An MIT development called femtocharge technology is said to yield the coolest, lowest-power A/D converters to date, but now the details had been a closely guarded secret. Now MIT spin-off Kenet Inc. (Woburn, Mass.) reveals that instead of using power-hungry amps 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, a former MIT researcher and the 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 power at each successive 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 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 going into other energy-conscious apps, 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 into an oscilloscope.
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 MIT spin-off has consumed $27 million in venture funding since its founding in '03, but it has been selling its low power A/Ds for only about a year.