These magical creatures crop up out of nowhere and fry your electronics or annoy your ear holes. Understanding them will doubtless save you money and hassle. The ground loop in a nutshell is what happens when two separate devices (A and B) are connected to ground separately, and then also connected to each other through some kind of communication cable with a ground, creating a loop. This provides two separate paths to ground (B can go through its own connection to ground or it can go through the ground of the cable to A and then to A’s ground), and means that current may start flowing in unanticipated ways. This is particularly noticeable in analog AV setups, where the result is audio hum or visible bars in a picture, but is also sometimes the cause of unexplained equipment failures.
One example is your cable TV. This is an analog signal that comes into your house and is grounded to earth in one place, usually outside your house. The cable snakes its way to your entertainment center, where it plugs into your receiver, which is grounded to earth in a different place. This creates a loop, and, through electromagnetic induction coupled to all kinds of AC signals around, a stray current which then leaks through various circuits. Another way to think of it is as one half of a transformer; it’s a single loop and a good portion of that loop is right next to the live wire of the building power with a constantly changing current. It’s not uncommon for there to be a 50 or 60 hertz hum in audio equipment thanks to the effects of ground loops.
Now that you’re an expert, solving the problem (or avoiding it entirely), is pretty straightforward. The most certain way is to cut the loop, which means removing the cable, or replacing it with something that isn’t a wire. You could switch to a wireless communication, like Bluetooth or WiFi. Some wired protocols use differential signals instead of single-ended signaling so that there isn’t a need for a common ground for reference. Move plugs around so that they are plugged into the same outlet, making your loop as small as possible. Another option is to use an isolator, which you could purchase for your cable of choice or design into your project with an optoisolator or isolation transformer. Do not use a cheater plug or remove the ground pin, as that just eliminates a safety feature and could create a dangerous situation with a chassis at live voltage.
When it comes to your oscilloscope, it’s likely that you will at some point want to probe something that is powered by mains, and then you get a completely different kind of ground loop. If your thing is battery powered, there’s no danger here; go nuts because there is no way to create a ground loop. If it’s plugged in to the wall but through an isolated power supply (something with only two prongs and an isolation transformer), you’re still ok, because there’s still no path for a ground loop, but you may see some noise from dirty power.
But if it’s connected to mains and has an earth pin (even indirectly, like a device powered by USB through a computer power supply), there is the potential to create a ground loop, because you’re connecting your grounded scope to another grounded device via the probe. The ground clip on the probe is connected straight to the ground pin, and the grounds on all the probes are connected to each other, and those ground pins are connected to the ground on your device. If that wasn’t clear, it’s better summed up as “all your grounds are already connected to each other and referenced to the same wire – the ground pin.” When you connect the ground clip to the device under test, you create a ground loop, which will add noise to your measurement and possibly hurt the scope.
If you get it wrong and attach the ground clip to something that’s not actually ground, then you’ll have all kinds of problems as the device is now shorted to a ground through your probe, which will quickly self-destruct. Testing devices that have a ground pin requires extra special care to prevent connecting things at different potentials. Breaking the ground loop is possible by simply not connecting the ground clip, though this has other consequences. Here, best practices are to use differential probes or hook the device under test up to an isolation transformer. Do not remove the grounding from your scope, though, because you will be touching it frequently, and it’s best that you don’t get shocked.
So to sum up: ground isn’t just ground. For measurement noise purposes, it’s best for each device to have one and only one path to a single ground point. When there are two or more paths to ground, they can form a loop that will pick up all sorts of environmental electrical and magentic interference. Fixing a ground loop is as simple as breaking it open, but to do so you have to have a good mental picture of all of the ground paths in play. What’s the trickiest ground loop that you’ve ever seen? Are we missing any good solutions?