Low-cost USB data isolation using a transformer?

[danieljfarrell]

Hi all,

I'm looking at using the https://www.analog.com/en/products/adum3160.html for isolating USB data, I would like full speed (12Mpbs) but actually slow speed (1.5Mbps) would probably be fine.

But I'm curious why there is not a cheap high-speed transformer approach that solves the same problem. For example, a 1:1 transformer for the D+ and another for the D-. The transformers would slew the rise and fall of the edges, let's guess a 2ns rise/fall time, the USB clock rate is 1/(24Mhz) ~ 40ns gives the duration of the data high or low. As long as the transformer does not delay the rise time longer than 1/10th of 40ns = 4ns, then I think this should work. I don't know much about high-speed transformers, but I guess this is too much of an ask, or is it?

There are a lot of transformer options for CAN, RS-232 etc, https://www.coilcraft.com/en-us/edu/series/understanding-and-selecting-isolation-transformers so why not USB?

Happy to be educated

[DavidJRobertson]

USB 1.0/1.1/2.0 (but not 3.0 onwards...) requires DC coupling on the diff pair, so a transformer alone will not work.

[moffy]

From my understanding USB also uses DC levels as part of the protocol and also for power demand levels, it's moderately involved.
https://www.engineersgarage.com/signal-and-encoding-of-usb-system-part-5-6/

[Irilia]

This is probably your best option.
> https://a.aliexpress.com/_EQ7w56Z

[danieljfarrell]

Thanks everyone,

I didn't realise DC coupling was in the specification for the data lines. Interestingly that is missing on USB-C so there might be some options there...

That ADUM4160 board is virtually as cheap as just buying the IC.

Thanks,

[exmadscientist]

Be mindful that USB-C connectors carry both the older D+/D- (aka USB1, USB1.1, USB2.0, Low Speed/LS/1.5, Full Speed/FS/12, or High Speed/HS/480) pairs and the newer SS (USB3.0 SuperSpeed/5Gbps aka ... okay, even this rant/joke has gone too far, just because it's a Universal bus didn't mean it had to get a whole Universe of silly nomenclature too) pairs.

So just because you're using Type-C connectors doesn't mean you're free of the original data signals. They're just present together in parallel.