Most reliable galvanic isolation technology for extreme EMI environments?
I'm working on another project with extreme EMC requirements as per various notorious military EMC standards. Civilian product but for military use, and as such subject to the toughest levels of conducted/radiated susceptibility, 200V/m fields across an 2MHz-18GHz range, peak pulses of far higher energy yet, extremely low radiated emissions down to 0 dBµV/m average @ 30-450MHz etc etc. The details aren't important since my question is conceptual, just assume that the EMC requirements are on the ridiculous side of things. We've done similar projects several times before, so we know what we are up against in general terms.
As one requirement, we need to use a CAN bus and it needs to have galvanic isolation. Usually not a big deal, I'm a fan of using inductive digital isolators such as these with a part like example datasheet ADµM1201. Since these a digital signals there's no worries about current transfer ratios and the like. These are cheaper, widely available and more rugged/reliable than optocouplers.
(One optocoupler part or another in our product range tended to go end of life/LTB once every full moon, until I banned them from use in new designs. Some ten years later we've successfully phased most of them out, good riddance. EOL announcements are now blissfully quiet.)
I plan to clock the CAN bus as slow as possible without messing up the real-time requirements, but realistically I can probably not get it below 50kbps. Toying with slope control will probably happen though, either directly on the CAN transceiver or indirectly though external filters.
My question is if I should be concerned when using these kind of digital isolators in a project with extreme EMI requirements?
They specify a bit of transient immunity etc in the datasheet, which won't be a concern since there will be external TVS and filters. I'm not concerned about transients or conducted EMI - that's one reason to use galvanic isolation in the first place. But other than that, I don't have much of a clue how it performs when it comes to radiated emissions/susceptibility, other than that they've passed industrial/automotive EMC testing flawlessly in the past (RS in levels of 50-100V/m, typically). But military EMC is another matter.
Whereas an old-school optocoupler is... optical. So maybe I should get back to using them for special projects like this?
1 answer
My first knee-jerk reaction was to use opto-isolators. There are some that have been around for decades and are unlikely to go obsolete any time soon.
I wasn't previously aware of these "digital isolators". They seem like they go thru a lot of trouble to ultimately do what opto-isolators do. I'd be more worried about sourcing problems for these than optos. They do seem to have advantages in niche cases, like lower current requirement and direct digital signal in and out. However, the description of your problem doesn't indicate these would be particular advantages.
Your main concern is radiated emissions. The main way to address that is to make sure signals are filtered to not include high frequencies. At 50 kbit/s, you have 20 µs per level. A 2-pole low pass filter that takes 5 µs to settle within 90%, for example, sounds good enough. The result would control the current thru the LED of an opto-isolator.
On the other end, don't make your circuit "snap" (hysteresis). I'd again use a 2-pole RC filter so that the output signal has a finite rise and fall time. Even with 5 µs time from 10% to 90%, a regular digital input should have no problem solidly interpreting the signal without bouncing. It would be OK to use a Schmidt trigger input as long as the resulting signal stays internal to a chip.
None of the above sounds hard to do, nor would require unusual opto-isolators so that you might have future availability problems.
CAN
The real problem is the way the CAN bus works. CAN is always bi-directional, and there are multiple things going on within a single bit time.
Whenever the sending node is "writing" (actually just letting the bus go) the recessive state, it checks to make sure that nobody else is writing the dominant state. This is how collisions are detected. The node writing the recessive state aborts the message. The node writing the dominant state never knows there was anyone else out there trying to write, and just keeps on sending its message. When the current message is over, all nodes that have something to send will try again. The ones that have recessive states where others have dominant states drop out, and one message is ultimately sent. When that message is over, the remaining nodes retry, etc.
Neither opto-isolators or your digital isolators allow for this. You will have to guarantee at a higher level that no node on the output side of the isolators tries to send while one on the input side of the isolators is sending.
Another problem is the ACK bit. Every node that sees a valid message on the bus asserts the bus during the ACK bit time. If the sending node doesn't see ACK asserted, it assumes a transmission error and tries again. If this continues to happen, it goes into various error modes with causes the node to get off the CAN bus, at least for some time period. You therefore have to make sure there is another node on the same segment that can provide the ACK bit.
These are all solvable problems, but you have to think about them. I won't go into more here because this has nothing to do with radiated emissions, and both your proposed isolation methods have the same issue.
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