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.

The Solution

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.

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.

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.

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