USB galvanic isolation module

In my line of work, I often run into situations where a device connected to a computer via USB must not have a direct electrical connection to it. Otherwise, it can cause anything from severe interference to complete loss of functionality, as well as interaction between the computer’s power supply and the device under test. There aren’t many ways to solve this, and the simplest is galvanic isolation.


I’ve written about adding galvanic isolation to one of my electronic loads. That approach is fairly simple too, but it still requires modifying the load and installing special software. Here are some alternatives:
1. Galvanically isolate the computer from the mains, but then you also have to isolate the monitor, the USB hub’s power supply, and the printer—in short, not really an option.
2. Use WiFi, Bluetooth, or Ethernet. The isolation is excellent, but implementation is complicated, and usually each device has to be modified separately.
3. A USB isolator. It’s an extremely simple device, completely transparent from both the software and hardware perspective, and can be used whenever needed. That’s what I’m going to talk about today.

Sometimes you really do just need to temporarily isolate a computer from a connected device, whether for normal operation or to figure out “who’s to blame.” That’s why I ordered this particular option, even though, as I mentioned at the beginning, whenever possible I try to build isolation into the devices themselves—for example, as I did with my power supplies.

When I bought it, the price was $7.59 plus $1.63 for shipping, about $9.20 in total. Not cheap, but I didn’t need a dozen of them.
After a while, this little package arrived. I’d also ordered a couple of WiFi-UART modules, but for some reason the seller decided I didn’t need them and didn’t ship them. What a shame—one useful review down the drain :(


Here it is next to a standard matchbox, to give you an idea of the size.


And here are the dimensions from the product page.


The build is quite neat. I could nitpick, of course, but I won’t, since I’m happy with it.


Besides the USB connectors, the board has three LEDs and a switch. Two of the LEDs indicate the operating mode (Low and High speed) and whether power is being supplied to the output. The switch selects the data transfer mode—Low or High speed.


All the high-capacitance capacitors on the board are tantalum. There’s a 400 mA resettable fuse on the input, so safety-wise, everything looks fine.


The device connected through the isolator is powered by an isolated DC-DC converter—in this case, 5 V to 5 V, rated at 1 W, so the maximum load current is 200 mA. The converter also powers the secondary side of the board itself, so you can’t ask too much of it. For example, when I tried connecting an external SSD, it kept rebooting in a loop.
I bought a handful of similar converters from another store to power voltmeters with VFD displays, and for various other uses. They’re handy little things.


The actual isolation is handled by ADUM3160. There’s an optocoupler next to it; the switch sends a command through it to the ADUM’s control input to set the data transfer speed.


According to the manufacturer (assuming the chip is genuine and not a clone), it provides galvanic isolation rated for up to 2.5 kV and data transfer speeds of 1.5/12 Mbit/s.


There’s nothing on the underside, but I should point something out. The ADUM3160 specifications say it can withstand 2.5 kV, but in this case the breakdown voltage will be limited not by the chip, but by the distance between the pins of the DC-DC converter. I don’t know what its insulation is rated for, but the gap between pins 2 and 3 is small, and I’d at least cut a slot in the PCB there.


I first measured the capacitance between the isolator’s two sides from ground to ground; the meter read 26 pF. Then I measured from ground to power, and got the same 26 pF, which seems more than acceptable to me.


Now let’s see how it works.
I tested it not with one of my devices, but with an ordinary USB flash drive. It makes no difference to the isolator what’s connected, since it’s completely transparent to the system and doesn’t need any drivers or anything else. Just plug it in and use it—that’s what caught my interest.
There are two LEDs on the primary side, blue and red. The blue one lights up when high-speed mode is selected (12 Mbit/s), and the red one when low-speed mode is selected (1.5 Mbit/s).
For some reason, though, only high-speed mode worked properly. In low-speed mode, the computer could tell that something was connected, but couldn’t identify it. I tested both flash drives and a USB-UART converter, switching both on the fly and by unplugging and reconnecting. Nothing worked in Low speed.


A few speed tests.
1. Card reader, connected to a USB 3.0 hub
2. Card reader, connected to the computer's USB 2.0 port
3. USB 2.0 flash drive, connected to the computer's USB 2.0 port.

The maximum speed I got was about 9 Mbps, which is perfectly reasonable. You can see that the speed is higher when connected directly to the computer.


1, 2. Current consumption in High speed mode is lower than in Low speed mode: 15 and 28 mA, respectively.
3. The no-load output voltage is about 0.1 V higher than the input voltage.
4, 5. Interestingly, in Low speed mode the load's current consumption drops almost to zero (photo 5). The tester is still powered, but the devices aren't detected, as I mentioned above.
6. The module has very little load capacity: at a load current of just 60–65 mA, the voltage drops to 4.6 V. For this test, the isolator was connected through a USB extension cable, but even when connected directly, the difference wasn't very large.


So, the positives: the build quality is decent, the module works reliably, and data gets through. It's very convenient to use because you simply plug it between the computer and the device under test—for example, the same electronic load, a power supply, a USB oscilloscope, and so on.
The downsides: speeds are limited to 12 Mbps, so it's only suitable for devices that don't need much bandwidth. For some reason, it also works in only one mode. Output load capacity is low, at just around 40–50 mA, so it's best to power the device under test separately. The isolation between the primary and secondary sides is also a concern. This can be fixed by replacing the isolated DC-DC converter; using a more powerful one would also increase the output load capacity.

In summary, if you're aware of its limitations, this module has its uses and can genuinely make life easier in some situations. Not long ago, I ran into a real case where a particular combination of electronic load, its power supply, the power supply under test, and a computer caused the USB connection to drop every 5–10 seconds. A USB isolator would have solved the problem, since everything worked reliably when the computer wasn't connected.

That's all from me. I hope you found the review useful.