Implementing an isolated interface

The local 5V/3.3V power supply also needs to cross the isolation barrier to power the RS-232 transceiver on the isolated bus side. This also needs to happen on the industrial PC side: both the data and power need to cross the isolation barrier.##IMAGE_2_C##

By isolating both devices from the RS-232 cable network, both the diagnostic port and the industrial PC circuitry are protected from transients that may get coupled onto the RS-232 cable in harsh environments. Ground loops that might occur due to different ground potentials at the diagnostic port and industrial PC side are interrupted by the isolation. The high common-mode voltages that might appear on the RS-232 bus are not allowed across the isolation barrier, protecting human users of the isolated data.

Fabricating transformers directly on chip, using wafer-level processing, allows low-cost integration of iCoupler channels with each other and with other semiconductor functions. An example of this is the ADM3252E, an isolated two-channel RS-232 transceiver in a single compact device. iCoupler isolation overcomes the limitations imposed by the older optocoupler solution in at least five ways: integration — which reduces overall solution size and system cost due to the bidirectional operation of iCoupler — higher performance, lower power consumption, ease of use, and higher reliability.

Until recently, transferring power across an isolation barrier required either a separate dc-to-dc converter, which is relatively large, expensive, and has insufficient isolation, or a custom discrete approach, which is both bulky and difficult to design. These approaches have been the only viable alternatives, even in applications such as isolated RS-232 data communications, where only a small amount of isolated power is needed.

To solve this problem, Analog Devices developed and manufactured a complete, fully integrated solution that combines signal and power transfer across an isolation barrier using microtransformers. This extension of our well established iCoupler technology, termed isoPower, is a breakthrough alternative. Signal and power isolation within a single component — good up to 2.5kV — eliminates the need for an isolated power supply that is bulky, expensive, and difficult-to-design. It can significantly reduce board space, design time, and total isolation system cost for a typical isolated RS-232 interface. The following sections describe two typical RS-232 design situations where this technology is applicable.

You can read the rest of this article in the September issue of Electronic Specifier Design by clicking here.