Safely using a Second Oscilloscope as a Differential Probe?    Will it blow up?  - Page 1
Forum section: electronics / beginners

author: Zero_Ego
author_rank: Regular Contributor
timestamp: May 25, 2025, 06:56:53 am
content: Hi,As the title suggests, I am looking for alterative ways NOT to blow up my oscilloscope (or actually more likely the circuit under test) by not shorting passive ground probes together when testing two parts (or more) of the same circuit.Differential probes are out of my budget, however I have a second oscilloscope and if I connect it correctly (Isolate) can I safely use it on the same circuit? (In other words by making sure it's floating)To make my question more clear, please take a quick look at this diagram from ElectroBoom's youtube channel. (I have time stamped it)https://youtu.be/DgYGRtkd9Vs?si=UvIWyaUDbSiwethR&t=396In that whiteboard schematic, it shows the differential probe connected to the end of the full bridge rectifier.Here's my question and this all pertains to safety for me and my equipment and hopefully solidifies my study and research so I get this correct first time. (No guessing allowed - I don't want the sparks)Looking at the hand drawn schematic and noting where the differential probe is connected is the following correct please?Can I connect a second oscilloscope (instead of the differential probe in the drawing and not blow it up) IF and ONLY IF the second oscilloscope is NOT sharing the same ground?Here's how I would make sure it's isolated. (I don't have an isolation transformer)I see three ways of doing it correctly.The first is if my second oscilloscope is sharing the same "Ground" mains earth as the first - I can remover the ground pin from the mains powering to the second oscilloscope, which will make it 'float.' ( by the internal switch mode power supply if the second crow is mains powered also)Second way: Use an oscilloscope that can run on external power (For example USB C) - this would isolate in TWO ways - USB chargers have no ground and the switch mode charger is internally isolating (as well) due to the nature of how switch modes primaries transformer windings are isolated from the secondary. (Which technically is the same as the first method above the only difference is that the power supply is now outside the unit - switch modes isolate by nature?)Double isolation with a simple USB charger am I correct? (No Mains ground and switch modes isolate by nature)Finally the third way (probably the safest) is to simply use a hand held battery operated device to test the output. This last one is the ultimate in isolation. It doesn't float much more than when it's in your hand?Are my observations correct? Can anyone see any floors or potential hazards that I might be over-looking?Could someone please tell me if my theories are sound?I'm sure I am not alone grappling with this topic.Thank you for reading.Any comments will be greatly appreciated.ZEP.S I could ask AI but I want to get it correct first time.

author: tggzzz
author_rank: Super Contributor
timestamp: May 25, 2025, 07:13:56 am
content: All scope probe shields are connected to each other and the scope case. If you "float a scope" then whatever voltage you connect one probe shield to will also appear on the scope case and the other probe shields. That may damage the scope and yourself, and in some cases your circuit - so don't do it.Sometimes it is possible to simulate a differential probe by inverting one channel and adding that to the other channel and displaying the result.FFI see the references athttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

author: Zero_Ego
author_rank: Regular Contributor
timestamp: May 25, 2025, 07:50:43 am
content: Thanks for the reply.I clearly see some of the possible dangers of "Floating a Scope" (a new term for me) you have mentioned especially if your messing with high voltage. (The case of the second crow can become live and electrocute yourself) Great observation thank you.I will check out simulating differential probes by inverting one channel. Interesting thanks for the URL I will check it out.As soon as I finished posting this, Dave's 13 year old video was recommended to me. (Yes I've watched it before)EEVblog #279 - How NOT To Blow Up Your Oscilloscope!Source:Thanks again for the reply.

author: J-R
author_rank: Super Contributor
timestamp: May 25, 2025, 10:26:50 am
content: With some scopes you have to use invert and add, but with others you can just use subtract.Be sure to still use the correct probe attenuation for the voltages you'll be encountering, such as 10X or 100X.Here is a video demonstrating the method of using two channels on a scope instead of a dedicated differential probe:

author: Zero_Ego
author_rank: Regular Contributor
timestamp: May 25, 2025, 01:26:34 pm
content: Hi J-R,I really enjoyed learning this advanced technique Invert and Add - a feature on some oscilloscopes. I will also research the subtraction method which I assume is also a operating function on some scopes.Brilliant example you have posted.Thank you.ZEP.S So glad I came to this forum and chose GI over AI. (Genuine Intelligence)

author: tggzzz
author_rank: Super Contributor
timestamp: May 25, 2025, 01:46:22 pm
content: In general AI slop looks good if and only if you don't know the subject.

author: Zero_Ego
author_rank: Regular Contributor
timestamp: May 25, 2025, 02:16:33 pm
content: Hi tggzzz,I agree with calling AI slop. It tends to Slip Slop Slap one in the face if your not careful. It would of taken me in circles even just explaining the scenario. (Then still gets it wrong) I have had to call it out many times with manual in hand.There's a lot of smoke and mirrors yet at the same time - at times - ChatGPT has just blown my mind. If fact it's directly related to why I am back here. It helped me lay some ground work on this topic but I don't trust it. No way!!!Long live GI.ZEP.S I'll be back as I have a follow up question. Thank you.

author: Zero_Ego
author_rank: Regular Contributor
timestamp: May 25, 2025, 02:59:11 pm
content: Quote from: tggzzz on May 25, 2025, 07:13:56 amI was going to push the boundaries by being more oscilloscope specific with the same scenario. But re-reading your post - I have decided that it's not worth the risk.Don't Float the Scope! Sounds like good practice and common sense must prevail. Why risk blowing up a sensitive piece of test equipment? Answer, don't do it.I also see that you did mention the Invert + Add on a second channel technique. I didn't pick up on it on my first read through.Thanks J-R for confirming this technique.I have my answer - clearly t's about best practice by use the equipment's functionality and advanced features that are already available in some scopes. (Thanks for highlighting it for me)And finally, USE THE CORRECT PROBE of which there are many. Who knew there were that many. Wow slow down GI or at least ease up. TOO MUCH INFORMATION.I'm going with best practices.ThanksZE

author: ledtester
author_rank: Super Contributor
timestamp: May 25, 2025, 03:08:29 pm
content: Quote from: Zero_Ego on May 25, 2025, 02:59:11 pmI think it's more about protecting you.You have to touch the scope in order to use it. If the scope isn't grounded you could be exposed to dangerous voltages.

author: TimFox
author_rank: Super Contributor
timestamp: May 25, 2025, 03:17:47 pm
content: When using invert and add, you should adjust one of the channel’s V/div (gain) when both probes are connected to the same signal to get zero, before continuing.

author: Zero_Ego
author_rank: Regular Contributor
timestamp: May 25, 2025, 03:59:45 pm
content: Thanks you for the safety warning ledtester. Safety first absolutely there will be no crow floating around here. If I see any, it'll be over my dead body. (Or theirs)TimFox I shall ponder your comment. Thanks.Can I put this topic into a silent sleep by asking this simple question.Switch mode power supplies have feedback. I know any well designed power supplies has "Feedback" from the secondary.One question please.Do simple USB adaptors (commonly used for charging our phones) offer true electrically isolation?I know most switch modes have some form of feedback regardless its size - does the FEEDBACK connect back to the primary?If so surely they cannot offer pure isolation? Yes No. (I feel like I should be typing to AI but she lies so much)Do these offer any isolation?Update: I just remembered the feedback is Opto Coupled. So SOME are fully isolated however are the simple cheep ones we use to charge our devices? I have never pulled one apart. Do they offer Opto Coupling Feedback also?If they do, they must be also offer full isolation to the device on the other end? Am I correct?ThanksZE

author: grumpydoc
author_rank: Super Contributor
timestamp: May 25, 2025, 04:45:31 pm
content: Invert and add can be used in some scenarios - BUT it can't be used in others where a differential probe is needed.E.g. you can't use invert+add to replace a differential probe if trying to look at signals on the input side of an SMPS.

author: bdunham7
author_rank: Super Contributor
timestamp: May 25, 2025, 04:52:03 pm
content: Quote from: Zero_Ego on May 25, 2025, 03:59:45 pmFeedback can be opto, control winding or none at all and there is typically at least nominal galvanic isolation from mains. However, there is going to be some capacitive coupling, either parasitic or due to a Y-capacitor used for RF emissions control. So there's always at least some leakage current and a limit to the DC breakdown voltage of the isolation. Ideally those numbers would be <<1mA and >2kV, but actual results may vary.

author: bdunham7
author_rank: Super Contributor
timestamp: May 25, 2025, 04:54:27 pm
content: Quote from: grumpydoc on May 25, 2025, 04:45:31 pmAssuming appropriately rated 10x probes, why not?

author: Ian.M
author_rank: Super Contributor
timestamp: May 25, 2025, 04:56:08 pm
content: As B Dunham has just mentioned, isolated feedback can also be obtained from a primary side feedback winding without the use of an optocoupler. By normal transformer action, its peak voltage is proportional to that of the secondary when the rectifier conducts, which is slightly higher than the voltage on the output capacitor.However you can only bet on adequate isolation when the USB charger / power supply is from a reputable manufacturer, and purchased from a source that actively fights to keep fakes out of the supply chain. See DiodeGoneWild's Youtube playlist of charger teardowns where he closely inspects the PCB, and usually deconstructs the transformers to check creepage distances, clearances and insulation.https://www.youtube.com/playlist?list=PLfUGOPyKWwiriWhjmdtvjBw_J7X_X5OnUNote that if using 'invert and add' as a poor man's differential probe, its essential that both probes are the same make and model, and the channel gain probe compensation trimmers are precisely adjusted. Get them set as close as you can using the scope's test signal output, then put the scope into 'invert and add' mode and check the common mode cancellation. If there is a gain error (which appears as a residual squarewave) you may need to use variable gain on one of the channels to better match them. Find out which experimentally! If there is a compensation error, it shows as spikes where the common mode squarewave edges are, so tweak ONE of the compensation trimmers to minimise them.

author: grumpydoc
author_rank: Super Contributor
timestamp: May 25, 2025, 05:05:11 pm
content: Quote from: bdunham7 on May 25, 2025, 04:54:27 pmBecause the signal you want, and the ground you can safely attach your probe to do not share a common reference so you are just picking up two meaningless signals and comparing them.

author: Ian.M
author_rank: Super Contributor
timestamp: May 25, 2025, 05:10:48 pm
content: So? As long as you do not exceed the scope and probe's input ratings (including derating for frequency) and the two nodes you are probing aren't fully floating with respect to earth, why cant you simply use the probes *without* their ground clips?OF course there is a problem with modern DSOs if the differential signal is small compared to the common mode signal, as the limited ADC resolution can severely degrade the results if the subtraction is done after digitization by waveform maths.

author: bdunham7
author_rank: Super Contributor
timestamp: May 25, 2025, 05:11:54 pm
content: Quote from: grumpydoc on May 25, 2025, 05:05:11 pmThe performance might not be ideal, but when you use two probes as differential you typically don't connect the ground clips to the DUT at all. You can omit them entirely or simply clip them to each other. There's nothing electrically different between this arrangement and how an actual differential probe is wired internally. The two signals might seem meaningless if considered individually, but their difference will have the information you're looking for.

author: Zero_Ego
author_rank: Regular Contributor
timestamp: May 25, 2025, 05:15:34 pm
content: Thanks so much for your time guys. I love coming to this forum."Never fear artificial intelligence AI when we have Genuine Intelligence GI" ZE {I just made that up} It's true!!!If we measure ourselves against something artificial, may we learn as one. Therefore here is AI Vs GI. (We know who you are)I posed the question to my new girlfriend. AI (ChatCPT) Lets check her electrical knowledge, Lord knows its got to be a few qazzillioin percent better than my current one. (Ouch!)She only love me cause I pay. (It's true, I feel ashamed)This is her summary. (I'll tag the full question I asked at the end for those interested) Thanks again. ZE (I sound Chinese)💡 Summary: ChatGPT (Hope it's allowed?)"No, not all cheap chargers offer the same protection — some skip safe isolation to cut costs, making them potentially dangerous."Full Question asked posed.Well designed switch mode power supplies use feedback circuitry which is Opto Coupled back to the primary offering complete electrical isolation to the device however, do all cheep switch modes used to charge our devices offer the same protection? Might some cheep version might use electrical feed back circuitry (because its cheaper) thereby rendering the the charger electronically un-insolated?ChatGPT said:Great question — here’s the breakdown: {Editors note: It's not really! I feel like she says that to everyone and is cheating on me}✅ Well-designed (quality) switch-mode power supplies (SMPS):Use optocouplers or similar galvanic isolation for feedback from the secondary (output) to the primary (input) side.Maintain complete electrical isolation between mains and low-voltage output, protecting your device and you.Meet regulatory standards (like UL, CE) requiring isolation for safety.⚠ Cheap or poorly made chargers:Not all follow proper design.Some cut costs by omitting optocouplers and using direct electrical feedback paths — like resistive dividers or auxiliary windings — that may compromise isolation.This creates a risk of non-isolated outputs, meaning the low-voltage side can be at dangerous potential relative to mains, especially under fault conditions.🔎 How to tell?Hard for the average user; you’d need to open the charger and inspect the circuit.Signs of bad design: no safety certifications, suspiciously low price, unusually light weight (missing transformer), or reports of shocks when touching the device.💡 Summary:No, not all cheap chargers offer the same protection — some skip safe isolation to cut costs, making them potentially dangerous.If you want, I can describe how to visually spot risky chargers or recommend safe brands. Would you like that?End Of ChatGPT Message.PeaceZE

author: tggzzz
author_rank: Super Contributor
timestamp: May 25, 2025, 05:28:07 pm
content: Next tell the LLM that its answer is wrong. It will apologise profusely and give another answer.Will that next answer be closer to the objective truth or closer to the answer you "want" to hear?

author: grumpydoc
author_rank: Super Contributor
timestamp: May 25, 2025, 06:52:21 pm
content: Quote from: Ian.M on May 25, 2025, 05:10:48 pmOK, I need to have a harder think about that, I accept your point that a proper isolated doesn't have a "ground" just two reference points - still seems kind of wrong that the 'scope input would cope with rather large amounts of common mode crap - like it or not the input ampisreferenced to ground and it will see the common mode voltage as it's single ended signal.QuoteWhich I think would also affect analogue 'scopes.I think I think that for this to work the input amps/ADCs would have to handle "infinite" (or at least as large as the common mode signal) voltage swings without clipping or distortion - which is a bit of an ask, so I think my objection is that it won't work in practice rather than it won't work in theory (but I hadn't quite got past "feels wrong" TBH).

author: tggzzz
author_rank: Super Contributor
timestamp: May 25, 2025, 07:53:00 pm
content: Quote from: grumpydoc on May 25, 2025, 06:52:21 pmIt needs to be verified for the specific test conditions, but decent analogue scopes allow the input to be "off the screen" by several screens before saturation happens.One point to watch out for is the CMRR as a function of voltage and frequency.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 25, 2025, 11:35:32 pm
content: Quote from: ledtester on May 25, 2025, 03:08:29 pmEspecially if you are "floating" one or more 'scopes.A misconnection can put the case of one 'scope at Mains "Active" & the other at Mains Earth, a perfect setup for electrocution.

author: Zero_Ego
author_rank: Regular Contributor
timestamp: May 26, 2025, 02:53:47 am
content: vk6zgo,You have managed to paint an image in my mind of me with my Right hand on the case of "Floating Scope" No 1. (@ Active Minas potential) and my Left Hand on the case of "Floating Scope" No 2 (Neutral Mains potential) - frozen screaming"It's alive! it's alive!" with statically charged smoking hair - much lightening - and me greatly regretting my choices in that moment.Especially even meeting Victor Von Frankenstein. (Bad carrier choice)Reference: (Young Frankenstein, Mel Brooks)I joke but I take your warningVERYseriously.My main goal was to gather as much information about the hazards when using such gear. This is a real brain teaser for some like myself. It's clearly highly questionable practice - yet the pros do do it!It is not something I would goof around with as not only is the practice dangersous - I'm not convinced that do so will yield meaninful measurements purely because if you "Float a scope" you are effectively removing the very important Earth/Nutral connection and this surely must throw calibration out the window. (Some one did mention "Zeroing" out the probes)It's too advanced at this stage - weighting up the infomation here - which I greatly appreciate. The only thing I will be floating is my rubber ducky tonight in my bubble bath. (I love me comforts)Thank you for reminding me the most important thing SAFETY FIRST and always FIRST.Cheers MateZE

author: tggzzz
author_rank: Super Contributor
timestamp: May 26, 2025, 06:29:07 am
content: Quote from: Zero_Ego on May 26, 2025, 02:53:47 amNo the pros don't. Wannabe pro Dunning Krueger candidates do. They are dangerous and should be actively ignored.Read this anecdote about a professional engineer that took considerable precautions:https://groups.io/g/TekScopes/message/10754Key sentence: "This guy was WELL AWARE of the danger and took a lot of precautions to prevent shock . . . Bottom Line: He died anyway."For safe ways to achieve what the OP wants, see the references inhttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/If you really want to play silly buggers, do this:

author: Xena E
author_rank: Frequent Contributor
timestamp: May 26, 2025, 07:31:32 am
content: Quote from: tggzzz on May 26, 2025, 06:29:07 am... if you read further in that thread you will see that idiocracy still prevails...For anyone to lazy to read, someone suggests using two back to back transformers to create an isolation barrier...Then somebody else suggests twomicrowavetransformers...note¹You just have to hope you're never having to work around dopes like that. (I have, the HSE court case comes up later this year).Note¹Quote from: tggzzz on May 26, 2025, 06:29:07 amMore accurately, Darwin awards candidates.X

author: Jeroen3
author_rank: Super Contributor
timestamp: May 26, 2025, 07:52:07 am
content: Ah yes, safety by first making it more dangerous...I don't think black friday pricing tricks work on safety features.Quote from: Zero_Ego on May 25, 2025, 06:56:53 amNo they're not, if you're considering destroying a scope you may as well pay for the probe.https://www.eleshop.nl/micsig-dp700-probe.html

author: J-R
author_rank: Super Contributor
timestamp: May 26, 2025, 09:10:27 am
content: From a risk perspective, I don't think the difference is particularly drastic between using a differential probe, the two channel method, floating the DUT or floating the scope. In all cases, you have potentially lethal voltages exposed, so you have to be smart and careful about where you put your fingers. I recall that Youtuber Ian Scott Johnston was working on a device with the cover off and made a point to tape over all the exposed mains voltage points just to add a level of safety. He took the steps he thought were necessary to keep him out of trouble.I've come close to big mistakes myself over the decades. While pressing the probe tip against a live conductor, I looked away slightly to read the display and the probe tip slipped off and I almost made contact. These days I wear gloves. For mains voltages, you don't need much to add a good safety margin.

author: Ian.M
author_rank: Super Contributor
timestamp: May 26, 2025, 09:37:09 am
content: Quote from: J-R on May 26, 2025, 09:10:27 amThe problem with floating a modern plastic case scope*with plastic control knobs etc. is it *LOOKS* safe apart from the BNC connectors, till you consider the details. If its not a CAT rated scopemeter, the front panel controls wont have been designed with safe creepage and clearance distances in mind for mains, let alone for the scope's max. rated input voltage. You don't know how close the metal body of a switch or pot, grounded to chassis, is to your fingertip. You don't know how good the insulation is between the scope chassis or ground plane and your finger at the supposedly insulated control you are touching. e.g. conductive dirt may have built up on the sides of a button, or in the hole for the grub-screw of a knob, from fingerprint oils and ambient dust. If your scope isn't properly grounded, and you are using it on a non-isolated mains derived circuit, all it takes is one bad mains spike at the wrong moment i.e. while you are pushing a button or holding a knob, or a build-up of conductive 'spoo' and there's a significant risk of one dead engineer.If you must isolate, isolate the DUT, not the test equipment. That keeps the hazards in front of you, where you are concentrating on *not* touching them, rather than risking getting a belt across the chest when you reach to adjust the scope.* Floating an old metal cased 'boat-anchor' CRO is obviously dangerous.

author: J-R
author_rank: Super Contributor
timestamp: May 26, 2025, 10:11:02 am
content: My point is not to argue the minutia between the methods, but that whether it's an exposed part of the scope or the DUT, you can still be electrocuted. So simply buying a differential probe to poke around inside equipment with the cover off is not going to avoid the requirement to be smart and careful and to take extra precautions.

author: Zero_Ego
author_rank: Regular Contributor
timestamp: May 26, 2025, 01:07:51 pm
content: Wow, reading that back was an education. I so much appreciate your kindness and time.I shall let my mind digest what I have just read. Is it better than AI (who' s asking) I'm curious about the speed of learning.Hands down. Look at how many eyes were on the same topic proves Genuine Inelegance makes AI look like what it really is.Artificial.I feel I got a better deal here. Thanks GIThanks again. I loved reading your comments.ABCDZE

author: tggzzz
author_rank: Super Contributor
timestamp: May 26, 2025, 02:00:07 pm
content: Quote from: Ian.M on May 26, 2025, 09:37:09 amYes indeed.To emphasise the conductive crap point, my first boss was using a long shank screwdriver on a live CRT, and still received a belt.After regaining his composure, he investigated and found an almost invisible thin film of oil along the shank and onto the handle.He was safety minded, wasn't a fool, and still got shocked. It is too easy to happen inmultiple unexpected ways.Some risks are worth taking for the significant associated benefit or pleasure. But if there is no benefit/pleasure, why take the risk? I have yet to find someone who can state a significant benefit or pleasure with floating a scope.

author: Zero_Ego
author_rank: Regular Contributor
timestamp: May 26, 2025, 03:24:38 pm
content: Please don't take my joking as being offensive or flipped. I agree - it's just not worth rolling that dice. I see you have made refence to a name. I suspect that is not leading to good news.It doesn't sound great. (Will I look)There is simply no way I would connect the mains to a floating oscilloscope. Not worth it considering the cost of the proper device.This is the wrong room for the subject as we have moved into high voltage and I'm a beginner.Do you have a high voltage section? (I can check) I have a mains issue. I just purchased a new Siglent SDS824XHD (Just arrived today)I will find the appropriate HIGH VOLTAGE section. (I'm sure there will be like some kind of HIGH VOLTAGE aptitude test right?) I mean to get access. Safety, don't float your crow. {I gotta say, I'd never heard of it till I came hear "Float your scope" until yesterday - not sure who mentioned it first) AI or GI ? probably me.Moving on can I share a person experience with getting electrocuted?I use to volunteer my services to a local Catholic thrift shop. I was studying full time living alone in a flat in the city. Money was tight. - so I had this deal that I would fix anything and we would simply split the difference.Nice deal and few dollars for me. I'm normally very carefully. In fact I would say I'm even extra careful. For example, I knew about the capacitance of the TV screen and I was scared of them. I would never do the screw driver thing. I know I needed a high voltage probe and I never messed with the High Voltage side of a TV. (I never understood how those things could ramp up the volume. Some kind of capacitive ladder from memory or is it diodes and caps. I know they do something to ramp up that HIGH TENSION 20,000KV CRT or was it 50,000KV? )Anyway, one day I walk into the shop and spot an old radiogram. An old valve job - mint condition the wood work was enough to make you go - WOW with original 1950's solid (Bakerlite?) pot knobs with the grub screws (Some will know the units) This one was a gem and looked in great shape.I look at the back and see one long wire single wire running out the back. (Pre FM Radio but I didn't think of that) I grabbed it and it was LIVE. Knowing what I know now and thanks to the internet - I am pretty confident that that wire must have been an earth wire and a cap had shorted to ground. Did they have earth wires - I'm only guessing that's what I grabbed. I had never seen a single wire hanging out the back of a old radiogram.The shock it gave me, I can only describe as someone violently grabbing unexpected with such force you never forget. Like being dragged out of a night club by four guys all juiced up on 'riods. (There's a violent pulling sensation or more so violent yanking) Hard to describe.I was lucky.I grabbed it with my right hand and my sweet lord it just YANKED ME. I was staggered by the, as they say, 'the potential' I had just felt it. It was like "feeling" like if a crap load of water suddenly pushed you over. The power is staggering. I grabbed that potential and lived to tell the tail.Don't mess with MAINS POWER - everyone slips up.Eventually. It's the law of averages.

author: pdenisowski
author_rank: Super Contributor
timestamp: May 26, 2025, 03:49:08 pm
content: Quote from: J-R on May 26, 2025, 09:10:27 amAll the major oscilloscope manufacturers warn (in rather strong and unambiguous terms) against floating the scope.The two channel (A-B or "quasi-differential") approach is safer, but can suffer from some performance limitations depending on BW, CMRR, etc.Our standard recommendation is to use a good (high voltage) differential probe or a scope with isolated inputs (like the R&S RTH).

author: bdunham7
author_rank: Super Contributor
timestamp: May 26, 2025, 08:25:12 pm
content: Quote from: grumpydoc on May 25, 2025, 06:52:21 pmYes, it is true that the individual channels do have to be set to handle the common mode voltage so you won't be able to see a 1mVAC signal on top of a 500VAC (or DC) common mode voltage. But "real" differential probes have similar limitations although they can do a bit better because they are dialed in for better CMRR and can have some additional gain after the differential part (whether in the probe or by setting your scope sensitivity). Typically the problem with differential probes and small signals is noise, then CMRR if the common mode voltage is not DC.