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ADCs reveal femtocharge secret

Posted: 03 Sep 2007 ?? ?Print Version ?Bookmark and Share

Keywords:femtocharge technology? lowest-power ADCs? charge packets? charge-coupled devices?

A Massachusetts Institute of Technology (MIT) development called femtocharge technology is said to yield the coolest, lowest-power ADCs to date, but the details had been a closely guarded secret. MIT spin-off Kenet Inc. has revealed that instead of using power-hungry amps between stages, the technique passes charge packets such as 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, a former MIT researcher and founder of Kenet. Those amps, Sollner said, are what make conventional ADCs "so power-hungry."

Ordinarily, an ADC 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 stageone per bit of resolutionthe conventional ADC amplifies its results by two, to account for the halving in size for each successive bit.

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

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

Originally developed for military customers interested in extending the battery life of portable devices, Kenet's high-sample-rate ADCs are going into other energy-conscious apps, such as software-defined radios, mobile digital video, 10Gbps networks over copper and portable instrumentation. For instance, customers can use Kenet's high-speed ADC in a USB-powered probe that can turn a laptop into an oscilloscope.

Kenet has also announced its fastest ADC yet: a 350MSps chip that it claims uses 30 percent less power than devices of comparable speed.

The MIT spin-off has consumed $27 million in venture funding since its founding in 2003, but it has been selling its low-power ADCs for only about a year.

- R. Colin Johnson
EE Times

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