scope differential probes - Page 1
Forum section: electronics / testgear

author: robint
author_rank: Regular Contributor
timestamp: October 13, 2023, 05:37:21 am
content: Hi Guys, can you help this board in clearing up the rubbish spouted about making differential measurements in circuits using a scopeSo this topic doesnt get convoluted - let us dispense here with concerns about electrical safety - that is probably well covered elsewhere. OBTW I deplore the cheap charlie way of just disconnecting the earth at the mains plugWhat we are concerned with here is protection of your scope and also the viability of making differential measurement (where the gnd croc cant be attached to PE)I have upgraded to a new DSO 4ch gear and (from my old school CRO scope bg) its quite a revelation and requires repurposing my habits so I can meddle with digital ccts safely.I spent several weeks re-learning this new hi tech quantum leap stuff so bear with me if I stumble. First I was made firmly aware that my DSO is a sensitive instrument and easily damaged by ignorant use. Fair play.So I came to the area of Differential probes and was shocked to discover that they can cost nearly as much as my new DSO.Back to basics so many rubbish vids out there were informercials pushing Diff probe kit yet there is a simple way and that is to use 2 channels (with the croc gnd leads connected) so the probes are then free to use on ccts without danger of shorting out or blowing up your scope by careless application of the croc gnd lead.It seems another way might be to use a cheap audio xformer 1:1 (£5) and use the primary as the device under test probes and the secondary for the scope leads. Total isolation. Of course this is no good for dc measurements (so use a DMM instead!) and will become inaccurate at mf-hf, but should be ok to poke around SWM PSUs.Anything wrong with that? Pls adviseAny Techs out there care to share wisdom on this

author: modoran
author_rank: Regular Contributor
timestamp: October 13, 2023, 06:07:45 am
content: I haven't seen anyone using audio transformers as probe input, this will load the circuit under test and get unexpected results, even blown it up.Cheaper option will be to use an isolation transformer for the DUT power supply.

author: tautech
author_rank: Super Contributor
timestamp: October 13, 2023, 06:50:19 am
content: Quote from: robint on October 13, 2023, 05:37:21 amCorrection made.You should also fit the probe condoms to further isolate the reference (gnd) rings near the probe tip from accidental short circuits when the grabbers are removed.

author: tggzzz
author_rank: Super Contributor
timestamp: October 13, 2023, 08:57:09 am
content: Quote from: robint on October 13, 2023, 05:37:21 amIf you think probes only cost as much as a cheap DSO, you haven't been lookingOr is thatThere are many types of differential probe. You need to characterise your requirements before this can be answered. What is are the differential and common mode voltages? What are the edge rates of digital waveforms or the bandwidth of analogue waveforms? What are the impedances where the waveforms are being measured?You will probably find the references athttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/helpful.

author: robint
author_rank: Regular Contributor
timestamp: October 13, 2023, 11:01:31 am
content: @tautech(with the croc gnd leads connected removed)I think the tiptster's purpose was that the croc leads didnt flop around - as they do and short to something bad

author: robint
author_rank: Regular Contributor
timestamp: October 13, 2023, 11:06:02 am
content: Quote from: modoran on October 13, 2023, 06:07:45 amThese xformers are rated 600 ohmWhat do techies use?

author: robint
author_rank: Regular Contributor
timestamp: October 13, 2023, 11:17:58 am
content: Quote from: tggzzz on October 13, 2023, 08:57:09 amYes indeed you can spend £2.5, £3k + on an MSO 200Mhz kit Agilent x2, Tek x4 PhewMy jolly DSO Sigol DS1054Z £350 - its enough to make my eyes water and not want to hurt my new baby.As for what user spec I am - say amateur (not prof techie) repairs/hacking something like SMPS from a PC. I am rarely involved in live Mains work and then I take proper precautions for personal safety - as you do. I am sufficiently schooled to know what I am doing there I hope which is why I hoped to divert comment away from HnS. IMHO if you have NOT got those basics you shouldnt be anywhere near a scope, or any other sensitive expensive kit.

author: alm
author_rank: Super Contributor
timestamp: October 13, 2023, 12:08:27 pm
content: Have you tried two channels in subtract mode with the probe grounds connected together (you can wrap electrical tape around the alligator clips so they don't detach and touch the DUT)? What was the limitation you ran into? This mode should be safe for both the user and scope as long as you keep ground away from the DUT and stay within the voltage ratings of probes and scope. And it's very cheap for anyone with a 2+ channel scope and some identical probes.Quote from: robint on October 13, 2023, 05:37:21 amAs already mentioned, in addition to frequency response there is circuit loading and voltage rating of the transformer.Quote from: robint on October 13, 2023, 11:17:58 amCH1 - CH2 or an off-the-shelf differential probe. The relation to the price of the probe and your scope is irrelevant. What it's important is how much does the probe costs vs how important is the problem that it's solving, and what the alternatives are.

author: robint
author_rank: Regular Contributor
timestamp: October 13, 2023, 12:37:37 pm
content: @tggzzzThnx for the link, OMG Pandora's box opened. 1ns rise time stuff or out of the processionJeez the comments went into rocket science or praps those super switches used to detonate atomic bums.Small boy here shat upon from a dizzy height ehI managed to ug my Rigol up to 100Mhz so herewith my humble attempt at rise time vs thehttps://www.eevblog.com/forum/testgear/pos-ebay-_scope-probes/msg398972/#msg398972.Posted tests byhttps://www.eevblog.com/forum/profile/?u=15682Grumpy dochmm pressing the rise time button I got 2.7us (=bw 200kHz) after fiddling the probe pot. Does this sound right?

author: robint
author_rank: Regular Contributor
timestamp: October 13, 2023, 01:16:24 pm
content: Quote from: alm on October 13, 2023, 12:08:27 pmI suppose one gotcha to be aware of is the common chassis earth (normally connected to PE but there are cowboys around). You now have 4 channels - an unheard of luxury in CRT scope days, so there may be a temptation to forget when using other channels elsewhere but thats show business

author: jonpaul
author_rank: Super Contributor
timestamp: October 13, 2023, 01:20:23 pm
content: Read and learn: 1960s..1990s Circuit Concets books written by Tektronix i staff.Calssisc and authoritative even today.https://www.w140.com/tekwiki/index.php/Concepts_SeriesOscilloscope Probe Circuits:https://w140.com/tekwiki/images/6/62/062-1146-00.pdfProbe measurementshttps://w140.com/tekwiki/images/1/19/062-1120-00.pdfEnjoy,Jon

author: bdunham7
author_rank: Super Contributor
timestamp: October 13, 2023, 01:37:21 pm
content: Quote from: robint on October 13, 2023, 05:37:21 amSure. There are many different situations that each require a specific solution to the probing problem. An audio transformer would not be a good general solution, but if you wanted to see the output of a bridged audio amplifier (where you can't ground either side), it would be appropriate. If you wanted to look at mains or a LF SMPS, an inexpensive HV differential probe might work well, but with higher frequencies you might have issues. Using two channels plus math as anad hocdifferential probe might work well enough in certain situations but isn't great in general. Tek IsoVu probes will work great in almost any case, but they're a bit pricey.https://www.testequity.com/product/32064-1-TIVP1?gclid=Cj0KCQjw1aOpBhCOARIsACXYv-dYwymIuOL7XosEPk0bZxmwy8rYyVzTf0YvQHztzusndY59d7vENBUaAic6EALw_wcBSo you need to be discussing a specific use case to really get any meaningful answers here.

author: tggzzz
author_rank: Super Contributor
timestamp: October 13, 2023, 02:00:52 pm
content: Quote from: robint on October 13, 2023, 11:17:58 amDon't think that £3k is the top end of probe prices: look at any major distributor, and sort on price. Or how about $5 every time a probetouchesa circuithttps://www.eevblog.com/forum/chat/what-do-you-use-prototyping/msg1309435/#msg1309435Some of the phrases you have used in this thread make me wonder whether you do know as much as you think you know. There are many subtle failure modes that aren't obvious to newcomers.Some parts of SMPSswillbedirectlyconnected to the mains. That isn't an area for an amateur to try to cut corners with cheap or inadequate tools.Mistakes, however short duration, can lead to loud noises, bad smells, - and deathly quiet.Having said that, it is your scope, your UUT, and your health. e.g. "I take proper precautions for personal safety - as you do". You have no idea what personal precautions I do - and don't - take.

author: robint
author_rank: Regular Contributor
timestamp: October 13, 2023, 02:36:47 pm
content: Thanx for your superior opinionQSome of the phrases you have used in this thread make me wonder whether you know as much as you think you do. There are many subtle failure modes that aren't obvious to newcomers.Some parts of SMPSs will be directly connected to the mains. That isn't an area for an amateur to try to cut corners with cheap or inadequate tools.Mistakes, however short duration, can lead to loud noises, bad smells, - and deathly quiet.UQIts always good to know that new members can benefit from such wisdom (as above) from elder statesmen and not waste their time on the all too regular rubbish that diverts and confuses important questions hoping that all may be helped (and avoid noxious smells)

author: tggzzz
author_rank: Super Contributor
timestamp: October 13, 2023, 03:28:34 pm
content: Quote from: robint on October 13, 2023, 02:36:47 pmGood luck, and have fun safely.BTW: there's a useful "Quote" button at the top right of each post.

author: robint
author_rank: Regular Contributor
timestamp: October 13, 2023, 05:44:16 pm
content: Quote from: jonpaul on October 13, 2023, 01:20:23 pmWOW JP this should be made into a sticky - thats real gem stuff, basics dont change and that was in the days when there was pride in pursuit of excellence in making an honest product and providing genuine customer service. I served my time with an Industrial Instrument Automation Company whose proud boast was that they made equipment and guaranteed spares and service to last the life of the plant (20 years)Imagine that document was produced in a typing pool (those legs) on mechanical Olivetti? typewriters using carbon paper.

author: tautech
author_rank: Super Contributor
timestamp: October 13, 2023, 07:16:03 pm
content: Quote from: robint on October 13, 2023, 11:01:31 amSure, but they are made detachable so use it !WRT to the cost differential probes......go price some current probes if this scares you.It has always been this way however cheaper differential and current probes for the hobbyist are available now but weren't when I was setting up....we floated scopes < not ever doing that again and shelled out $200 for 2nd hand P6021 current probes.Buying the scope isneverthe last cost.

author: alm
author_rank: Super Contributor
timestamp: October 13, 2023, 08:17:36 pm
content: Quote from: tautech on October 13, 2023, 07:16:03 pmExcept that attaching them together will give you better performance. Tektronix madea special type of ground leadback when they were making passive probes specifically for use with the differential amplifiers used in the 7000 series scopes. They surely didn't design that because their customers couldn't figure out how to detach their ground lead.

author: tautech
author_rank: Super Contributor
timestamp: October 13, 2023, 08:31:09 pm
content: Quote from: alm on October 13, 2023, 08:17:36 pmWhen/if you need it.A tech I delivered a DSO to never used probe reference leads and instead had a dedicated reference lead back to the scope which although not giving wonderful performance suited his needs and also allowed reference connectionanywhereon the DUT and not be constrained by the length of the probe reference lead.Of course with a proper differential probe their long leads also offer this advantage.QuoteYou'd be surprised by the # that initially don't know reference leads can be detached.

author: alm
author_rank: Super Contributor
timestamp: October 13, 2023, 08:41:49 pm
content: Quote from: tautech on October 13, 2023, 08:31:09 pmBack then probes shipped without ground leads attached. So I think the number of people knowing that they could be detached was higher then

author: jonpaul
author_rank: Super Contributor
timestamp: October 13, 2023, 08:42:07 pm
content: Bonjour a tous, mille mercis pour le mention.Have used diff probes since 1970s, on diff RF/IF tuners (Sequerra) and off line SMPS: a few moments please to clarify...The FET input wideband probes like TEK P6046 are VERY delicate electrically and mechanically.https://w140.com/tekwiki/wiki/P6046Their 100 MHz BW is seldom needed. The 25V peak V restriction, difficulty to repair a blow FET input, and huge size are limitations.For off line SMPS, bridge, the probes are designed for such use, with > 1 kV rating and dedicated probes.Fluke, Teledyne, Keysight, TEK,We use similar to Yokogwa 701927https://tmi.yokogawa.com/us/solutions/products/oscilloscopes/oscilloscope-accessories/701927-differential-probe-150-mhz1400-v/Since 1970s we designed, debugged and consulted on many off line SMPS, PFC, EBU and seldom used these costly devices.Use of battery operated portable scopes and a 2 kVA 1:1 mains isolation transformer on the primary side.eliminated the need for such probes.Just my experience and notes...HAVE AN ABSOLUTELY FANTASTIC DAYJon

author: tautech
author_rank: Super Contributor
timestamp: October 13, 2023, 08:48:33 pm
content: Quote from: alm on October 13, 2023, 08:41:49 pmYup, that had crossed my mind however today some would get into trouble probing without a reference and we'd just have to offer more support.Scopes are pretty hardy these days since Tek learned to place robust PCB ground planes on the TDS200 series and not have worldwide recalls for fused traces.A few sparks or melted reference leads are lessons newbies never forget.

author: robint
author_rank: Regular Contributor
timestamp: October 13, 2023, 09:46:56 pm
content: JP merci mille foisHave followed the Tek Probe doc 128pp, I stopped at active probes - not going down there.I am suitably apprised and now have the wisdom to know how little I know. Will tackle Probe measurements soonest tout de suiteYou suggested that a battery powered dso can solve many problems. Its a pity the my Rigol DS1054z doesnt have a battery supply port (maybe its hackable?). In the meantime its power is 17VA so it could be run from a typical PC UPS (with PE disconnected) - has anyone done this? Also my scope is a total plastic case with only the channel i/p sockets exposed (of course care not to connect ethernet or usb cables - which I would do with Mains power around.)Around Covid time I rigged up an sine wave 24v inverter connected to a 50 Ah Lead battery deep cycle (what a con trick) Dont get me started on LABs blood pressure alarm!!! In essence whatever is claimed as Ah capacity divide by 4 - that how bad they are in practice.Anyway it was there in case of a prolonged power cut. My APC UPS would allow me say 15mins for an orderly shut down. I could run my router and a laptop to maintain comms to the outside world for a few hours say - never yet put to the test. But I could use this stand alone system to power my scope - ca 0.7A from a 17 Ah battery bank, so I might get a day's use say?Is this a techie solution?

author: tautech
author_rank: Super Contributor
timestamp: October 13, 2023, 09:51:22 pm
content: Quote from: robint on October 13, 2023, 09:46:56 pmHalf arsed, yes.But it only offers 1 isolated channel from PE as all BNC's shells share a common connection.Scope chassis and BNC's become elevated to reference lead connection potential = risky to the operator.

author: tggzzz
author_rank: Super Contributor
timestamp: October 13, 2023, 09:57:38 pm
content: Quote from: robint on October 13, 2023, 09:46:56 pmBattery power is beneficial, but not sufficient.Yes, people have run scopes with PE disconnected. But that's the correct answer to the wrong question.Read the references in the page I gave you; they will help you understand the widely differentclassesof scopes and probes, and the reasons it is necessary for them to exist.

author: tggzzz
author_rank: Super Contributor
timestamp: October 13, 2023, 10:02:57 pm
content: Quote from: tautech on October 13, 2023, 09:51:22 pmYup.And if a probe shield is connected to an "unfortunate" voltage, that voltage will materialise throughout the scope and its case. Then the operator attaches/removes a probe from the scope, or uses the touchscreen or membrane button or similar, and...As for the UUT, it may not appreciate a large capacitance to ground appearing on one of its nodes, and oscillation or delayed switch on/off may occur with "interesting" consequences.

author: tautech
author_rank: Super Contributor
timestamp: October 13, 2023, 10:09:36 pm
content: Quote from: tggzzz on October 13, 2023, 10:02:57 pmExactly, added with an edit while you were typing.Again we have these threads about differential probe cost and need when there is a real need for operator safety when they don't fully understand the risks.To address this basic need we stock the well known and well priced DP-25 which while not impressively spec'd suits most requirements.

author: tggzzz
author_rank: Super Contributor
timestamp: October 13, 2023, 10:46:52 pm
content: Quote from: tautech on October 13, 2023, 10:09:36 pmMessages crossing in the æther is a standard problem. It was worse (w.r.t. UK and Australia) before we used the æther for messagesQuoteYup. Such ashttps://www.eevblog.com/forum/projects/your-test-equipments-**oscilliscope**-very-best-friend/msg4712102/#msg4712102But there are the Tek A6902 probes for people that need a couple of isolated channels. Some variants come with standard probes, some with impressively chunky (?manly?) probes.I ilke to point out the kinds of precautions you need to take to float a scope that wasn't designed to be floated, e.g.

author: JeremyC
author_rank: Regular Contributor
timestamp: October 14, 2023, 02:49:56 am
content: Quote from: robint on October 13, 2023, 05:37:21 amYou should check the DP differential probes from Miscig, on Amazon US they cost ~$190.https://www.micsig.com/Differential%20Probe04/

author: robint
author_rank: Regular Contributor
timestamp: October 14, 2023, 03:08:58 pm
content: Yes GentsThe bottom line is that these diff probes cost ca £200, just so you can poke around some live DUT - and - at that price if I needed to do that then clearly it would be better to buy a separate standalone battery operated DSO Nano - I bought one couple years back ca £50, ok for poking around a SMPS - it does the job.I think if I really needed to expose myself and my precious Rigolthen I could power it from my existing standby Battery/inverter system which might give me a days work/ charge overnight. Yes a much derided floating scope but the safety issue seems well over blown here (commercial opinions selling). A knowledgeable Tech worth his salt would knowthe risks and precautions. This board is after all for those with some electrical schoolingPosted by: jonpaul« on: Yesterday at 08:42:07 pm »he has the solutionso many thanks JPand Ive learnt so much that I was quite unawarehttps://w140.com/tekwiki/images/1/19/062-1120-00.pdf.You could spend 200hrs theory/lab work in a Uni course just to cover all the issues raised. So I now know what I dont knowI'm a city shopping car type not a 1000hp EVcar boy racer. Hi spec hi cost solutions are not my bag.My Rigol has catapulted me from a prewar Austin 7 to a Toyota Aygo (lovely little mover btw)

author: tggzzz
author_rank: Super Contributor
timestamp: October 14, 2023, 03:32:19 pm
content: This is for anybody new that comes and reads this thread, not for the OP.Quote from: robint on October 14, 2023, 03:08:58 pmThat's offensive bovine excrement. None of us have tried to sell you anything.QuoteWhat makes you so sure of that?You seem to have not read things like "Here’s an anecdote about someone thattook precautions and still killed themself."QuoteYou mean just enough schooling to be dangerous? Yes, the Dunning-Krueger effect is very real, unfortunately.You are new here. Those of us that have been here a while know some posters have a habit of posting truly hare-brained points.

author: jonpaul
author_rank: Super Contributor
timestamp: October 14, 2023, 04:44:53 pm
content: Rebonjour:RE floating scope had old TEK 212https://w140.com/tekwiki/wiki/212Built in NiCd. But both channel rtn are common.Most of the diff proble needs disappear if you have a good mains isolation trsf.We used Signal/BelFuse DU series like DU-2, DU-10Bon soiree,JonJon

author: David Hess
author_rank: Super Contributor
timestamp: October 15, 2023, 02:14:27 am
content: Quote from: alm on October 13, 2023, 12:08:27 pmTwo probes in add and invert mode works. The DC common mode rejection must be trimmed with the oscilloscope's variable control, and then the AC common mode rejection can be trimmed with the probe compensation adjustments. Tektronix made probe pairs with adjustable DC attenuation for exactly this application.The primary limitation is the oscilloscope's own input range. The vertical sensitivity must be set to accommodate the input common mode range, which limits differential sensitivity.Is it worth doing? You can definitely pick out signals which would otherwise be buried in common mode noise, but the improvement is not quite as good as with a dedicated differential probe. Digital oscilloscopes, with some old exceptions, do not work quite as well as analog oscilloscopes because they also have to remove quantization noise.

author: jonpaul
author_rank: Super Contributor
timestamp: October 15, 2023, 05:53:23 am
content: Hello: any scope CH1 -CH2 has a common earth between the two probes and forms only a single differential display.Result depends on the CMRR and CM V of the scope and probe balance and matching from DC up to freq of interest.Expect CMRR 20:1 or 50:1 By nulling one can improve the DC CMRROld analog TEK mainframe 7000 scopes have some fine diff amp plugins with filters and very good CMRR. See 7A22, 7A13https://w140.com/tekwiki/wiki/7000-series_plug-ins#Vertical_plug-insSome special probe sets are specified for diff use.Jon

author: David Hess
author_rank: Super Contributor
timestamp: October 15, 2023, 09:41:10 am
content: Quote from: jonpaul on October 15, 2023, 05:53:23 amThe disadvantage of the 7A22 and 7A13 is that they have no built in way to adjust the DC or AC common mode rejection of an attenuating probe. Attenuating probes with a DC gain trim are required and these have not been made for more than a decade. This does not apply to a 1x probe except when the high impedance input attenuator is engaged as the attenuators lack a DC trim also.So these are great differential input vertical amplifiers, however they are not a panacea unless you have probes with DC trims.

author: alm
author_rank: Super Contributor
timestamp: October 15, 2023, 10:55:56 am
content: Quote from: David Hess on October 15, 2023, 09:41:10 amThe 7A22 and 7A13, or 7000 series scopes, have not been made for decades either. Lecroy still sells the DA-1855A which is basically a stand-alone version of the 7A13. They also sell attenuating probes to go with it. I don't know if they would work on a 7A13. They require a specific BNC jack spacing, and the input capacitance may or may not be in range.

author: jonpaul
author_rank: Super Contributor
timestamp: October 15, 2023, 11:13:58 am
content: Bonjour,The 7A22/7A13 are NOT designed for use with probes....precision matched pair diff probes were made at one time.Jon

author: adam4521
author_rank: Regular Contributor
timestamp: October 15, 2023, 01:36:52 pm
content: For people on the forum learning from this thread, the 'common mode rejection' that members are talking about isn't an esoteric thing that only matters to purists -- having a balanced probe I've found is actually really important when trying to measure across a switching transition on a power supply. That's why floating the scope and using single ended probes is not a great plan even ignoring the safety point: the ground lead and the probe 'pin' have very different impedances from the perspective of the circuit you are measuring and therefore the disturbance you are trying to null out breaks through to your trace and you don't see the truth. So the advice here:1. Use two identical probes, connect the grounds, use the 'subtract' function or2. Get a differential probeare your two practical options.Option 3, isolate the DUT with an isolating transformer might sometimes be practical but can have similar cost as (2) and will change source parameters that might sometimes be important.Watch for differential probes on eBay, if you are patient you will eventually find a deal! If you look after them they will hold their value.

author: David Hess
author_rank: Super Contributor
timestamp: October 15, 2023, 07:49:32 pm
content: Quote from: jonpaul on October 15, 2023, 11:13:58 amThey were made to work with either 1x probes, or two of the Tektronix P6055A or later P6135A 10x probes which have DC and compensation adjustments.There are also some earlier probes which had attenuation adjustments like the 100x P6009, which means they could also have worked with high voltage 1000x probes.The 7A13 was also intended for comparator applications where there is no requirement for probe matching, and any probe can be used.