Secret is out on femtocharge-based low-power ADC

Femtocharge technology is claimed to yield the coolest, lowest-power ADCs achieved to date, but until now the details have been a closely guarded secret. Now MIT spin-off Kenet Inc. is revealing that instead of using power-hungry amplifiers 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, former MIT scientist 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 stage—one per bit of resolution—the conventional ADC 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 ADCs 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 ADCs."

Originally developed for military customers interested in extending the battery life of portable devices, Kenet's high-sample-rate ADCs are now finding use in other energy-conscious applications, 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 computer into an oscilloscope. The dual-channel probe would have been single-channel if not for Kenet's low-power ADC.

"If you use a normal ADC, you can chew up most of the USB's 2.5W power budget when running at 275MSps, but our ADCs use just 270mW," said Mike Ziehl, VP 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 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 only been selling its low-power ADCs for about a year.

- R. Colin Johnson
EE Times