Electronic Design

Digital Isolators Turn To RF

For a long time, there was little activity in the universe of nonoptical digital isolators. But the past two months have seen three developments (See ED Online 11998 and 12231 at www.electronicdesign.com).

The most recent is a 2500-V digitalisolator family that probably represents the shortest-range sub-light RF-transmission path in history. In fact, Silicon Laboratories' Si844x is quite similar to an optocoupler, except 2.1-GHz transceiver pairs replace the optical components.

Each of the four channels consists of an RF transmitter and receiver separated by a transformer. Each input modulates the carrier provided by an RF oscillator using on/off keying and applies the resulting waveform to the primary of its transformer. The corresponding receiver demodulates the input state according to its RF energy content and applies the result to its output via an output driver.

The design uses chip-scale transformers fabricated in a standard, 100% CMOS process technology to create a four-channel isolator with a footprint that's one-third the size of competing optocouplers. At the same time, the Si844x is twice as fast as existing digital isolators while consuming less than 12 mA per channel at 100 Mbits/s (see the figure). And as for electromagnetic interference, measurements are down better than 4 dBµV/m from FCC specs from 2.094 GHz on up. The company expects the isolators to be used in digital power supplies, where "one of the challenges to lowering system cost \[has been\] to find a way to costeffectively transfer a lot of data from layer to layer within the system with negligible propagation delay," says iSuppli analyst Chris Ambarian.

There are three members in the product family. The Si8840 has four forward channels, the Si8441 has three forward channels and one reverse channel, and the Si8442 has two forward and two reverse channels. All three come in a 16-pin wide-body small-outine IC that's pinforpin compatible with standard optoisolators. They also fully comply with UL, VDE, and CSA specifications.

Pricing begins at $2.44 in 1000-unit quantities. Samples are available now.

Silicon Laboratories

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