Differential measurements using isolated oscilloscope - Page 1
Forum section: electronics / beginners

author: Aners
author_rank: Newbie
timestamp: December 12, 2019, 03:54:16 pm
content: How much difference in measurment accuracy can I expect from using an oscilloscope with isolated channels and single-ended probes versus using a non isolated oscilloscope but with differential probes?I understand that it depends on the scope and the probes, but from a general perspective, what affects the measurment outcome?

author: Benta
author_rank: Super Contributor
timestamp: December 12, 2019, 06:28:13 pm
content: What exactly do you mean by "isolated channels"?If you mean isolating the 'scope power supply by using an isolation transformer: don't do it. It's the most certain way to kill yourself.There are numerous threads on this forum on this subject, as well as videos.

author: Aners
author_rank: Newbie
timestamp: December 12, 2019, 06:36:11 pm
content: Scopes like these:https://www.tek.com/oscilloscope/tps2024bhttps://cleverscope.com/products/CS448

author: David Hess
author_rank: Super Contributor
timestamp: December 12, 2019, 07:07:32 pm
content: There are two major considerations:1. Differential probes have high input impedance on both connections but with common mode rejection limited both inherently and because of mismatched source impedances.2. Isolated probes have low input impedance on the common side, chiefly because of common mode capacitance to ground, but essentially unlimited common mode rejection.The result is that at higher frequencies where the common mode rejection of the differential probe falls, an isolated probe has an advantage in accuracy. But an isolated probe can only be used where one of the connection points has a low impedance capable of driving its common mode capacitance to ground which can be 100 picofarads or more.Note that common high voltage differential probes can have horrendous common mode rejection after a time because of a poor choice of substrate for the high impedance dividers and there are no provisions for the user to adjust it. Isolated probes do not have this issue so might be considered more reliable.

author: Wimberleytech
author_rank: Super Contributor
timestamp: December 12, 2019, 07:19:56 pm
content: Not an answer but a comment related.Not too long ago, I had a case where I needed to make a differential measurement on an off-line SMPS. I decided to build my own differential amplifier using the AD624. The output went to a single-ended probe to a TDS2022B.

author: David Hess
author_rank: Super Contributor
timestamp: December 12, 2019, 09:50:27 pm
content: Quote from: Wimberleytech on December 12, 2019, 07:19:56 pmI have done that also when I did not have a suitable differential or isolated probe available. A test circuit can be designed to drive a 50 ohm transmission line connected directly to the oscilloscope or coaxially drive a common high impedance probe. But for off-line designs, the test circuit can be non-trivial and will not provide galvanic isolation so a special probe is a better choice.

author: GeorgeOfTheJungle
author_rank: Super Contributor
timestamp: December 12, 2019, 10:19:40 pm
content: Quote from: Benta on December 12, 2019, 06:28:13 pmThere are portable scopes that run on batteries and float... and there are signals to be 'scoped that are NOT at lethal potentials... and many other reasons why your answer (even if he floats a mains powered scope) isn't accurate and IMHO is a bit much fear mongering-ish.

author: tggzzz
author_rank: Super Contributor
timestamp: December 12, 2019, 10:35:38 pm
content: Quote from: GeorgeOfTheJungle on December 12, 2019, 10:19:40 pmSigh.The OP, who isalsonew to this forum, would be well advised to read, learn, and inwardly digest this:https://www.eevblog.com/forum/projects/isolation-transformer-for-scope/msg2259465/#msg2259465Especially the anecdote including "This guy was WELL AWARE of the danger and took a lot of precautions to prevent shock . . . Bottom Line: He died anyway."

author: Wimberleytech
author_rank: Super Contributor
timestamp: December 12, 2019, 11:08:14 pm
content: Quote from: David Hess on December 12, 2019, 09:50:27 pmI floated the SMPS with a transformer. Then I eliminated the galvanic isolation (made the grounds common). Thought through it very very carefully before flipping the switch. It worked.

author: David Hess
author_rank: Super Contributor
timestamp: December 13, 2019, 03:35:36 am
content: Quote from: Wimberleytech on December 12, 2019, 11:08:14 pmSure, I have done the same when I had no better option. However the issue for the test circuit goes beyond providing galvanic isolation; when a high side measurement is needed, it is difficult to do a 340 volt level shift with high bandwidth. So instead the low impedance high voltage node needs to be grounded which impresses a 340 volt high voltage switching waveform across the common mode capacitance of the isolation transformer.The whole situation is not impossible however things can quickly get out of hand for certain measurements which a high voltage differential probe or isolated probe make trivially easy.Another way to handle it is to use a high bandwidth current probe. The test circuit converts the measurement to a current and the current probe provides common mode rejection and galvanic isolation. This is not ideal but it can work if there are no better options.

author: Aners
author_rank: Newbie
timestamp: December 13, 2019, 09:19:45 am
content: Quote from: David Hess on December 12, 2019, 07:07:32 pmI am still a little new to some of this stuff but I use it daily and I feel that I need to better my understanding.Could you give me an example of a meansurment with two high impedance signals?

author: David Hess
author_rank: Super Contributor
timestamp: December 13, 2019, 12:43:58 pm
content: Quote from: Aners on December 13, 2019, 09:19:45 amThe signals only have to be high impedance compared to the probe impedance. And since an isolated probe is shunted by 100 picofarads or more to ground on its common side, that is not all that much. Even a low impedance high speed balanced transmission line would qualify in that case.High side gate drive circuits are the common measurement case however their output is very low impedance so isolated probes work well for them; they are almost the ideal case. But if you are measuring the difference between two nodes inside the driver itself, there could be problems with an isolated probe.

author: Aners
author_rank: Newbie
timestamp: December 13, 2019, 02:09:40 pm
content: Quote from: David Hess on December 13, 2019, 12:43:58 pmOk, when you say it is shunted by 100pF what do you mean then? Because I have been looking at simplified probe schematics for single ended probes like the one below, and I dont see what you mean.

author: GeorgeOfTheJungle
author_rank: Super Contributor
timestamp: December 13, 2019, 02:19:11 pm
content: Quote from: tggzzz on December 12, 2019, 10:35:38 pm(Never Ask A Question If You Don’t Know The Answer...)@tggzzz, have you never ever floated your 'scope? Everybody else: raise your hand if you've never done it.

author: tggzzz
author_rank: Super Contributor
timestamp: December 13, 2019, 04:25:20 pm
content: Quote from: GeorgeOfTheJungle on December 13, 2019, 02:19:11 pmNo, I have never floated a scope. I've used my skill and imagination to find ways not to float a scope.How many people have you watched die in front of you fromavoidable accidents? That changes your perspective.

author: capt bullshot
author_rank: Super Contributor
timestamp: December 13, 2019, 04:36:28 pm
content: Quote from: tggzzz on December 13, 2019, 04:25:20 pmNot that difficult. Just keep your scopes away from trees and rivers.

author: magic
author_rank: Super Contributor
timestamp: December 13, 2019, 05:36:52 pm
content: Love those scope floating threadsQuote from: Aners on December 13, 2019, 02:09:40 pmYour picture is wrong, it shows a conventional scope. Cut the connection from input probe ground to oscilloscope ground. Replace with a bit of parasitic capacitance. That's a scope with isolated input channels.

author: Aners
author_rank: Newbie
timestamp: December 13, 2019, 08:04:55 pm
content: Quote from: magic on December 13, 2019, 05:36:52 pmOh yeah, I just assumed the ground is floating so the symbol in it self could be ignored, but still the same idea. I was more wondering about the 100pf shunt, and what that is.

author: David Hess
author_rank: Super Contributor
timestamp: December 14, 2019, 03:35:58 am
content: Quote from: Aners on December 13, 2019, 02:09:40 pmIn your example, an isolated probe has the ground connection on the right hand side cut. The shunt capacitance is then the capacitance from the probe "ground", more properly referred to as common, and the environment including the isolation barriers it uses for power and signal. This also includes the entire length of the probe cable.If an isolation transformer is used, then it has its own capacitance from the primary to the secondary or from the secondary to an electrostatic shield and it will likely be considerably more than 100 picofarads although they do make expensive "low capacitance high isolation" transformers. An isolated probe has a lot less "surface area" contributing to its common mode capacitance.

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 14, 2019, 05:45:40 am
content: Quote from: tggzzz on December 12, 2019, 10:35:38 pmI agree about the dangers of high voltage, but I want to stress that a random quote from a post forum has absolutely no value as a proof.... that smells so much of urban legend...

author: tggzzz
author_rank: Super Contributor
timestamp: December 14, 2019, 08:44:53 am
content: Quote from: not1xor1 on December 14, 2019, 05:45:40 amIf you read the reference, that anecdote was written by theengineerthat investigated the incident as part of hisprofessionalduties. It is much more credible than urban legend.Yes, of course it is an anecdote - but one that is a concrete example of sound proven theory and practice. It is much more credible than urban legend.It is worth repeating anecdotes, because some people - particularly those that distrust "theory" and "experts" - tend to place more reliance on personal stories than impersonal theory. Strange, but true, and worth understanding.And I would still be interested to hear if "GeorgeOfTheJungle" has watched anybody die in an avoidable accident. Somehow I doubt it

author: magic
author_rank: Super Contributor
timestamp: December 14, 2019, 08:56:13 am
content: I have almost been killed myself in an avoidable accident and still don't see it as a reason to become a cowardly wimpThe best way to avoid high voltage accidents is to become a web developerHate to say it, but once you open the cover of some mains powered appliance youalreadyneed to be careful around it, not matter how you probe it.

author: Aners
author_rank: Newbie
timestamp: December 14, 2019, 10:03:07 am
content: Guys, I really appretiate the warning about floating my oscilloscope but I was not planning on doing it and also I would rather this thread contain information about the question I asked. Even if it does not specifically state a by-design isolated oscilloscope I did post which types of scopes I meant in the reply I made. I still have some followup questions, I just need to write them down.I just bet there are many many threads already about how dangerous it is to float a scope and another one is not needed.

author: tggzzz
author_rank: Super Contributor
timestamp: December 14, 2019, 10:13:53 am
content: Quote from: magic on December 14, 2019, 08:56:13 amThat's a stupid non-sequiteur.I don't avoid risks and have deliberately placed my daughter in risky situations, e.g. backpacking around India when 13yo, climbing and skiing, and being a solo pilot in aircraft that have to make forced landings (before she could start to drive a car in her case).The point is to learn tounderstandwhat's risky, how torecogniseincipient risks, and how todealwith them when they occur.People that advocate floating a scopedon'tunderstand,can'trecognise and hencecan'tavoid the risk.I really don't care if such people injure themselves.I do care if they ignorantly teach bad pactices to other people.QuoteTrue but trite and unhelpful.Maybe you think statements such as "do not be on fire" areuseful? Yes, I've seen/heard that statement too often - usually by people embracing the Dunning-Kruger syndrome.

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 14, 2019, 04:31:05 pm
content: Quote from: tggzzz on December 14, 2019, 08:44:53 amOK... I had missed that... you're right... in that case that's a reliable source

author: tggzzz
author_rank: Super Contributor
timestamp: December 14, 2019, 04:42:13 pm
content: Quote from: not1xor1 on December 14, 2019, 04:31:05 pm... and itisalways worth being aware of the possibility of FOAF urban legends

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 17, 2019, 05:56:44 am
content: Quote from: tggzzz on December 14, 2019, 04:42:13 pmThe main problem is that old scopes had a metal case and it was much easier to touch a ground connected part.Modern scopes (at least the cheap ones I know) have very few, easily touchable, ground exposed parts, in most cases just the BNC connectors.Although not as safe as a good multimeter, they are much safer than the old scopes.In any case, even with the right equipment, multimeter or isolated probes, you must be careful when dealing with dangerous voltages. No specialized equipment can protect you from some stupid mistakes.

author: magic
author_rank: Super Contributor
timestamp: December 17, 2019, 08:23:40 am
content: Quote from: tggzzz on December 14, 2019, 10:13:53 amAs always, you are welcome to speak for yourselfThousands of people survive this horrible experience. I'm not even sure if the accident ratio per hour flown isn't lower than in amateur aviation

author: tggzzz
author_rank: Super Contributor
timestamp: December 17, 2019, 09:34:57 am
content: Quote from: not1xor1 on December 17, 2019, 05:56:44 amYes, absolutely true.It isn't even necessary to invokestupidmistakes. We all make mistakes.

author: tggzzz
author_rank: Super Contributor
timestamp: December 17, 2019, 09:38:26 am
content: Quote from: magic on December 17, 2019, 08:23:40 amI'm speaking of people that exhibit the Dunning-Kruger syndrome.Weknow who they are but, by definition,theydon't.Thousands of people survive walking out into road traffic without looking; more so nowadays. By your "reasoning" we shouldn't point out that it is dangerous.

author: GeorgeOfTheJungle
author_rank: Super Contributor
timestamp: December 17, 2019, 09:45:01 am
content: Just let the natural selection do its thing...

author: magic
author_rank: Super Contributor
timestamp: December 17, 2019, 10:09:23 am
content: Quote from: tggzzz on December 17, 2019, 09:38:26 amNobody says you should do it without looking. I say it's safe enough if you look, or at least no worse than any other poking in live HV electronics because that's what it boils down to.

author: tggzzz
author_rank: Super Contributor
timestamp: December 17, 2019, 08:32:01 pm
content: Quote from: magic on December 17, 2019, 10:09:23 amYour statement, which you chose to omit, is equivalent to that in the way I noted.Beginners don't understand the dangers; that makes it dangerous to advocate risky practices.Plus many experienced professionals don't understand the dangers - or rather they incorrectly think they understand the dangers.