Older AWG with ground isolation? - Page 1
Forum section: electronics / testgear

author: ballsystemlord
author_rank: Frequent Contributor
timestamp: January 11, 2025, 06:03:49 pm
content: Hello,Recently, I was using my MSO5000 to measure switched current in a circuit. I wanted to control the circuit, so I connected up the built-in AWG. I turned the AWG on and suddenly my aluminum 30AWG wire smoked. Like, the whole thing went up in smoke. Picture attached.Now, I was of the understanding that oscilloscope manufacturers would have the good sense to galvanically isolate their built-in AWGs from their analog scope inputs. But I guess not...I have a few options to fix this, for now my solution has been a 1Kohm resistor on the AWG's ground (which limits the circuits current to 5mA max).But eventually I'd need an AWG, so I was wondering if there is any good older used AWGs out there? Like, do you have any models you'd recommend?Thanks!

author: shabaz
author_rank: Frequent Contributor
timestamp: January 11, 2025, 06:17:43 pm
content: QuoteIf you were switching current, then that's the sort of thing that could have been easily solved. Possibly most AWGs are non-isolated from ground. Some possible solutions are:(a) Optocoupler(b) Transformer(c) A very low-cost AliExpress-type AWG powered from DC, which can be isolated from ground by using an appropriate power supply(d) A more expensive AWG with isolation as standard, e.g. Tektronix AFG31000.If you're switching high power stuff, you really ought to strongly consider option (a) since it's cheap to do, and eliminates risk.

author: coppercone2
author_rank: Super Contributor
timestamp: January 11, 2025, 06:50:45 pm
content: There is also transformer coupling like the HP 600 ohm ones. 5KHz to 600KHz.They plug into banana jacks and galvanically isolate your signal. Not a high current device though.In your case, I think you would need some kind of pulse transformer.

author: tggzzz
author_rank: Super Contributor
timestamp: January 11, 2025, 09:10:36 pm
content: Quote from: ballsystemlord on January 11, 2025, 06:03:49 pmExposed metal absolutely must be connected to protective mains earth. That includesallthe BNC shields.If your "switched current" involves an SMPS, especially mains, then you should consider an HV differential probe.For when to use different classes of probes, see the references athttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

author: RoGeorge
author_rank: Super Contributor
timestamp: January 11, 2025, 09:12:54 pm
content: Usually, all the bench lab instruments are grounded, not isolated, not floating (unless in some rare exceptions like isolated power supplies and such). The idea is that all lab instruments are supposed to be grounded by default, unless clearly specified otherwise.The usual way to go is to have grounded instruments, and if isolation is needed, to float the device under test (DUT).If floating the DUT is not enough or not possible, look for handheld instruments, or for bench generator that can be powered from a battery pack. Or maybe DIY a battery powered generator.

author: nctnico
author_rank: Super Contributor
timestamp: January 11, 2025, 09:14:22 pm
content: Quote from: tggzzz on January 11, 2025, 09:10:36 pmNope. There are quite a few AWGs out there with isolated outputs. Tektronix and GW Instek have several models. And it makes sense because it is easy to couple noise in through a ground loop.As long as exposed metal parts are seperated from mains by a proper isolation barrier, there is no danger.

author: tggzzz
author_rank: Super Contributor
timestamp: January 11, 2025, 09:21:08 pm
content: Quote from: nctnico on January 11, 2025, 09:14:22 pmThe OP's general purpose AWG is inside the scope. It is not a separate very specialised instrument.

author: Someone
author_rank: Super Contributor
timestamp: January 11, 2025, 10:42:46 pm
content: Quote from: tggzzz on January 11, 2025, 09:10:36 pmObviously not required, as there are AWGs with isolated outputs that still present a regular BNC, both Tek AFG31000 and Keysight 33500 for starters.

author: Someone
author_rank: Super Contributor
timestamp: January 11, 2025, 10:50:32 pm
content: Quote from: tggzzz on January 11, 2025, 09:21:08 pmWhich still does not require every piece of accessible metal to be connected to mains earth, your original claim. An oscilloscope could have an isolated AWG output, it has practical value, and is not prohibited by any safety standards, but it would add some significant cost.

author: ballsystemlord
author_rank: Frequent Contributor
timestamp: January 12, 2025, 01:22:40 am
content: QuoteMaybe this sounds dumb, but wouldn't all you need is an extra (isolated) rail on the PSU (which has 4 rails already IIRC), a separate regulator for the AWG (probably there already), and some optocouplers to-from the main IC which controls the AWG?Like, it seems to me that this could be done for under $20 USD.

author: nctnico
author_rank: Super Contributor
timestamp: January 12, 2025, 02:00:00 am
content: No. The trick / problem is not to couple switching noise from the PSU into the output and have a low capacitive coupling between the floating and grounded section. This is not cheap and easy.

author: RoGeorge
author_rank: Super Contributor
timestamp: January 12, 2025, 06:27:56 am
content: Another way, if the application allows, is to couple the AWG to the DUT with a transformer. I did that a few times. Doesn't has to be a big transformer. A few turns 1:1 through a ferrite tor core.

author: tggzzz
author_rank: Super Contributor
timestamp: January 12, 2025, 10:22:22 am
content: Quote from: Someone on January 11, 2025, 10:42:46 pmInteresting.I note the "isolation" is less than might be necessary, depending on application. Unsurprisingly all the output's shields are connected together. For the Keysight the shield to ground must be <42Vppeak, and the shield is connected to the ground with 1Mohm//50nF. User manual p253 discusses how that affects the waveform seen at the load.In addition, that's not a low capacitance, and would be entertaining if it was put on an inopportune place in an SMPS.

author: 2N3055
author_rank: Super Contributor
timestamp: January 12, 2025, 11:43:11 am
content: Quote from: ballsystemlord on January 11, 2025, 06:03:49 pmI would ask you why you where under impression that would the case.I went and checked, your first posts about "my MSO5000" are from 2022..Did you not, in a few years, ever bothered to read user manual that clearly states (in very hard to ignore fonts and graphics) that instrument is grounded (as it should be)?I am not criticysing. Simply stating that you need to learn a bit more about how things work.There are special scopes that have galvanically isolated inputs. I know of one benchtop scope (TPS2000B, not being sold anymore), Cleverscope USB scope and several handheld scopes (Siglent, R&S, Fluke, Keysight) that have isolated inputs...First you tried to do what is called differential measurement (measuring a potential between two points that are both floating at some potential) with grounded instrument, meaning one side of your probe created direct short circuit where you put it.Now you want to float AWG.Instead of inventing instruments that does not exist or are rare and exotic, ask yourself a question: what am I doing wrong? What knowledge do I miss ?How others do this task by using grounded instruments?Also you should learn about isolation and what it means.Is it only galvanically isolated at DC but will pass some AC frequencies?What is capacitive coupling?How much coupling is to ground, how much is to signal, how much is common mode?Is your isolated output/input single ended with isolation or is it truly differential?If you don't want to go fully down the rabbit hole, that is fine.Then you should ask questions in more specific way.Like "how do I measure current with a scope?"Or " I have to inject 1 Khz 1V P-P sine wave between two points in circuit. How do I do that?"Don't take this as me criticising you. I am trying to point you in the right direction.Absolutely read article on Tggzzz site about probes, that he linked.. He has some good stuff there.As to actual isolated instruments, Cleverscope has scope that have isolated inputs and outputs. It is very expensive scope, quite average otherwise. As a testament how easy it was, the designer made a video where he explains how they had to invent new way to design power supply to be able to have good isolation (AC and DC). All manufacturers of isolated scopes have dozens of patents just for that and for channel implementations. That is how easy it is.And, like I said, price is such that buying differential voltage probes and isolated current probes for your existing scope start to look cheap.

author: ballsystemlord
author_rank: Frequent Contributor
timestamp: January 12, 2025, 04:40:27 pm
content: Quote from: 2N3055 on January 12, 2025, 11:43:11 amTo my credit, I did and do know that the instrument and it's chassis are earth grounded. I expected the AWG to be floating. Probing a shunt resistor is only one of several uses of a scope + AWG where you'd want the AWG to be floating.Quote from: 2N3055 on January 12, 2025, 11:43:11 amWhich is why I'm a student.Quote from: 2N3055 on January 12, 2025, 11:43:11 amI knew that too. I used my DMM to check the PSU's cold side rails. They were not connected to earth ground, so doing a differential measurement with a regular scope probe seemed to be an okay idea.Quote from: 2N3055 on January 12, 2025, 11:43:11 am1: Nothing that I was aware of (I'm now of another opinion.)2: A lot, I'd imagine.3: I'm not sure. Most stuff I've read or seen has been direct scope-to-DUT. Very few people do differential measurements, much less using the AWG at the same time. Many people I've seen/read about doing measurements with an oscilloscope use a dedicated AWG -- even if their scope has a built in one. I'm not sure why. IMSA guy has the exact same MSO5000, but chose to purchase a dedicated AWG. IIRC Mr. Smith, who frequents these forums, has several AWGs, some of which are specialized which he uses with an old Lecry scope (it's unclear if that scope has a built-in AWG). Dave seems to prefer to use higher end scope's built-in AWGs.Articles on EE I've read don't use an AWG, so not applicable. They're mostly discussing various points of EE or what's going on in the industry.The book series I've read on EE had one chapter on oscilloscopes. No mention was made on the subject of probing technique. It was just about how to operate a scope and interpret the data.Granted, I can certainly look up all the above, I just didn't think it was that complicated.Quote from: 2N3055 on January 12, 2025, 11:43:11 amOh no. I've taken this personally. This is war.If you accessed the URL that Tggzzz provided, you'd know that it's a list of articles/video to read/watch. Some of them I've read/watched. The others I'll certainly get to.Thanks

author: bdunham7
author_rank: Super Contributor
timestamp: January 12, 2025, 04:49:27 pm
content: The Feeltech FY6600/6800/6900 are either isolated-ish or can be easily modified to be so with little difficulty. They can also be adapted to battery use if you want to avoid the noise issues. However, it is per-unit isolation so that both channels and all of the external BNC connections are all floating together. Like some of the A-brand "isolated" AWGs mentioned earlier, this is only acceptable for SELV-level isolation so 30-40V at most. It is not anything like the isolation of the in-guard portion of a bench multimeter.