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Tuesday, February 5, 2008

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


"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."

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