head-on collision) and a minor one (going over a curb)

Assess the size of the opposing passenger and deploy accordingly

Minimize early and late deployments

Auto companies are also experimenting with CMOS-enhanced mirrors, hoping to eliminate blind spots. Similarly, they are researching development of collision avoidance systems. If a driver tailgates, for instance, failing to observe the old "one car length per 10 miles per hour" separation rule, Mr. Zippy might find his car automatically slowing down. He might find his headlights subtly adjusting, too, to avoid shining in the rearview mirror of the driver in front of him or directly into the eyes of an oncoming driver.

CMOS is entering medicine and industry, too. The sensor has uses in endoscopy, dental cameras, veterinary endoscopes, industrial cameras, wireless videophones, and telemedicine.

Copyright Instrument Society of America Mar 2001