Differential Scope Primer is a good introduction to oscillospe measurement. This web posting is a shortened version of the information on that document related to ground loops. I have also added here some of my own comments.
Most oscilloscopes are designed to measure voltages that are referenced to earth ground, which is connected to the scope chassis. These are referred to as “single-ended” measurements. Ground loops can corrupt such scope measurement easily.
A ground loop results when two or more separate ground paths are tied together at two or more points. The result is a loop of conductor. Connecting the ground lead of an oscilloscope probe to the ground in the circuit-under-test results in a ground loop if the circuit is “grounded” to earth ground. Typically the metal chassis of both scope and device under test are connected to safety ground and internal power supply common. Scope probe ground connects to scope chassis at the input BNC connector.
Image source: http://www.tek.com/Measurement/App_Notes/DiffOscPrimer/Introduction.html
In the presence of a varying magnetic field, this loop becomes the secondary of a transformer which is essentially a shorted turn. The magnetic field which excites the transformer can be created by any conductor in the vicinity which is carrying AC or changing current. The potential difference seen on oscilloscope probe ground can range from microvolts to as high as hundreds of millivolts.
In these situations, it’s often tempting to remove the probe ground lead to get rid of the noise. This method really works sometimes, but this technique is only effective when measuring very low-frequency signals. At higher frequencies, the probe without good ground contact on both ends begins to add “ring” to the signal. Keep in mind that the current must always form a loop and the smaller the loop are the better. Mysterious ground article gives you more details on what happens and what errors you can see. It is possible that the position of the probe cable can have an effect on the shape of the signals you see on the scope (Try it). Another nasty artifact of a no-ground probe arrangement is the resonance associated with the combination of the rather large inductance (loop inductance of L1=500 nH) and input capacitance of the probe (for example C1=1-10 pF). This resonance is called a probe resonance. A short, explicit ground connection made between the scope ground and the equipment under test shunts around both CI and L1, eliminating their influence on the measured result and pushing the probe resonance up and out of the band of interest.
The next technique often tried to break ground loops is to “float” the scope or “float” the circuit being measured. This practice is inherently dangerous, as it defeats the protection from electrical shock. Idea of “floating” the scope is generally a bad and unsafe idea with a normal oscilloscope (usually metal case and touchable metal parts in it, all in contact with probe ground). Some battery-operated portable scopes allow safe floating operation and you can get rid of ground loop problems and neither side grounded problems with them.
In case of small circuit being measured powering the circuit through safety isolation transformer that breaks the ground connection could be useful. Powering the circuit being measurede though safety isolation transformer is a proven method used at electronics repair shops.
Even when the measurement system doesn’t introduce ground loops, the “ground is not ground” syndrome may exist within the device being measured. Large static currents and high-frequency currents act on the resistive and inductive components of the device ground path to produce voltage gradients. These effects have challenged designers of sensitive analog systems and fast digital systems for years.
If the voltage to be measured is between two circuit nodes, neither of which is grounded, conventional oscilloscope probing cannot be used. There are several types of differential amplifiers and isolation systems available for oscilloscopes with different propertied (targeted for different applications).
29 Comments
DigiTech DF-7 Distortion Factory Demo | Online Car Stereo says:
[...] Scope and ground loops « Tomi Engdahl’s ePanorama blog [...]
Jon Rector says:
Hey incredibly great website!! Man .. Gorgeous .. Wonderful .. I’ll bookmark your blog and take the feeds also…
Graig Salvitti says:
I nevertheless could not recognize some of the thoughts after reading this report. Saturday for me, negative evening, because yesterday I worked all evening. great luck
Dani Khauv says:
Purchasing carts are quite critical to any E-Commerce website. You may need a user friendly looking cart with all necessary attributes in purchase to convert your website traffics.
Alana Surber says:
Intriguing website. My co-workers and I were just discussing this another evening. Also your web page looks fantastic on my aged sidekick. Now thats uncommon. Decent work.
Hayabusa brother says:
Just cause you got the monkey off your back doesn’t mean the circus has left town.
Sent from my Android phone
Fred Scotts says:
I am so grateful for your article.Thanks Again. Want more.
Megger Test Equipment says:
shares use a wonderful web page decent Gives cheers for the working hard to help out people
Annuaire généraliste says:
Hey i think,that your blog is very Helpful and i’m very glad to be here with you ! My passion, is internet and technology. I have just turn on my Internet to search some information and i arrived on your blog. I am impressed by the quality of your ticket and of the information you bring me. It shows as you manage the information. Thank you for your awesome post, I am going to share it with my friends and come back soon.
surround home theater: Eliminating Home Theater Hum and Noise says:
[...] Scope and ground loops « Tomi Engdahl’s ePanorama blog [...]
Japanese Teen says:
What a great article! I enjoy reading your weblog. It is great to find someone who can create such well written articles!
Kaitlyn Codding says:
Laccetti (professor of humanities at Stevens Institute of Technology) and Molsk, in their essay entitled The Lost Art of Writing,[12][13] are critical of the terms, predicting reduced chances of employment for students who use such slang, stating that, “Unfortunately for these students, their bosses will not be ‘lol’ when they read a report that lacks proper punctuation and grammar, has numerous misspellings, various made-up words, and silly acronyms
Carol Shea-Porter says:
Epic. I found this place on google poking around for something else entirely- and now I’m gonna have to go all the archives! So much for free time this morning, but this was a truly great find.
pharmacy technician says:
Keep posting stuff like this i really like it
Peter Donahoe says:
This is just the sort of info I was looking for! Thanks
Where should I ground guitar pickups to? | Electronics In Car says:
[...] Scope and ground loops « Tomi Engdahl’s ePanorama blog [...]
Hp Test Equipment says:
Your blog keeps getting better and better! Your older articles are not as good as newer ones you have a lot more creativity and originality now keep it up! New from Danny | Thermal Scope
omron says:
Hi and thanks for the great details, It was just what I was searching for, and really helpful.
Antony Vangyi says:
Thanks for the nice post. Keep up the good work. I’ll spread the word around.
Posseidon says:
congratulations
.. you have finally won a new reader 
http://www.mysiteu.com/2009/10/test-post.html says:
Great blog post.Really looking forward to read more….
Your blog is so informative ¡ keep up the good work!!!!…
Avery Alosta says:
Great info! I recently came across your blog and have been reading along. I thought I would leave my first comment. I don’t know what to say except that I have enjoyed reading. Nice blog. I will keep visiting this blog very often. http://www.read-news.info/ 8
mypadmedia says:
Great post!
nursing schools says:
found your site on del.icio.us today and really liked it.. i bookmarked it and will be back to check it out some more later
Camille Gethers says:
Hy everybody! I and my wife are probably quite fascinated over today’s universe, My brother and I just like towards take a look closely of each stars throughout any nighttime. Right now there generallya large number of planets just simply here in the solar system combined with marvelous lots more found in that entirely universe.Can now any person indicate to me and my brother plenty of good website pages from which we can down load selected picture clips or else images? It might be truly great in the instance that somebody possess several answers available for me personally.
quittez é says:
I want to express my affection for your generosity for people that should have help on this particular concern. Your real commitment to getting the message around was certainly invaluable and has in most cases helped some individuals like me to achieve their goals. Your own warm and helpful suggestions means so much to me and still more to my mates. Thanks a ton; from everyone of us.
Bağevleri says:
You made some respectable factors there. I seemed on the internet for the difficulty and located most people will associate with together with your website.
iherb coupons at www.youtube.com says:
Nice blog here! Also your site loads up very fast! What host are you using?
Can I get your affiliate link to your host? I wish my website loaded up as fast as
yours lol
Tomi Engdahl says:
WTF are Ground Loops?
http://hackaday.com/2017/03/09/wtf-are-ground-loops/
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.