New Isolator IC from Analog Devices has Integrated DC-DC ConversionJanuary 24, 2020 by Robert Keim
The ADuM642x series offers four independent isolation channels and is suitable for high-temperature industrial environments.
Optoisolators, also called optocouplers, used to be an essential component for those who work with applications that require galvanic isolation. By converting electrical current into light and then light into electrical current, an optoisolator allows two different portions of an electrical circuit or system to communicate and interact in the absence of a direct electrical connection.
I maintain a certain respect for optical isolation, though there’s no denying that optoisolators come with some undesirable characteristics. Manufacturers have attempted to offer improved methods of galvanic isolation by thinking up clever ways to move information from electrical point A to electrical point B without electrons or light. The solution favored by Analog Devices is to use magnetic coupling combined with specialized encoding and decoding circuitry; they call this iCoupler technology.
As shown in the following diagram, a magnetic isolator intended for digital signals can be built around a pair of very small coupled inductors—in other words, a transformer.
A magnetic isolator can be built around a pair of small coupled inductors. Image used courtesy of Analog Devices
When we’re working with direct current, a transformer is just a barrier to current flow. To move electrical signals or energy from one side to the other, we need alternating current, which produces the varying magnetic fields that happily pass through the insulation between the two inductors. Whether we’re dealing with high-voltage power transmission or low-voltage embedded devices, the same restriction applies: transformer-based isolation (unlike optical isolation) is not directly compatible with DC systems.
In the above diagram, AC signaling is achieved by converting rising and falling edges into digital pulses (one pulse for a falling edge, and two pulses for a rising edge). The next diagram shows a different approach.
Digital signals converted into differential on-off keying waveforms. Image used courtesy of Analog Devices
Here, digital signals are converted into differential on-off keying waveforms; sustained oscillation occurs during logic high, and no oscillation occurs during logic low. This is how the ADuM642x achieves magnetic-coupling-based electrical isolation.
Isolating Both Signals and Power
The ADuM642x devices are four-channel digital isolators that are compatible with signal frequencies as low as DC and as high as 100 Mbps. That upper frequency limit is an impressive spec for those who are accustomed to optical isolation. Though a small selection of high-speed optocouplers may attain speeds that are at least in the same ballpark as 100 Mbps, optocoupler technology is, in general, much more restrictive in terms of bandwidth.
Systems that require isolation are inconvenient not only because of the need for non-electrical transmission of electrical signals but also because power supplies must be isolated. The goal here is to have no electrically conductive connection between the two portions of the circuit, and this means that the supply voltages and even the grounds must be separated. A helpful feature of the ADuM642x series is the on-chip isolated DC/DC converter.
Low-radiated emissions DC-DC. Image (modified) used courtesy of Analog Devices
With a 5 V primary-side supply voltage and an isolated output voltage of 3.3 V or 5 V, this DC/DC converter can supply 100 mA of current. This is more than enough for powering the isolated side of the ADuM642x as well as various devices (such as a microcontroller) that might come in handy for processing the transferred digital data.
However, you do need to be careful with this converter’s derating. The ADuM642x operates at ambient temperatures as high as 125°C, but the output-current capability of the on-chip converter decreases significantly as temperature increases:
Output current levels of ADuM642x. Image used courtesy of Analog Devices
Do you still use optoisolators when they provide the required functionality or have you gravitated toward the non-optical technologies? If you have any part or implementation advice, feel free to leave a comment and let us know.