Oscilloscope ground noise concepts - Page 1
Forum section: electronics / projects

author: leonhart88
author_rank: Regular Contributor
timestamp: June 25, 2015, 02:12:38 am
content: Hi All,I think this is a fairly simple concept, but I want to make sure I'm understanding it right. I have 3 scenarios (see the attached pic).Obviously, for scenario 1, with a floating supply, I need to hook up the probe ground clip to the negative line. This gives me a clean signal.For scenario 2, if I tie my negative line to mains ground and I probe the positive side without attaching the ground clip, I get a noisy signal.For scenario 3, if I tie the ground clip to the negative line of the supply, the noise diminishes.Why does this happen? Aren't both the supply and scope tied to mains ground already anyway in scenario 2? My guess is that although they are both tied to mains earth, they could technically be at different potentials due to the earth plug location and have different common mode noise pickup. Therefore, when you use the ground clip to tie the grounds together, you force the ground potentials to be the same. Is this a correct explanation for what's going on?Thanks all!

author: nitro2k01
author_rank: Frequent Contributor
timestamp: June 25, 2015, 02:50:31 am
content: One way to look at it is magnetic loop area. All the scope "knows" is it's local ground potential, and it measures the voltage present at the channel input, relative to that ground potential. With the ground clip hooked up, the lowest impedance ground path goes parallel to the signal line, and you have a small loop area. If you disconnect the ground clip but have mains earth present (scenario 2) the loop area becomes a lot larger, the full stretch of mains earth wire between your DOT and your scope. This has two effects:1) You've basically built a big loop antenna (a 1 turn coil) which is subject to pick up random crap, both from the parallel conducting wires, and radio signals.2) As a "bonus" the extra stretch of wire adds a lot of lead inductance, which gives a noticeably higher impedance for high frequency signals, as if you had connected a coil in series.Another case worth mentioning is what happens when you have no ground reference connected at all. Given a high enough frequency of the signal, you may still pick upsomething. In this case your ground reference is only capacitively coupled through the stray capacitance of everything around you. Think putting a capacitor between the two otherwise isolated grounds. (And realistically also a coil to account for various sources of stray inductance along the line.) Not a recipe for good signal integrity to say the least!Finally, here are a couple of interesting videos. Old EEVBlog videos about an unusual phenomenon where sitting down on your chair is picked up as noise on your oscilloscope. Not directly answering your question, but a real life example of what an accidental magnetic loop antenna can do to your measurement.Also a video by Mikeselectricstuff (whose other videos you should definitely check out, and subscribe to) about making custom probes for reverse engineering. Again not a direct answer to the question, but some real life application of what an extra piece of ground wire does to the impedance of a high frequency measurement.

author: tggzzz
author_rank: Super Contributor
timestamp: June 25, 2015, 07:39:19 am
content: Good to see someone thinking about what is happening.In addition to the points made above, in your second example there is a very long wire between the UUT and scope, and that will have inductance. A fast edge on a digital signal in the UUT will interact with the inductance and stray capacitance, and ringing will be visible. That can be a problem even with a 150mm probe lead.Have a look at some of the references noted athttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/for a solid background in probes and probing.

author: dom0
author_rank: Super Contributor
timestamp: June 25, 2015, 08:08:10 am
content: Quote from: tggzzz on June 25, 2015, 07:39:19 amThis will be a problem even with a 150mm probe lead. The inductance of the ground lead essentially forms a LC circuit with the input capacitance of the probe. The resonant frequency of that thing is in the range of a few dozen MHz, which matches (not coincidentally) with the ringing you can observe on a fast edge.

author: tggzzz
author_rank: Super Contributor
timestamp: June 25, 2015, 08:34:05 am
content: Quote from: dom0 on June 25, 2015, 08:08:10 amIn my experience at ~90MHz, though it would be lower if, for some strange reason, you were using a 1:1 probe!The problem can be significantly reduced by the addition of a simple homebrew mod to a standard 10Mohm 10:1 probe. Seehttps://entertaininghacks.wordpress.com/2015/04/23/scope-probe-accessory-improves-signal-fidelity/for a design inspired by commercial probes.

author: dom0
author_rank: Super Contributor
timestamp: June 25, 2015, 08:44:36 am
content: Interesting design. I just wrap some 0.5 silver-plated wire around the ground shaft behind the probe's tip. Works for me.Another probing tip: the usual x10 probes have interchangeable tips and come with two tips. I have one "clean" one to use with hooks, and one tip I just solder to whatever I want to observe.

author: tggzzz
author_rank: Super Contributor
timestamp: June 25, 2015, 11:17:14 am
content: Quote from: dom0 on June 25, 2015, 08:44:36 amThanks. The 3D printed plastic only cost, IIRC £4 + postage. I don't mind letting people have the OpenSCAD if they want it; it reasonably clean and parameterised but I can't be bothered with full documentation!I've tried the DIY wire approach, but it seemed to fall off too easily.QuoteMy expensive 500MHz 6.5pF probes have replacable tips, but I've never steeled myself to attack them with heat! I prefer to solder loops onto the UUT, which has the added advantage that I can still use both hands!

author: leonhart88
author_rank: Regular Contributor
timestamp: June 25, 2015, 06:14:30 pm
content: Thanks for all the responses guys! The loop area was my 2nd guess, but I still wasn't entirely sure.I'll give that document a read and watch those old EEVblog videos. The EEVblog has come such a long way, I remember watching some of the first episodes!I also have a 3D printer of my own, so perhaps I will get around to making one of those adapters =)Another interesting observation I made while playing around with the oscilloscope is that my track lights on the ceiling add an insane amount of common mode noise that's picked up by my probes. I noticed this while trying to measure small pulsing voltages across a 0.1 ohm resistor current sense (20-40mV range). With the track lights on, I couldn't see the pulses at all and everything was consumed by noise. With the lights off, the signal was much cleaner and I could see the small pulses.Quite fascinating stuff. Thanks again!

author: Bud
author_rank: Super Contributor
timestamp: June 25, 2015, 11:43:04 pm
content: Putting a snap-on ferrite choke on the probe close to oscilloscope connector should reduce common mode noise so you can keep your lights turned on.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: June 25, 2015, 11:58:53 pm
content: Quote from: tggzzz on June 25, 2015, 11:17:14 amUm, doesn't every probe worth the title come with a little baggy with the springy bit in it? I have so many of those.

author: tggzzz
author_rank: Super Contributor
timestamp: June 26, 2015, 01:03:51 am
content: Quote from: Alex Eisenhut on June 25, 2015, 11:58:53 pmMany do, but often they are lost. In any event they require a ground to be available at precisely the right distance from the signal. That's probably valid for an analogue board with a ground plane and without a solder mask, but otherwise it is often a problem.Having used wire springs, wire supports/sockets, and spears such as I describe, they are all valuable in different situations. Spears are particularly valuable on tightly packed digital boards.

author: onlooker
author_rank: Frequent Contributor
timestamp: June 26, 2015, 02:13:10 am
content: I also use DIY ground pins. They are made of copper wire and heat shrink tube.There is nothing spring about them; They are also not loose. You can have the pin coming out from the front or back, or bend or pull out a little to suit the use. You can make one right when you need it.

author: leonhart88
author_rank: Regular Contributor
timestamp: June 26, 2015, 04:10:32 am
content: Quote from: Bud on June 25, 2015, 11:43:04 pmGreat suggestion, I will have to try this!Quote from: onlooker on June 26, 2015, 02:13:10 amThanks for sharing!

author: tggzzz
author_rank: Super Contributor
timestamp: June 26, 2015, 08:39:06 am
content: Quote from: onlooker on June 26, 2015, 02:13:10 amThose look neat and useful in many cases.When you need to rapidly probe around digital ics where there is a variable distance between the pin and an available ground, an adjustible spear is more convenient. Seehttps://www.eevblog.com/forum/testgear/eevblog-707-ringing-on-the-rising-edges-is-there-a-way-to-eliminate/?action=dlattach;attach=133653for a commercial example, orhttps://entertaininghacks.wordpress.com/2015/04/23/scope-probe-accessory-improves-signal-fidelity/for a homebrew equivalent.