Designing an ethernet isolator

Question

I'm trying to galvanically isolate a device which is controlled by ethernet (1000base t) via computer. I've been looking and apparently the ethernet standard provides some isolation, but after I connect the cable to my otherwise isolated device to my computer, my multimeter reads short between its ground and earth ground. So clearly it's not isolating enough.

I'm wondering how I should proceed; according to wikipedia, gigabit ethernet is 8-bits, so a digital isolator won't work. Can anyone suggest a kind of chip that would work? They don't seem to make 8 bit digital isolators. In any case, it seems the ethernet signal can go below ground, so I'm not sure opto couplers would be a (direct) option, although I suppose I could bias them before and remove the bias after with a cap? It also seems like they are not fast enough for gigabit ethernet...

Answer 1

Buy a pair of gigabit fiber media converters and a few meters of fiber. That will give you more than enough isolation.

Answer 2

Ethernet does provide 'transient' isolation (to around 1.5kV but don't quote me on that) as distinct from functional isolation (a permanent ac or dc bias, which is a different spec), via a transformer which may be a discrete transformer on the PCB, or integrated into the RJ45 connector ("integrated magnetics"). This provides a floating differential pair driven/received by the transformer at each end - 2 of them for 10/100BaseT, or all 4 pairs (of Cat5/6) for 1000BaseT. Unless this is a PoE scenario, I would expect open-circuit between the "ground" of the main circuitry at each end. If you don't, I think there's something wrong somewhere.

A separate issue is whether the "ground" of the circuitry of your PC & mains-earth are also connected, which varies - desktop PCs usually are, laptops usually aren't. This has nothing to do with the 'galvanic isolation' you're seeking between PC & peripheral.

GigE isn't "8 bits", it uses all 8 wires of Cat5/6 cabling, 4 (floating) differential pairs. True 'Ethernet Isolators' can provide true 'functional isolation' between each end, for (rare) situations where the "ground" at either end has a permanent ac or dc differential.

Forget about digital isolators or optocouplers, this isn't your problem or your path forward :)

Answer 3

Well, this was a lot simpler than I thought. Thanks The Photon for the comments giving me the idea; it turns out the cat6 cable I was using is metal shielded. The metal shield is connected to ground at the computer end, and to the ground on my microcontroller. Using a cable with no metal shield breaks the connection.