Floating an oscilloscope for measurements - Page 1
Forum section: electronics / testgear

author: System Error Message
author_rank: Frequent Contributor
timestamp: September 05, 2016, 09:05:10 am
content: Hi it seems i absolutely have no choice but to disconnect oscilloscope from mains. I have AV2000 running and it has been feeding noise back into the oscilloscope and messing up readings. DC readings now appear as AC around 0V rather than the expected volts and the consistent negative voltage noise as well.How would i go about floating the oscilloscope? The AV2000 uses the mains and ground.

author: tggzzz
author_rank: Super Contributor
timestamp: September 05, 2016, 09:14:38 am
content: Quote from: System Error Message on September 05, 2016, 09:05:10 amPrecisely why do you have to float the scope?What other techniques and technologies have you assessed and deemed to be inadequate, and why?Who is making you indulge in dangerous practices?Have you got it in writing that they are forcing you to indulge in dangerous practices? ... safely stored in a place your dependents can find it?If it kills you, presumably innocent third parties will have to deal with your body. Will they also be at risk of the same electric shock, and if not, why not?Have you made a will? The UK intestacy laws can be surprising.

author: System Error Message
author_rank: Frequent Contributor
timestamp: September 05, 2016, 09:31:39 am
content: I told you why, i have the powerline adapter av2000 running about.I cannot make measurements on the same power grid with those adapters running as they communicate using the ground link also so it messes up measurements from the oscilloscope. So when measuring DC instead of say a +12V straight line with the usual noise i get a 2V peak to peak AC wave. I checked all the settings on the oscilloscope as well.

author: nctnico
author_rank: Super Contributor
timestamp: September 05, 2016, 09:38:15 am
content: Use a differential probe. Another solution would be to use a 600 Ohm telecom (modem) transformer between the DUT and your scope but this is AC only.Still I can't imagine a power line modem using ground because ground may be absent. There may be another problem lurking like faulty mains wiring in the building.

author: nfmax
author_rank: Super Contributor
timestamp: September 05, 2016, 09:43:02 am
content: Are you trying to measure the AV2000, or is it simply that the presence of the AV2000 signals is interfering with the normal operation of your oscilloscope? If the latter, you need to replace either your oscilloscope with one that is not susceptible to the EMI generated by the AV2000, or disconnect the AV2000 (all of them) while you are measuring. These mains networking boxes are the devil's own invention, anyway

author: System Error Message
author_rank: Frequent Contributor
timestamp: September 05, 2016, 09:47:45 am
content: differential probes dont help, you have modulated digital signals on the ground that change as voltage is applied. Have any of you guys ever tried the av2000 powerline before? The AV2000 uses 2x2, the 2nd link using ground. Older powerline adapters do not use ground.Using a transformer and differential probes on the front side of the scope isnt going to help as the problem is on the rear end so i will still get weird readings.I didnt have this issue before installing the av2000. I also dont have a choice of removing them because im just renting so i cannot install cables for my network (already asked). Basically its modem -- powerline -- router. Is the bandwidth of the av2000 necessary? The answer is yes as it also has the lowest latency among powerline adapters. I actually get 1/3 of rated speed on the powerline adapter itself so downgrading will seriously hurt internet performance.So i do need to remove the ground reference from the scope, so my question is how about would i do that? Do i get a UPS and run it on battery? Do i just use a 2 pin plug instead?Im not trying to measure the av2000, the av2000 is interfering with the scope.

author: tggzzz
author_rank: Super Contributor
timestamp: September 05, 2016, 10:03:34 am
content: Quote from: System Error Message on September 05, 2016, 09:31:39 amI have no idea what an "AV2000" is (as per your OP), and if you think I am going to search for a service manual and then read it to find how it workd, you will be sadly disappointed.QuoteIf they lift the ground relative to something-or-other, then presumably that would also affect other equipment.My other questions are rather important as well.

author: tggzzz
author_rank: Super Contributor
timestamp: September 05, 2016, 10:12:40 am
content: Quote from: System Error Message on September 05, 2016, 09:47:45 amYou should remove the source of the problem, not introduce other problems.There are other newfangled ways of getting short-range network connectivity without cables. I suggest you investigate them.Floating a scope may not affect internet performance, but it may seriously hurt other things' performance.QuoteYou can do anything you like, but not safely. The key question is "when a scope probe shield is accidentally connected to a dangerous voltage, what stops the entire external chassis being at that dangerous voltage?". Batteries won't make that safe!Please consider the safety of the "first attenders" that may have to deal with your body.

author: forrestc
author_rank: Unknown
timestamp: September 05, 2016, 10:21:59 am
content: Quote from: System Error Message on September 05, 2016, 09:31:39 amAre you sure what you think is happening is what is actually happening here? (Or stated differently: Are you sure that you don't have something else broken?)First of all, the scope circuitry itself should be isolated from the AC power line current carrying conductors completely. This is a safety issue if it isn't.The shield on the scope BNCs will most likely be attached to the ground - BUT... if used properly this shouldn't cause what you're describing - simply because the 'reference' for the measurement should be the ground connector on the probe itself, not the AC power line. So measuring a DC power source should effectively reference the ground potential on the scope, eliminating any noise on the ground line. Think about it this way: The ground noise you're getting in the back becomes the reference for the scope. It doesn't matter what that reference voltage *is*... it could be a very large sine wave.... once you connect ground from the front of the scope to your circuit, the reference voltage for the scope is identical to the reference voltage in your circuit. (I'm ignoring potential safety issues with a large sign wave living on your ground, as that shouldn't ever happen).Every time I've seen something like this, it's a situation of an improper or missing connection between the circuit under test and the shield on the BNC connectors. Theoretically you could end up with some weird ground currents if your circuit is also ground referenced, and that *might* cause some problems. But almost certainly not at the levels you're describing.I'd check your scope, and your probes, etc., before going down the path you think you need to go down. Heck, start by finding a chunk of cable with a BNC connector on it and shorting it and then hooking it into the scope input.I apologize if you've already been down all of these paths, it's just that I'm very skeptical that ground noise on the AC line would cause this problem (with the magnitude you describe) if the cabling on the front of the scope is working and connected properly. I think it's far more likely that either a) your probes are defective (or hooked up wrong) or b) Your scope needs some help. What scope is this? Can you post a pic of say the scope hooked up to a 9V battery and the accompanying waveform?Specifically in relation to ground isolation: There are ways to do this, but very few are safe. The scope ground is connected to the mains ground for good reason. Only when you understand the reasons behind the connection, and how grounding keeps you safe. Once you understand this, you'll also understand how to remove this isolation if you are willing to accept the risk. I've never felt the need to do so.

author: tautech
author_rank: Super Contributor
timestamp: September 05, 2016, 10:27:15 am
content: SEM, you're between a rock and a hard place and I understand your dilemma but be assured what you're proposing has the potential to kill, so don't go there.Find another safe solution no matter how painful it may be.

author: singapol
author_rank: Frequent Contributor
timestamp: September 05, 2016, 10:42:59 am
content: Quote from: System Error Message on September 05, 2016, 09:05:10 amYou don't float the scope but use proper differential probe or differential amplifier which may be out of your budget. Don't be reckless. Manufacturers of powerline networking don't follow industry standards and use different chipsets in their products resulting in some compatibility issues like this:http://www.techhive.com/article/3011197/home-networking/zyxel-av2000-homeplug-av2-model-pla5456-powerline-adapter-review-speedy-hardware-dodgy-marketing.htmlIf you are curious to know how floating measurements are made then have a look at these: Remember that your life is precious.Floating osciloscope measurements:www.tek.com/dl/51W_10640_1.pdfhttp://www.tek.com/document/application-note/three-facets-floating-measurement-solutionshttp://www3.telus.net/Harley_Davidson/oscilloscope/differential%20techniques.pdfhttp://www.davmar.org/TE/TekConcepts/TekProbeCircuits.pdf

author: Alex Nikitin
author_rank: Super Contributor
timestamp: September 05, 2016, 10:53:32 am
content: If a simple mains filter does not resolve this interference, you may have a problem with the ground connection on your mains. Under no circumstances you should have a noise, capable to interfere with a scope operation, to appear on the mains ground wires. Something is not right here.CheersAlex

author: System Error Message
author_rank: Frequent Contributor
timestamp: September 05, 2016, 12:46:44 pm
content: well whats not right is a DC voltage out from a bench PSU that i tested less than a month ago appearing as AC on the scope itself (scope is set to DC). I still havent tested plugging the scope behind the powerline adapter itself or a noise filter on ground. If the PSU's voltage was AC and less than 10 volts peak to peak it would not be able to power the equipment i was testing.In the UK things are very tight in terms of space and supply that interference comes from various places. Short of a faraday cage for wireless interference it seems like i will need a filter for ground.So aside from filters, plugging behind the PLA, what else can be done?

author: Fungus
author_rank: Super Contributor
timestamp: September 05, 2016, 01:00:39 pm
content: Quote from: System Error Message on September 05, 2016, 12:46:44 pmSure, but floating the 'scope won't fix that.Why not float the PSU instead?Quote from: System Error Message on September 05, 2016, 12:46:44 pmCheck your mains wires. Maybe there's a loose wire in a plug or something.Are both the 'scope and PSU plugged into the same power strip? If not, try that.Have you got any new light fixtures in the last month. Cheap LED lights can do things like this (Dave made a video on it).Take both the 'scope and PSU to somebody else's house and try it there.

author: System Error Message
author_rank: Frequent Contributor
timestamp: September 05, 2016, 01:08:41 pm
content: Quote from: Fungus on September 05, 2016, 01:00:39 pmI use philips LED bulbs, not sure if you call them cheap.If there is a loose wire it will spark as i have seen it before when dealing with poor quality terminals which you have to screw the cable down on. Cable got loose and there are a few sparks between the wire and terminal (very small gap, insignificant sparks).I could certainly try the same power strip or test against batteries.

author: Fungus
author_rank: Super Contributor
timestamp: September 05, 2016, 01:12:41 pm
content: Quote from: System Error Message on September 05, 2016, 01:08:41 pmDid you get them recently?One of them may be failing or have a bad capacitor in it. That can do all sorts of bad things to the power.Quote from: System Error Message on September 05, 2016, 01:08:41 pmAlso try it at a friend's house (seriously!)

author: System Error Message
author_rank: Frequent Contributor
timestamp: September 05, 2016, 01:39:21 pm
content: LEDs are easy to dismiss, just turn them off. They are LED bulbs, not the fancy LED light strips or lights that use pulsing and other tricks to be bright and power saving.Nearest friend's house is extremely far away so not an option.Edit: i just tested measuring a battery by both either using only 1 terminal on probe and both. With connecting both the probe and ground i got the accurate voltage, with just the probe itself the result is in the screenshot.So now my concern is when i cannot use ground to probe the circuit.

author: nfmax
author_rank: Super Contributor
timestamp: September 05, 2016, 01:49:43 pm
content: Have you triedtemporarilyunplugging the AV2000 units (both of them) and confirming the oscilloscope then operates correctly?

author: System Error Message
author_rank: Frequent Contributor
timestamp: September 05, 2016, 01:53:54 pm
content: unplugging the AV2000 would be very very inconvenient especially for putting them back. Not only are things plugged into them and losing internet but to have to go down some floors unplug, and repeat is tedious including the annoyed looks i will get. I used to be able to just connect probe and not ground and get a near accurate voltage of what i was measuring.

author: nfmax
author_rank: Super Contributor
timestamp: September 05, 2016, 02:00:43 pm
content: Wait - are you saying your standard way of measuring was to connect only the probe signal lead and not the ground lead, relying on a common mains earth to complete the circuit? That is just asking for trouble!Edit to add:Is either your scope or the UUT actually connected to mains through the feedthrough socket on the AV2000? Have you tested the AV2000 for continuity between mains earth in and mains earth out (while disconnected from the mains supply, of course!)

author: Fungus
author_rank: Super Contributor
timestamp: September 05, 2016, 02:04:37 pm
content: Quote from: nfmax on September 05, 2016, 02:00:43 pmI'm also worried about the "go down some floors and unplug" part.I think we need more information about how things are connected.

author: System Error Message
author_rank: Frequent Contributor
timestamp: September 05, 2016, 02:23:44 pm
content: Its not easy trying to hook up the ground of the probe at times to measure.For example in this pictureThere is a hole for you to insert probes but the ground for the probe is a crocodile clip so hooking it up and making sure the crocodile clip doesnt touch the board or circuit and accidentally cause a short is another because for one thing its an expensive equipment, another is that it has no ground connection. So trying to measure the voltage of it while it is powered up is tricky so i dont always connect the ground probe.Besides isnt the ground probe itself connected to ground being more likely to be dangerous to connect both leads if the circuit is grounded?

author: nfmax
author_rank: Super Contributor
timestamp: September 05, 2016, 03:06:35 pm
content: Quote from: System Error Message on September 05, 2016, 12:46:44 pmStop right there! If I understand what you are trying to say, it looks as if when you plug your oscilloscope into the output of your AV2000the earth pin of the oscilloscope mains plug is no longer connected to mains earth. Stop. No I meanSTOP! Disconnect the mains supply to the AV2000, disconnect the AV2000 completely from the mains and everything else, and check for continuity between the 'plug' earth pin and the 'socket' earth contact of the AV2000. I think you might have a faulty unit which could kill you. You should be able to return it for a correctly operating unit or a refund.

author: System Error Message
author_rank: Frequent Contributor
timestamp: September 05, 2016, 04:21:32 pm
content: Quote from: nfmax on September 05, 2016, 03:06:35 pmQuite wrong, as i said i am not trying to measure the AV2000.The oscilloscope and AV2000 are on seperate plugs, both are connected to earth. If i were to plug the oscilloscope into the plug on the av2000 they will both be connected to earth.

author: nctnico
author_rank: Super Contributor
timestamp: September 05, 2016, 05:08:11 pm
content: None of us understands the relation between your problem and the AV2000. Why are you mentioning the AV2000?Can you post a picture of your test setup and what you are trying to measure? Are you sure you probe isn't broken?

author: Fungus
author_rank: Super Contributor
timestamp: September 05, 2016, 05:45:10 pm
content: Quote from: System Error Message on September 05, 2016, 04:21:32 pmThat's true but there's a big piece of unshielded wire between them with all sorts of switching power supplies, etc. in the vicinity (there's one in the 'scope for a start).That wire has inductance, it picks up the local radio station radio and cosmic rays, it will produceexactlythe sort of noise you're seeing on screen.

author: System Error Message
author_rank: Frequent Contributor
timestamp: September 05, 2016, 09:50:48 pm
content: i do get more noise when i connect the probes but what bugs me is when not connecting ground probe for measuring.If i understand right with the ground probe connected the voltage travels from one end of the circuit to the other but if not it will go to ground? Usually when i dont connect ground to the circuit the measurement is less accurate (to a constant degree)and shows the correct waveform but recently it seems to always fails to measure DC when i dont connect the ground which is a bother to me because arent you only supposed to connect ground to circuit if you are entirely sure there wont be some sort of loop if circuit is not isolated?

author: forrestc
author_rank: Unknown
timestamp: September 06, 2016, 12:28:27 am
content: Quote from: System Error Message on September 05, 2016, 01:39:21 pmSorry if the following sounds harsh, just trying to be clear:What you are doing is wrong. Not even close to correct.The way you're using the scope will frequently result in what you are seeing. Let me try to explain.Your scope isn't magic. The scope needs *two* connections to the circuit. Just like a light bulb needs two connections to a circuit to turn on, or a voltmeter needs two connections to the circuit to read a voltage (or ohms, or whatever). The oscilloscope is EXACTLY LIKE THAT, it's just a much faster voltmeter which can graph the voltage over time.What you're effectively doing is plugging your 'voltmeter' onto the circuit by sticking the red lead on the circuit and the black lead into the ground lead on the mains plug. In order for you to get any sort of reading, your device under test has to also be grounded to the mains, and the 'voltmeter' is going to read the combination of the voltage from your circuit and any noise which is on the ground wire. This is basic electronics. Now substitute 'scope' conceptually into this thought process and you'll see why you're getting the noise.In your 'use just one probe on the battery' example, you'll never get a reading. Do you expect a voltmeter to read if you're only connecting the red lead? You shouldn't expect any more from your scope.Along the same lines, if you did isolate the ground from the oscilloscope like you were asking, and not connect the ground lead, there is absolutely no way for the oscilloscope to EVER get a reading - because you've just eliminated the one, very crappy, return path for the signal. Just like unhooking the black lead from your voltmeter.The *CORRECT* method is to ground using the ground clip on the probe that the oscilloscope gods gave you. It's so important that there are numerous scholarly papers on it. Even the 'how to use an oscilloscope' document athttps://learn.sparkfun.com/tutorials/how-to-use-an-oscilloscope/using-an-oscilloscopecontains this gem:QuoteI noted down a ways you mentioned that sometimes you don't feel that you can find a useful, or safe, ground on the device under test. I've probed a lot of devices, and can't say that I've ever encountered a situation where I couldn't figure out a better ground than just hoping that the AC ground is good enough. I have encountered a lot of places where I wished I had a *better* way to connect the ground.So here are some notes which might help:The ground wire is short for a reason. In an ideal world, you would ground the clip electrically close to the signal you are probing, and use a separate ground for each signal to help eliminate any ground noise in your measurement. With that firmly in mind, there isn't any reason you can't cheat a little bit where you just care about basic readings. Just be aware that any noise you encounter may be an artifact of how much you've cheated. The problem you're running into is that what you're trying to do (hope the electrical ground will suffice) is way over the line as far as what is likely to work.Personally I try to ground each oscilloscope probe to a common ground point on the circuit. I will use various alligator clip test leads or similar to extend the cables just as needed. This seems to be the best compromise, especially if I'm watching signals on an ongoing basis, and not just probing around the board.In the case where I am experiencing difficulty probing using a probe while keeping it's ground clip connected, I'll occasionally make sure I have at least one probe attached to the circuit (and oscilloscope) with a solid ground, and then use a second probe without the ground lead connected to probe signals. This way, the return path is at least 'close' - i.e. through the second scope probe. I know when doing this I WILL end up with more noise on the signal - but this is perfectly fine for where you're looking at signals of a reasonable amplitude, as long as you recognize this as a likely problem.In your case, I would suggest that a good assortment of useful grabbers and clips and adapters would help greatly - this would help you get a solid ground point. Every device has somewhere which you can connect to. For the larger devices, don't overlook the metal chassis in some cases - although that too may have more noise on it.Any time you see noise, you should instantly think 'need better ground'.Some useful links for grabbers:http://parrot-invent.com/shop/http://muellerelectric.com/product-category/grabbers-plungers/https://www.soselectronic.com/test-probes?brand=HIRSCHMANN-SKSOne other item which I purchased was a new set of 4900 series scope leads and an assortment of adapters from probemaster. Seehttp://probemaster.com/oscilloscope-probes/4900-series/accessories/for a nice chart of what you can do with the adapters they make.

author: tggzzz
author_rank: Super Contributor
timestamp: September 06, 2016, 08:21:48 am
content: Quote from: System Error Message on September 05, 2016, 01:39:21 pmAnd that is a good illustration of why it isextremelyunwise to tell beginners how to do anything safety related, e.g. a recent thread about how to measure 2kV when repairing an oscilloscope. There is simply no way that people can remotely diagnose what's going on inside a beginner's head, nor any mistakes there may be in their experimental setup.It is necessary to presume they haven't told you everything relevant because they simply don't understand what might be relevant!It also shows why the simple, safe statement is "never float an oscilloscope". If you think you need to float a scope, it is usually best to rethink what you are doing. Doubly so if you are a beginner.Once the OP has understood the basics of measuring a voltage, they will then move on to the concept that "ground" is a convenient fiction, earth even more so.Let's start that process by considering something on a PCB; clearly anything with longer wires will be even worse than this standard example. Given six inverters in a 16 pin DIL package driving a 50ohm transmission line load. When they simultaneously switches 5V in say 5ns, then the output current will change by 6*5/50=600mA in 5ns = 120MA/s (not mA/s). Now the ground lead inside the package is 10mm long so it has an inductance of 10nH. Since V=Ldi/dt, the voltage induced across the ground lead inside the package is 1.2V. That's ground bounce, and it does cause real problems in TTL circuits!If you don't like that, consider probing a digital circuit using a standard 15pF *10 probe with a 6" ground lead, where the latter is effectively a 150nH inductor. That LC combination will resonate at ~100MHz. That's a very real probing problem that hits beginners all the time. Seehttps://entertaininghacks.wordpress.com/category/probes/for the theory, plus commercial and homebrew solutions.

author: System Error Message
author_rank: Frequent Contributor
timestamp: September 06, 2016, 12:43:41 pm
content: Thanks, a lot of helpful information which is quite different from EEVblog's video on not blowing up an oscilloscope. The different thing he mentioned was not connecting the ground probe/clip if the circuit is grounded incase a loop would form.If i wanted to measure current on the scope how would i go about it?

author: tggzzz
author_rank: Super Contributor
timestamp: September 06, 2016, 01:00:10 pm
content: Quote from: System Error Message on September 06, 2016, 12:43:41 pmThat is a standard topic. Rather than refer you to lmgtfy.com, I'll point out that definitive professional answers are in the relevant references athttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/Once you have understood those you will be in a good position to ask pointed and interesting questions.

author: Fungus
author_rank: Super Contributor
timestamp: September 06, 2016, 01:04:58 pm
content: Quote from: System Error Message on September 06, 2016, 12:43:41 pmThe video is correct: The ground clip can be dangerous in the wrong hands. Clip it to the wrong thing and you can short-circuit it.OTOH you can expect very wobbly readings if you rely only on the mains earth to provide a common reference between the 'scope and the device under test. You need to find a middle ground.If you have a multimeter you can look for a big AC voltage difference between oscilloscope ground and device-under-test ground. If the reading looks safe (ie. close to zero) then switch to amps mode and see if any current flows (10A range). If it doesn't then there's no loop, go ahead and connect the ground clip. If you're paranoid and your meter has crocodile clips then you could put a resistor in series for this step.If you *do* see a big voltage difference then:a) The equipment could be floatingorb) Your mains wiring is bad(a) Is safe for ground clips, (b) isn't, find out which case you're dealing with before proceeding.

author: Muxr
author_rank: Super Contributor
timestamp: September 06, 2016, 01:42:57 pm
content: Quote from: System Error Message on September 06, 2016, 12:43:41 pmSome more reading material.. in particular it covers the A minus B method of current measurement with a typical oscilloscope:http://www.techni-tool.com/site/ARTICLE_LIBRARY/Tektronix%20-%20IsolatedInputOscopes.pdf