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MIT spin-off tips secret behind low-power A/Ds

PORTLAND, Ore. — An MIT development called femtocharge technology is claimed to yield the coolest, lowest-power analog-to-digital (A/D) converters achieved to date, but until now the details have been a closely guarded secret. Now MIT spin-off Kenet Inc. (Woburn, Mass.) is revealing that instead of using power-hungry amplifiers between stages, the technique passes charge packets, such as charge-coupled devices (CCDs).

“The trick is that we keep our intermediary results in the charge domain, like a CCD, rather than amplify them with an op amp at every stage,” said Gerry Sollner, former MIT scientist and founder of Kenet. Those amps, Sollner said, are what make conventional A/Ds “so power-hungry.”

Ordinarily, an A/D will compare the analog output with each bit in the digital output in succession from the most-significant to the least-significant bit. In between each stage—one per bit of resolution—the conventional A/D amplifies its results by two, to account for the halving in size for each successive bit.

Kenet's femtocharge circuitry instead passes incrementally smaller charge packets from stage to stage, demanding higher precision from its components, but using less and less power at each stage.

“We have to keep each stage very precise, whereas normal A/Ds can relax precision at successive stages,” said Sollner. “But charge is easier to handle precisely than amplifying a voltage, and the end result is worth it, because we consume much less power than conventional A/Ds.”

Originally developed for military customers interested in extending the battery life of portable devices, Kenet's high-sample-rate A/Ds are now finding use in other energy-conscious applications, such as software-defined radios, mobile digital video, 10-Gbit/second networks over copper and portable instrumentation. For instance, customers can use Kenet's high-speed A/D in a USB-powered probe that can turn a laptop computer into an oscilloscope. The dual-channel probe would have been single-channel if not for Kenet's low power A/D.

“If you use a normal A/D, you can chew up most of the USB's 2-1/2 watt power budget when running at 275 megasamples [per second], but our A/D converters use just 270 milliwatts,” said Mike Ziehl, vice president of marketing and business development. “That enables applications to go to dual channels or to higher sampling rates.”

This month, Kenet will announce its fastest A/D yet: a 350-Msample/s chip that it claims uses 30 percent less power than devices of comparable speed.

The Massachusetts Institute of Technology spin-off has consumed $27 million in venture funding since its founding in 2003, but it has only been selling its low power A/Ds for about a year.

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