National ends the long wait for the filter-free converter

It has taken 15 years to hit the market and it has arrived two years after the company that started development said it would be ready, but National Semiconductor has started sampling versions of the first continuous-time sigma-delta converter. It is an analogue-to-digital converter that does away with the front-end analogue filter that competing designs need.

National Semiconductor has started supplying samples of the first commercial continuous-time sigma-delta analogue-to-digital converter some two years after the company that developed the technology expected to have them on the market, and 15 years after the basic principles behind the converter were first explored.

Aimed at ultrasound equipment, the eight-channel 50Msample/s ADC uses a design that does away with the need for front-end filtering and which consumes less power than a pipelined converter. National claimed that its design needs about 30 per cent less power than a pipelined converter. But the continuous-time sigma-delta converter has proved problematic to bring to market: the concept was first floated in research papers some 15 years ago.

National bought the creator of the core circuit design in 2007. German startup Xignal said it had developed the technology for a commercially viable continuous-time sigma-delta ADC in 2005, claiming that the first products would be released in 2006. Salvatore Napolitano, marketing manager for National's signal-path group in Europe said the core architecture worked well but that the intervening time was needed to turn that into a complete product, citing package design and immunity to electrostatic discharge as crucial to its development.

Napolitano claimed the time taken to bring the product to market was as expected: "Normally, the move from the first concept to real product takes a certain amount of time. This is what you expect. People started work on continuous time in the 1990s and Xignal started in 1997/98. Now we are sampling: it exists, it works and it is manufacturable."

National has stuck with the 130nm process from TSMC as the basis for the ADC12EU050 that Xignal chose originally for its first part. In principle, the technology should port down to 90 or 65nm processes more easily than pipelined ADCs. However, Napolitano said the sigma-delta architecture is not a direct replacement for its pipelined converters despite the power savings.

"The key market is ultrasound imaging: most medical with some non-medical applications," said Napolitano. "They are mostly going to portable and smaller systems, so they require long battery lifetime and the ability to squeeze more into a smaller space."

Although the continuous-time sigma-delta architecture could be used for radio communications, where soft-radio designs are beginning to appear, the circuitry needs to be altered to suit those applications. "They are not low-pass applications, they are band-pass applications," said Napolitano: the current generation of ADCs assumes that the system needs a low-pass response. "A different modulator is needed for band-pass."

A further limitation is that the sigma-delta architecture is not suited to systems that use undersampling. "They can't work in any way other than the band you set," said Napolitano. "It is a technological issue that will evolve with time."

Although National is the first to have a product reach the sampling stage, NXP Semiconductor is working on a similar architecture intended to reduce the power needed to drive FM radios. Last year, the company said it had developed the core technology but has yet to launch a product.

Image: The main target for National's ADC is ultrasound equipment

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