Nead an insulated probe for oscilloscope - Page 1
Forum section: electronics / beginners

author: FriedMule
author_rank: Frequent Contributor
timestamp: June 05, 2019, 12:39:12 am
content: Hi I am just looking for a "floating" probe where the ground lead is not grounded trough my 350MHz oscilloscope.I do not need high voltage measurements, but only the possibility to measure between two points that are not at ground voltage.Hope someone please can help.

author: ArthurDent
author_rank: Super Contributor
timestamp: June 05, 2019, 12:49:34 am
content: Do a search in the forums. This topic is discussed about once a week.

author: rstofer
author_rank: Super Contributor
timestamp: June 05, 2019, 01:40:19 am
content: Hint: search for 'differential probe'

author: Brumby
author_rank: Unknown
timestamp: June 05, 2019, 01:45:18 am
content: Quote from: rstofer on June 05, 2019, 01:40:19 amThis ^ ^ ^

author: Mr.B
author_rank: Unknown
timestamp: June 05, 2019, 01:48:16 am
content: Buy one of Daves.Thats what I did... Does not have to be high voltage to use as a differential probe.https://www.eevblog.com/product/hvp70/

author: FriedMule
author_rank: Frequent Contributor
timestamp: June 05, 2019, 10:27:25 am
content: A differential probe looks like they costs as much as a beginner scope and often have 50X or even 500X attenuation.I think what ia am looking for may be some DIY I maybe can connect my passive probes to?I have tried to search but I can't seem to find something, I am looking for, is not to technical to understand or not for higher power / attenuation.

author: tautech
author_rank: Super Contributor
timestamp: June 05, 2019, 10:47:48 am
content: Quote from: FriedMule on June 05, 2019, 10:27:25 amYep and that's just the cheap ones !QuoteNope, don't do that if you value your safety or scope.QuoteLook for the DP-25, a Micsig one or get Daves one.If you think just buying a scope was the end of cost, think again !Really want a fright then look for a price on a 100 MHz current probe.Then venture into RF and have another fit at the cost of getting set up.Buy wise, once and have it for life.

author: mikerj
author_rank: Super Contributor
timestamp: June 05, 2019, 11:14:20 am
content: Commercial differential probes are expensive, but even DIY versions will not be particularly cheap. Good DC precision, good common mode performance and high bandwith require careful design and trimming.I recently needed a x1 differential probe for low-ish voltage use and an EEVBlog member pointed me to the Pintek model in the link below which is quite reasonably priced compared to many, though still not cheap compared to the cost of a entry level DSO.https://www.globalmediapro.com/dp/A25XX7/Pintek-DP-30HS-Differential-Probe-30MHz-65V/

author: FriedMule
author_rank: Frequent Contributor
timestamp: June 05, 2019, 11:48:01 am
content: Thanks a lot!Am I understanding right if I am saying that I can either build a crappy probe that will be of no use anyway, or dig deep in my pocket once for all and get a decent one?But what about that 500X attenuation, would that not eliminate the 2uV noise I may look for in an circuit?

author: AndersJ
author_rank: Frequent Contributor
timestamp: June 05, 2019, 01:01:18 pm
content: You will most likely never see 2 uV signals on a oscilloscope,regardless of which probe you use.

author: macboy
author_rank: Super Contributor
timestamp: June 05, 2019, 01:44:20 pm
content: You mean "Isolated" not "Insulated". A differential probealmostthe same as an isolated probe, but not really. Each input is still referenced to ground, and has a specified impedance to ground (as well as to the other input), and is limited to be some voltage +/- w.r.t ground. An internal differential amplifier rejects the common mode signal, providing the difference at the output, so the reference point need not be ground. High voltage differential probes commonly handle several hundred volts or more. Low voltage, high bandwidth differential probes may be specified for signals of say +/- 4 V,. and can be damaged by as little as 7 volts (to ground) on either input. These would be used to directly view LVDS and similar high speed digital signals, or may be used with an optional input attenuator to extend the useful range.You said you have a 350 MHz oscilloscope but said nothing about the bandwidth of the signals you want to probe. For low bandwidth, you should be able to easily build up a simple instrumentation amplifier type of circuit. The specifications (bandwidth, common mode rejection, gain, DC accuracy, slew rate, etc) will depend on the type of op-amps and passives that you use and the physical construction. For low voltage and audio bandwidth (or even a few MHz), the job is easy. If you want 10's or 100's of MHz, you are up for a challenge. High voltage (beyond op amp rails) is another can of worms which adds not only difficulty in the design, but demands a firm grasp of the safety issues and of the consequences of getting it wrong.

author: SteveyG
author_rank: Unknown
timestamp: June 05, 2019, 02:02:25 pm
content: Take a look at a couple of my videos on the Micsig diff probes. They're good value

author: Nerull
author_rank: Frequent Contributor
timestamp: June 05, 2019, 03:50:44 pm
content: A poor mans diff probe is to use two channels simultaneously and a math function. If you're worried about accuracy this won't be nearly as good as a diff probe, though.

author: bob91343
author_rank: Super Contributor
timestamp: June 05, 2019, 04:20:27 pm
content: It's also possible to use a dual trace oscilloscope in differential mode with two channels, invert one and add. This won't have such great bandwidth or common mode rejection but can be useful.Just don't try using an isolation transformer. You could experience some jolts while touching the chassis.

author: FriedMule
author_rank: Frequent Contributor
timestamp: June 05, 2019, 05:28:25 pm
content: Again thanks, I can see that my lack of knowledge about the English and electronic is a bit of a problem here so I'll try to explain what I want.I want to have the possibility to measure without thinking about the black lead going to ground, and I would like to be able to measure two independent places at once, without them "cross-talking".I know that I could use two probes and a bit math, but I was just hoping to be able to point my probes, measure and next.Maybe you could call it a floating probe for maybe only 20MHz, would referrer 100MHzI have no nead for higher voltage then what a scope can handle and if I could get no attenuation, I think that it would be best.

author: JxR
author_rank: Unknown
timestamp: June 05, 2019, 05:54:01 pm
content: As others have already pointed out, the answer is still an active differential probe for what you are describing. Yes, they are expensive unfortunately. There are certainly ones made that are not for high voltage either. Some of those tend to support higher bandwidths. Some support both.

author: FriedMule
author_rank: Frequent Contributor
timestamp: June 05, 2019, 06:44:07 pm
content: Quote from: JxR on June 05, 2019, 05:54:01 pmYes I think I thought that I understood that but macboy said that a differential probe is not isolated and do go to ground and it may be possible to make an low frequency one. There is also the part about 500X attenuation, and me mixing insulation with isolation and 1uV.So I felt that I had to correct my mistakes and follow up upon the answers. :-)

author: bdunham7
author_rank: Super Contributor
timestamp: June 05, 2019, 07:23:32 pm
content: Quote from: FriedMule on June 05, 2019, 05:28:25 pmThis is the root of the problem that many new users seem to have. If you are poking around in your mains panel or your tractor's electrical system, with a 10M input DMM, you can do that "without thinking" as long as you don't screw up the range or short the system out by leaving the leads hooked to the AMPS socket. This is because the DMM has high enough isolation and input impedance and low enough input capacitance to not have any significant effect on those circuits AND because dV/dt is low enough on all those circuits to as to not pose a hazard to you or your instrument.That all changes when you get an oscilloscope. Unless you are scoping your tractor's generator or your refrigerator's light switch, the scope and probe become a significant and relevant part of the circuit. You simply have to think about it and understand what you are dealing with. That is all a bit complicated at first and unless you sit down and understand it all you will be like the guy in the "medium-priced scopes" thread and spend forever looking for that perfect all-in-one solution. IT DOESN'T EXIST!So tell us what you are trying to measure.

author: Mr.B
author_rank: Unknown
timestamp: June 05, 2019, 07:51:59 pm
content: Quote from: FriedMule on June 05, 2019, 11:48:01 amDaves one is x10 and x100.

author: capt bullshot
author_rank: Super Contributor
timestamp: June 05, 2019, 09:36:15 pm
content: Quote from: FriedMule on June 05, 2019, 05:28:25 pmThere's some scopes that have isolated inputs (Tek THS720, TPS20xx, Micsig whatever model, Cleverscope). Reading your requirement, that's what you want.And there are low voltage differential probes suitable for non-isolated scopes, offering a few V to a few 10V common mode range and 1:1 or 10:1 attenuation, for example the good old P6046 (http://w140.com/tekwiki/wiki/P6046). These offer better common mode rejection and more bandwith than the typical Pintek or Sapphire brand higher voltage diff probes, but aren't suitable for voltages above maybe 10V, depending on the particular model. Modern ones of this kind have real wide BW (1GHz and more), good CMR and can be very expensive and you'd need to treat them very carefully.Then there're these "high voltage" differential probes, as others pointed out above. Mostly intended for safe measurements in mains connected circuits, not really appropriate for small signals. These have usually mediocre CMR, and BW up to 100MHz available. There's no black magic inside these, just two matched attenuators and an differential amp. One can find schematics of these on the web. BTW, there's maybe two or three manufacturers of this kind of diff probe (Pintek, Micsig and Sapphire afaik), all the others are just rebranded. And IMO many of them too expensive for what they offer.A simple differential probe with limited voltage / BW / common mode rejection capabilities isn't too hard to roll your own. For e.g. 20MHz BW and a few volts of input signal and common mode, one could try a modern instrumentation or differentail amplifier IC as offered by ADI / TI.

author: Electro Detective
author_rank: Super Contributor
timestamp: June 05, 2019, 11:33:37 pm
content: Quote from: Nerull on June 05, 2019, 03:50:44 pmNo contest on thatBUT.. with a bit of fiddling about getting both channel gains to matchand 'square as square can be' 10x probes calibrationone can get a lot of poor persons non critical tasks done in dual channel diff/invert/ADD mode,with no wallet biting probe croc clips fitted or BANG potential earth/ground BNC/scope chassis connections necessary.I've had good enough results doing this with 15 to 200mhz scopes, and on mains***with extreme cautionOP may get some serious bandwidth doing this with a 350mhz killa scopeBest to practice this stuff*** on very low AC and or DC voltages FIRST before every session,and take physical notes of the setup for repeatability and or future use,before diving in and over-driving the probes and or channels into oblivion with higher voltages,..whilst the scope screen displays no signal, due to misadjusted or bumped controls == $ $ $ $EDIT:It would be handy to have some sort of knockup external box or easy internal mod (switchable) that can invert the signal on channel 4 on a four channel DSO,so an impoverished usercan have two poor persons differential channels using four calibrated x10 probesor is this pushing imagination and hope beyond DIY (+cheap n easy) feasabilty?..assuming this comment made any sense

author: FriedMule
author_rank: Frequent Contributor
timestamp: June 06, 2019, 12:30:33 am
content: great replyes! :-)Lets say I am using probe 1 to measure input ripple before the full bridge rectifier and the smoothing of the DC 5V input and put probe 2 in the other end, to measure how much ripple there are left. If I do understand it correctly, this will give some strange result, due to the common ground connector?How would you do that?

author: tautech
author_rank: Super Contributor
timestamp: June 06, 2019, 12:54:14 am
content: Quote from: FriedMule on June 06, 2019, 12:30:33 amWhy ?What could you possibly learn from looking at the LV AC ripple from the transformer. We all know it will be at mains frequency.What matters is the ripple at the PSU output under various states of load and for which you'd just connect across the output terminals (after checking 0V doesn't create ground loop issues with a DMM) and use a 1x probe for the additional sensitivity it offers.For a low impedance source like a PSU rail the capacitive loading of a 1x probe is never an issue.

author: Electro Detective
author_rank: Super Contributor
timestamp: June 06, 2019, 12:57:09 am
content: OP may wanna look at both while he tweaks

author: David Hess
author_rank: Super Contributor
timestamp: June 06, 2019, 03:41:36 am
content: Quote from: FriedMule on June 05, 2019, 11:48:01 amThat pretty much sums it up. The cheapest way if you have the time is to build a specialized probe into the circuit at the measurement point.The old way was to use an isolation amplifier between the probe and oscilloscope:http://w140.com/tekwiki/wiki/A6902QuoteA high voltage differential probe will not be useful in that case because of its input attenuation but there may be other ways if that is your requirement. An isolated probe will not measure 2 microvolts noise either.An active differential probe might be able to resolve 2 microvolts under the right circumstances.Quote from: AndersJ on June 05, 2019, 01:01:18 pmA Tektronix 7A22 differential amplifier can resolve 2 microvolts over a range of +/-1 volt but only at a bandwidth significantly lower than its rated 1 MHz. Doing better than this even 40 years later would be a significant challenge but it could be done.

author: FriedMule
author_rank: Frequent Contributor
timestamp: June 06, 2019, 03:41:51 am
content: Quote from: tautech on June 06, 2019, 12:54:14 amAs Electro Detective says, I would like to have the ability to measure two places in a circuit and then tweak, tinkerer and play. Why, because I think that will be a great way for me to learn what happens in a circuit, before and after, a way to follow the power, from input to output, trough an opamp, logic circuit or what not. :-)

author: FriedMule
author_rank: Frequent Contributor
timestamp: June 06, 2019, 03:49:28 am
content: Quote from: David Hess on June 06, 2019, 03:41:36 amTThe 2uV was an error from my side, I was trying to illustrate that I think that a 500X atteniuatet probe, may not be usefull for my usage of playing with beginner circuits at maybe 1.5V to 15V or to show ripple in a DC output from a power supply circuit. Sinse I have about no idea of what I am doing, am I looking for a way to look before and after some components, or maybe save my circuit, if I by error connect plus to oscilloscope ground.Simply a way to poke around.

author: David Hess
author_rank: Super Contributor
timestamp: June 06, 2019, 04:08:42 am
content: Quote from: FriedMule on June 06, 2019, 03:49:28 amHigh voltage probes are available with lower attenuation factors yielding lower noise and that is the modern solution but not particularly inexpensive. Even a 500x attenuation probe might be acceptable for making a floating measurement of everything interesting across a bridge rectifier. Dave's probe is x10 and x100 and will certainly work.Either a Tektronix 7A13 (100 MHz) or 7A22 (1 MHz) could make that measurement easily and I have used them to do it. But they are not an inexpensive method either.An isolated probe would also work great for this but today that pretty much means buying an oscilloscope which includes isolated inputs as a feature.Bob91343 mentioned combining two oscilloscope channels by subtracting one from another and that will work well in this case. Some oscilloscopes are better at this than others.

author: FriedMule
author_rank: Frequent Contributor
timestamp: June 06, 2019, 04:52:54 am
content: Quote from: David Hess on June 06, 2019, 04:08:42 amWhat is bothering me is that I may have to pay more then a beginner scope costs:-)I my naivity, I had hoped for a simpl-ish solution on a circuit, that could isulate my 1x/10x pasive probes for my noob usage.I thought that a few mega Hertz and voltage under what the scope is build fore, could be handled by some sort of transformer, optocoupler or something like that, could handle it:-)

author: David Hess
author_rank: Super Contributor
timestamp: June 06, 2019, 05:01:32 am
content: Quote from: FriedMule on June 06, 2019, 04:52:54 amThat is exactly how this works:http://w140.com/tekwiki/wiki/A6902

author: FriedMule
author_rank: Frequent Contributor
timestamp: June 06, 2019, 05:32:04 am
content: Thanks I'll read about that system:-)Just found this US patent:https://patents.google.com/patent/US6603891Sounds complicated but perhaps something useful?Is it even possible to build?

author: capt bullshot
author_rank: Super Contributor
timestamp: June 06, 2019, 05:39:25 am
content: Quote from: FriedMule on June 06, 2019, 04:52:54 amYou wrote about measuring the AC input of a bridge rectifier and the output of a 5VDC regulator. One can achieve this with an 4ch beginners scope, using 3 probes. One single ended for the DC part, and the two others for the AC part, scope does subtract these channels.Otherwise, I'm really often watching more than one signal in a circuit, and have access to various kinds of isolated channels on scopes and differential probes, but rarely need them. Most of the measurements can be done with all scope inputs connected to one common GND in the circuit. Speaking your simple example, I'd connect one channel to the regulators output and the other one to its input to see the rectified ripple - this usually gives you the same information as watching the AC side.Of course, there are situations where you _need_ an isolated input or a differential probe - e.g. watching gate driver signals on large IGBT modules, or debugging PFC stages and the like. Most small signal measurement can be achieved with your standard single ended probes.Edit: BTW, the isolation technique used by the A6902 is the same as the THS720 uses - a HF transformer path and a LF optocoupler. The TPS20xx somehow replaced the optocoupler part by transformer and encoding / modulation stuff. Splitting and re-combining the frequency band can be hard to get right. You'd still have to place the step attenuator (say the 10mV 20mV ... 20V / div switch) on the isolated side.The most modern Cleverscope does it the digital way, placing the ADC on the isolated side. Same did I when I developed rather low frequency isolated input channels for power line transient recording usage in a former job.

author: David Hess
author_rank: Super Contributor
timestamp: June 06, 2019, 05:39:44 am
content: The A6902 service manual at that page has a detailed description and full schematic. It works like you suggested with a parallel path using transformer isolation at high frequencies up to 20 MHz and a linear optocoupler for low frequencies down to DC. It would not be difficult to duplicate but it would be a lot of work.

author: capt bullshot
author_rank: Super Contributor
timestamp: June 06, 2019, 05:51:40 am
content: Quote from: FriedMule on June 06, 2019, 05:32:04 amTek did a probe that uses optical fibers only to transfer power, the signal and some control data. I've seen this probe working, it was quite bulky and you wouldn't ask for the price tagOtherwise quite impressive specs.

author: FriedMule
author_rank: Frequent Contributor
timestamp: June 06, 2019, 08:41:24 am
content: Quote from: David Hess on June 06, 2019, 05:39:44 amWary interesting!If we ignore the part of 3000V and instead say maybe 100V, could that not be made with SMD 1206 in a much smaller box with maybe some standard probes attached?You write "not difficult to duplicate but a lot of work" ehm I am not sure I understand? To me it look like a wary difficult task to duplicate the schematic, since I have about no idea on what does what, so I have to duplicate every line and comma. :-)

author: capt bullshot
author_rank: Super Contributor
timestamp: June 06, 2019, 09:55:44 am
content: Quote from: FriedMule on June 06, 2019, 08:41:24 amSince this is quite an old design, using parts that you cannot buy anymore, you'd have to duplicate the working principle, not the schematic.With some luck, an A6902 might appear at the evil bay within the next ten yearsEdit: not within ten years, but today (you see me surprised):https://www.ebay.de/itm/TEKTRONIX-A6902B-Isolator/233187964339

author: David Hess
author_rank: Super Contributor
timestamp: June 06, 2019, 05:08:12 pm
content: Quote from: FriedMule on June 06, 2019, 08:41:24 amThe A6902 actually works with standard 1 megohm passive oscilloscope probes. It included special probes which had reinforced insulation for safety. Modern oscilloscopes which have isolated inputs use the same type of probes with reinforced insulation for the same reason.Honestly if you are measuring after the line isolation transformer but before the rectifiers, lifting the ground connection to your oscilloscope is relatively safe. However this only gives you one isolated domain; all of the oscilloscope probes share the same common connection to each other. Also the common mode capacitance from the probe commons to Earth ground still exists and would be high enough to disturb some measurements. The A6902 specifications say less than 100 picofarads of common mode capacitance which is only that low because the A6902 was designed to minimize it.QuoteI mean the concept and design is straightforward for someone skilled in the art. A modern implementation would be considerably simplified because integrated operational amplifiers are now commonly available with sufficient performance to replace most if not all of the discrete circuitry. (1)I think a transformer would still be required for the high frequency path. Modern isolation amplifiers are only good up to 500 kHz or so. But an isolation amplifier could replace the low frequency linear optocoupler path and yield better performance.(1) I am not a fan of using shunt feedback amplifiers in oscilloscope applications because of poor overload recovery time but at only 20 MHz this would not be a problem.

author: David Hess
author_rank: Super Contributor
timestamp: June 06, 2019, 05:19:35 pm
content: Quote from: David Hess on June 06, 2019, 05:08:12 pmIncidentally, Tektronix also made the A6901 Ground Isolation Monitor for exactly this application.http://w140.com/tekwiki/wiki/A6901

author: FriedMule
author_rank: Frequent Contributor
timestamp: June 06, 2019, 05:52:24 pm
content: LOL WOW sometime luck is smiling to you! :-)I hope that it is still there when I have all the money.I can see that handheld scopes have galvanic isolated channels, would it be possible to build one, I mean a unit where you plug the standard passive probe in one end and a coax out in the other end to the scope, maybe with external power source?https://hackaday.com/2016/03/31/diy-active-sub-ghz-differential-scope-probe/

author: ggchab
author_rank: Frequent Contributor
timestamp: June 06, 2019, 08:04:53 pm
content: This also seems interesting:http://www.paulvdiyblogs.net/2017/10/differential-amplifier-probe-buy-or-diy.html

author: David Hess
author_rank: Super Contributor
timestamp: June 06, 2019, 08:56:25 pm
content: Quote from: FriedMule on June 06, 2019, 05:52:24 pmThat is what the A6902 is. The probe plugs into one side and the oscilloscope plugs into the other.

author: FriedMule
author_rank: Frequent Contributor
timestamp: June 06, 2019, 09:34:09 pm
content: Quote from: David Hess on June 06, 2019, 08:56:25 pmOkay yes, sorry for being so noob:-)Do you think it is realistic for me to try to build it?Quote from: ggchab on June 06, 2019, 08:04:53 pmDo look interesting, again, thinking of I can manage to build anything like that?

author: David Hess
author_rank: Super Contributor
timestamp: June 07, 2019, 03:45:30 am
content: Quote from: FriedMule on June 06, 2019, 09:34:09 pmBreak it down into parts and start with the simple stuff. We have the full description and schematic of the A6902.Could you design and build a high impedance amplifier which accepts a probe signal? That is the first stage. If not, then no although I can think of old oscilloscopes which could provide this part inexpensively. It just gets more complicated after this.