Author Topic: Floating an oscilloscope for measurements  (Read 8385 times)

0 Members and 1 Guest are viewing this topic.

Online nctnico

  • Super Contributor
  • ***
  • Posts: 21127
  • Country: nl
    • NCT Developments
Re: Floating an oscilloscope for measurements
« Reply #25 on: September 05, 2016, 05:08:11 pm »
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?
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline Fungus

  • Super Contributor
  • ***
  • Posts: 12243
  • Country: 00
Re: Floating an oscilloscope for measurements
« Reply #26 on: September 05, 2016, 05:45:10 pm »
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.

That'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 produce exactly the sort of noise you're seeing on screen.
 

Offline System Error Message

  • Frequent Contributor
  • **
  • Posts: 470
  • Country: gb
Re: Floating an oscilloscope for measurements
« Reply #27 on: September 05, 2016, 09:50:48 pm »
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?
 

Offline forrestc

  • Supporter
  • ****
  • Posts: 585
  • Country: us
Re: Floating an oscilloscope for measurements
« Reply #28 on: September 06, 2016, 12:28:27 am »
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.

Sorry 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 at https://learn.sparkfun.com/tutorials/how-to-use-an-oscilloscope/using-an-oscilloscope contains this gem:

Quote
The first key to probing a signal is finding a solid, reliable grounding point. Clasp your ground clip to a known ground, sometimes you may have to use a small wire to intermediate between the ground clip and your circuit’s ground point. Then connect your probe tip to the signal under test. Probe tips exist in a variety of form factors – the spring-loaded clip, fine point, hooks, etc. – try to find one that doesn’t require you to hold it in place all the time.

I 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-SKS

One other item which I purchased was a new set of 4900 series scope leads and an assortment of adapters from probemaster.  See http://probemaster.com/oscilloscope-probes/4900-series/accessories/ for a nice chart of what you can do with the adapters they make.
 
The following users thanked this post: System Error Message

Online tggzzz

  • Super Contributor
  • ***
  • Posts: 13558
  • Country: gb
    • Having fun doing more, with less
Re: Floating an oscilloscope for measurements
« Reply #29 on: September 06, 2016, 08:21:48 am »
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.

And that is a good illustration of why it is extremely unwise 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. See https://entertaininghacks.wordpress.com/category/probes/ for the theory, plus commercial and homebrew solutions.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 

Offline System Error Message

  • Frequent Contributor
  • **
  • Posts: 470
  • Country: gb
Re: Floating an oscilloscope for measurements
« Reply #30 on: September 06, 2016, 12:43:41 pm »
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?
 

Online tggzzz

  • Super Contributor
  • ***
  • Posts: 13558
  • Country: gb
    • Having fun doing more, with less
Re: Floating an oscilloscope for measurements
« Reply #31 on: September 06, 2016, 01:00:10 pm »
If i wanted to measure current on the scope how would i go about it?

That is a standard topic. Rather than refer you to lmgtfy.com, I'll point out that definitive professional answers are in the relevant references at https://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.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 

Offline Fungus

  • Super Contributor
  • ***
  • Posts: 12243
  • Country: 00
Re: Floating an oscilloscope for measurements
« Reply #32 on: September 06, 2016, 01:04:58 pm »
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.

The 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 floating
or
b) Your mains wiring is bad

(a) Is safe for ground clips, (b) isn't, find out which case you're dealing with before proceeding.
« Last Edit: September 06, 2016, 01:09:36 pm by Fungus »
 
The following users thanked this post: Trader

Offline Muxr

  • Super Contributor
  • ***
  • Posts: 1345
  • Country: us
Re: Floating an oscilloscope for measurements
« Reply #33 on: September 06, 2016, 01:42:57 pm »
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?
Some 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
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf