How to Make a DIY Differential Oscilloscope Probe

For an upcoming project, Industry Expert Consultant Mark Harris needs a differential probe that can handle high voltages. But finding one to suit his needs is an expensive endeavor. Join Mark as he goes the DIY route with this differential oscilloscope probe project.
0:00 Intro
0:31 What is a Differential Probe?
1:34 Our Differential Probe Overview
3:07 Board Layout & Assembly
5:20 Circuit Alterations
6:41 Testing
10:03 Isolating the Issue
12:10 Checking the Probe's Response
12:56 Wrapping Up
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[@str8upkickyaindanuts289]

Well that's interesting, I've never considered myself an expert and not to brag but my DIY differential probe worked first shot. I probably spent 10x the time designing mine so not sure it's apples to apples. Mine included safety features this design is lacking, no TVS, GDT or MOV on the inputs, no clamping diodes or transistors on the buffers, even with the high impedance I wouldn't feel that safe using it on mains. I really wish you went over the CMRR and zero offset on the buffers as well, or at the very lest applied common mode DC voltages to show the CMRR performance. It's a key design parameter to insure accurate results, I know I struggled with matching the high impedance inputs because I only have 5.5 digit DMM and measuring 4Mohm in series it doesn't have the resolution to show the least significant digits needed to reduce the potentiometer value for lower noise. If we wanted to get extra fancy, isolating the opAmp rails would insure no ground loop issues as well. Only mentioning all this because you pointed out the SI consideration of rotating the layout for dielectric effects, IMO that's far less important than the considerations I pointed out above. In no way am I saying this in negativity, I greatly appreciate the effort to produce the training material Altium provides.

[@joet4348]

Appreciate your comment. Thanks for providing all of these details.

[@filipnevezi]

Thanks for sharing your thoughts. Is there a write up or a video of your design anywhere? I feel that it would be a great contribution to the community if you shared your design with all the thoughts that went into it.

[@666aron]

Nice writeup. I'm also lining up for a design file or two if there are any.

[@Gengh13]

Also interested in seeing your design, if you uploaded it somewhere.

[@tfinmoraes]

Those are nice points, and they certainly make an overall better characterized probe. Not to mention protected. Since you mention, I'd also like to see your previous work, if you are willing to share.

[@sebastian_harnisch]

I'd like to mention that there *are* decent low-priced HV probes like the Micsig's. Sure, those are only 100 MHz and therefore wouldn't meet your bandwidth requirement of 250 MHz. On the other hand you didn't test/validate your probe for anything higher than 25 MHz ;) Anyway, we like to diy things and this might be reason enough. That being said, I recently tried to build a differential probe using the ADA4817, but with two ranges (x10, x100). I faced many issues starting with the power supply. 1) I opted for an isolated DC/DC converter, but at the time I was surprised to see its rather poor common mode current performance. Didn't really think about it at the time... 2) Finding a good adjustment that works with both ranges has proven very difficult if not impossible with my circuit design. So I would have to either get rid of the 10x range or redesign the front end. 3) This was my first high-speed pcb (I'm an power systems engineer; not much experience with PCB layouts at all, it's just a hobby). So with my probably poor PCB layout I ran into opamp stability issues in one of the two ranges and found a rather poor frequency response (whereas the simulation showed great performance). For now I decided to stop working on this just-for-fun project, because building one or two more prototypes would probably cost me significantly more than just buying a Micsig probe ;) Still, I learned something, and this is always a good thing...

[@SurvivalSquirrel]

Damn! Its a great example, of how important the correct measurements and signal generators are.

[@zvotaisvfi8678]

Pretty reasonable sequence of steps when troubleshooting ! Thanks for releasing this !

[@Dieseleux]

I work with high voltage and i frequency power converter, i use Fluke Scopemeter 196 and 199C because is isolate between probe and also between power supply, in extreme case, Scopementer run on battery, is very nice scope to probe 600Vpkpk on hot side at 100khz square wave when probe B are on cold side in feedback circuit.... Scopemeter are great but also have some common mode trouble... sometime, i run probe with few ferrite on the cable to reduce emi pickup and solve false trig and clean the trace, is a good trick to do with any scope in fast wave mesurement. I found your differenciel probe interresting and testing this type of design are not really easy, push the circuit to the limit need very high end test equiment and specific lab environment! Thanks for this video.

[@K.D.Fischer_HEPHY]

Very cool video. A man that has fun working with Altium and with the skill to actually produce the stuff himself is a rare sight. ;-)

[@0xTJ]

This is a really cool video! The project is simple in concept, but very nicely executed! And it's fun to see the struggles with testing the result.

[@AltiumAcademy]

Thank you very much!

[@susantosukadi5143]

You got a like, a subscriber and a buzzer on from an old guy. TNice tutorials is the best soft soft tutorial I've seen so far. You covered a lot of

[@tfinmoraes]

Hi Mark! That was a great presentation, thank you for all the work you put in here and shared! The way the content was scripted, and the storytelling was awesome! I really enjoyed it! Now, if I may, a brief constructive feedback. I believe you didn't bring up some key points involving protection and characterization because it would not fit on the length of the video you and your team probably had in mind. Also believe that you wanted to frame the video to the "average" guy in the lab that may not be concerned with a deeper understanding of all the issues concerning diff probes, and/or that does not have high end equipment for testing/debugging. If that is the case, that's alright, you have a broad audience. Now, from a personal perspective, I’d like to see some more information about the specs. Things I am sure you thought about and dealt with, but were not presented: some more thought on the input voltage divider, frequency and safety wise: input capacitance, clearance, cut outs, distances, resistors voltage ratings - 1.2kV is a serious matter, as you have mentioned; CMRR analysis; proper EMI filtering (since you had some issues with it); shielding; proper frequency response and slew rate dv/dt limitations; battery monitoring - or at least some warning when batteries are low, possibly compromising the measurements; ESD protection, since someone will certainly touch the probe at some point; noise floor; AC and DC accuracy and a brief error budget analysis, including attenuation error, op amps errors and drifts. And since we are at it, who knows I can bargain with you an input voltage range selector. 😅 I know these points would make a much longer video, but the thing is that this is a topic most of the lab addicted electronic engineers, like many of us here, love to dissect. Having a well spec’ed and tested diff probe no doubt pays off, since every measurement using it is subject to its behavior. To be honest, when I first saw the title of this video from this channel, I had hopes I'd see some of those points covered. So, this was a great introduction and aroused a lot of interest! Maybe a follow up video will be a lot of fun!

[@paulmeynell8866]

It always amazes me we can accurately measure ns and the smaller and faster we try to measure the harder it gets! Thanks for the video

[@user-zc8sd8jx8s]

pretty much like accelerating to the speed of light or cooling down to the absolute zero temperature.

[@dimonasua]

Thanks for a nice video! Looks like someone is having fun here :) Looks like you are unstoppable even using a hobby grade lab for this development. Well done!

[@Danielastronomo]

Thank you for the video! It is very educational. Could you make a tutorial on how to do hierarchical design? I find harness connectivity between functional blocks confusing.

[@gerryjamesedwards1227]

A Schmitt-trigger oscillator, a hex jobby with one oscillating and the feeding the other 5 with their outputs summed, can give a surprisingly fast-edged square wave, if you need to test rise-times.

[@BHBalast]

It was great showing the process and fails :D

[@Robertkopp84]

Can't wait for that one. I wanted to pick one up for a long while, but I did not for the given reasons. The same is true for low amperage ac/dc current clamp.

[@str8upkickyaindanuts289]

What bandwidth are you needing that current clamp to be? Ti has a 20KHz hall effect sensor that I successfully used to get very accurate results with only an iron core toroid notched to fit the sensor. I calibrated it using a DMM and bench power supply, at room temperature it's been very accurate and stable for the past year or so. For my application I included the current clamp on my differential probe circuit board to have a single unit power measuring ability for DC-20KHz.

[@Bianchi77]

Nice info, thanks for sharing it :)

[@shuashuashua1]

Great vid, thanks for sharing

[@AltiumAcademy]

Thanks for watching!

[@AmpworksCoUkRepairs]

Trimmer capacitor is: Mfr. Part No. JZ400 Mfr.:Knowles Description: Trimmer/Variable Capacitors 8 -40.0pF 110V

[@yveslesage8525]

Very nice and simple design, thank you for sharing. But there is something I don't understand about the diff amp stage: the 348 ohms resistor from inv input to ground gives a differential gain of 1.9 dB (instead of 0 dB). Is it done on purpose or is it an error on the pdf schematics ?

[@martinvasilev9809]

Great video! I like the idea very much.🔥❤️ Someone get this man some measuring equipment! Where is Zach Peterson when you need him?

[@johnyang799]

Have you matched the output impedance of the ground and the output from the generator? Or you could also use two outputs to make a true differential output that way you eliminate the ground. Lastly I don't know if two-conductor only coaxial cable is capable of delivering differential signal. It should be two wires in side with a shield on the outside and ground joints at the output end of the cable.

[@tejonBiker]

Nice video, I worked for a company that makes induction heating machines and we used a lot of diferential probes, That machines can generate square wave signals in the range of 30 kHz and and 620 Vp, I always wondering how to make a differential probe DIY (because this devices are expensives for my income), I found some useful schematic but in the end the problem was the signal to test (frequency and voltage) I think is more harder to generate the signals for the DIY diff. probe than building the probe itself.

[@gabrielbrunheira]

Great video! Loved all the little details you went across during the testing part! It's the first time I hear about signal chains running diagonally across the board for a integrity purpose, and I actually didn't understand it very well. Could you please provide some references to get deeper on this topic?

[@vonnikon]

Just search for "PCB weave effect". It is really just an issue for routed transmission lines. Not component footprints like in this design. The differential input on this board is not even a controlled impedance path. So weave effect is not at all a concern here.

[@gabrielbrunheira]

@@vonnikon Thank you! I've stumbled upon a cool article, from which I understood that this effect is relevant for much higher frequencies than the 250 MHz bandwidth spec from this design. And this is related to your comments above, right? So even if this diagonal routing was considered to address common mode rejection loss, maybe it wouldn't have any difference, because the affected frequencies would already be attenuated. Is it correct?

[@vonnikon]

@@gabrielbrunheira weave effect can cause a small change in impedance. When you are routing a 50ohm microstrip transmission line. But the differential inputs stage of this design is not a controlled impedance line, so it does not matter.

[@Bob-zg2zf]

great teacher, thank you.

[@AltiumAcademy]

Glad you liked it!

[@maks886]

I really enjoy these project videos, thanks!

[@AltiumAcademy]

Glad to hear it!

[@haraldnoack4139]

You can make a high speed high voltage pulse generator with a coaxial mounted and shielded reed contact in a coaxial cable. One side is charged from a high voltage power supply by a resistor in the kilo ohm range. The length of the coax cable correlates to the pulse lenght. The other side is loaded by 50R and is the pulse output. The reed contact is switched by a coil outside of the shield. I got pulses with 1000V 20ns long into a 50R load. Repetition rate 10Hz, rise time

[@MrNoobed]

I'd be interested to see if putting a BNC with a terminating resistor on the unused outputs cleans up your signal generator.

[@amirb715]

since your function gen has two channels, you could have used both to generate your differential signal instead of connecting its 'gound' to one of the floating diff inputs of your circuit.

[@ariewestland3523]

You take care to provide multiple high voltage components in series with the inputs, but this is completely defeated by only one jump from SMA signal to shield plus one jump across the 0R (sometimes not mounted) 1206 footprint.

[@ninjapiroxD]

Great video, and thanks for sharing you desing projects. couple of questions, so the capacitors that are in parallel with the string of resisters at the input, are they to compensate for inductance of the pcb and/or resistor? would also be nice to be able to by pass some of the input resistors to have an ajustable max input/ input impedance, to get less noise on smaller signals, how to do that with a switch without affecting the reponse i dont know, ¿small jumper wire? sominthing that dosent require soldering idealy, thanks again :)

[@JohnUsp]

Awesome, Thanks..

[@AltiumAcademy]

You're welcome!

[@1202Sid]

Can this probe be used to measure a voltage drop across a resistor on a 12v system ?

[@alexwang007]

My differential prob so far is just two of my scope channels lolol

[@stevebabiak6997]

That’s what I used to use back in the day when I made such measurements. And an isolation transformer for situations where that was needed. Had to spend enough time testing and fixing whatever project I was working on, so not wasting time on fancy stuff like in this video when it wasn’t necessary (and I did have to design test devices for cases where couldn’t get by with off the shelf stuff).

[@jonaslithen7240]

Nice project! Looks like there are 2 EDAs that can 3D render boards properly. KiCAD and Altium. PADS looks prehistoric and Cadence makes a woodlike board with layers wrong.

[@jeremyglover5541]

Diptrace does a half decent job as well, as does Eagle, since it is now owned by autodesk and a module of fusion360

[@gaozihan4]

where is the design file?

[@filipnevezi]

github. check the description.

[@magnuspihl6974]

Nice. How much noise does it have?

[@The_Unobtainium]

wow!

[@micbanand]

waw Great video. You got the bell and follow butten pushed :)

[@AltiumAcademy]

Thanks so much

[@joeshmoe7899]

Where is this on Aliexpress?

[@KD0CAC]

Not familiar enough to take on a unfinished project , was hoping to find a build ?

[@massriver]

🌚Not sure if I should subscribe or not, omg .

[@samuelemmettbray]

Way too much info in way too short a time. MORE about the schematic. MORE about doing the layout. OK with mistakes and bad designs so long as discovery and fixing are done too. This would have made several great videos. Make it long; give us all the dirt and info. Show us the grueling task of problem solving and trouble-shooting and learning. Much more of this.

[@Momixxxxx]

What is roughly the cost of this probe?

[@BHBalast]

Why the caps on divider network?

[@robdavis3220]

Compensation. Without them the high value resistors combined with and capacitance at the end (and all along) of the resistor chain will form a low pass filter with a cutt-off way lower than you would want.

[@sebastian_harnisch]

@@robdavis3220 and not to mention the input capatitance of the FET opamp...

[@jimviau327]

Why aren't scope inputs floating by default? I mean, who doesn't need, at least, once in their career, to measure a 10vac signal at 500Vdc away from ground? It's a fundamental must for any serious electronic endeavor. Building a scope input amplifier and mendatory ADC must output a digital signal. All that is needed from there is an insulated way of transferring those bits to the main computer that is running inside the scope. That couldn't cost prohibitive extra money ? And so, following the same idea, why not build an insulating amplifier using preamp, ADC, insulated bit transfer, and back to analog? Two of those would be much more flexible than a dual input differential probe. I have not the abilities myself, but the idea seems quite evident to me.

[@vonnikon]

You should not need a 50ohm series resistor on the BNC output, as long as you have 50ohm termination enabled in the oscilloscope.

[@gkasprow]

the oscillations are caused by the capacitive load of the opamp. Probably a few Ohms would help to decouple the cable from the opamp

[@vonnikon]

@@gkasprow maybe... But he is using a Rigol. Many Rigol oscilloscopes don't even have 50 ohm termination built-in. The frequency of the reflections correspond quite well with the propagation delay of a short unterminated coax cable. On another related note: SMA for 5 Mohm differential inputs is an interesting choice... Those differential inputs must be kept really short, or there will be the same kind of ringing problem on the input side. SMA connectors certainly don't make it easy to keep the inputs short. Pin headers would have been a better choice.

[@gkasprow]

@@vonnikon yep. But impedance mismatch looks different than decaying oscillations :) OFC, both issues add up.

[@gabrielbrunheira]

I tend to agree with Greg, including the suggestion of using "few Ohms", instead of 50 Ohm, because with any 13 pF capacitance (cable + connector + osciloscope input?), it would form a RC filter with a cut-off frequency lower than his 250 MHz bandwidth.

[@robdavis3220]

My Rigol 2000 series does have 50 ohm selectable option. I'm sure that scope does as well. I'd love a decent diff probe , but cost has always been a issue

[@blablabla7796]

Couldn’t you just use three well spec’d op amps in an instrumentation amplifier configuration to mimic a diff probe? Having so many RF conditioning circuit components feel like you’re just adding more things that could go wrong, no?

[@coisasnatv]

How a person can call it self an "expert" if he can't cover the basics? Like some one said in another video, today, any disqualified person can call it self "expert" or "coach". It is worse when the channel name has "academy" on it.

[@cvspvr]

hey mark, did you know that i'm using a cracked version of altium designer? ;)

[@VorpalForceField]

I would like to point out the extreme voltage range targeted .. There has been practically no safety features built into this design for working at such high voltages .. The sma connectors chosen for inputs, at their 500V rating, cant handle even half the target voltage .. I can appreciate the attention to detail used for signal integrity, but safety should come first ... The front end of this needs to be completely redesigned before any one attempts using for high voltages.. I can point to a similar project, by a content creator who has experience in high voltage design practices, to help address these concerns .. Granted the back end will need to be redesigned to meet the bandwidth requirements for this project, but following video would be a better guide for creating the front end.. Hope this helps .. Stay Safe .. Cheers :) Prototyped: kzbin.info/www/bejne/lYC9Zoubp51nm9E Finalized: kzbin.info/www/bejne/ZqXLgJmhaa5mp9U

[@Alpine_flo92002]

Instead of a differential probe you could also get a cheap USB scope and use a laptop...tho you may wanna get a data and power isolated scope aswell as else you might still kill your laptop if you get a too high voltage

[@cashbull59]

you have too many of everything!!!! but.. forgot the simple transformer!!! all that hi tech jazz can not match the simplicity of a transformer! thanks for posting!

[@robdavis3220]

Tracking down common mode noise can be a real PITA. You also need to be careful about what you see on you scope screen. Incorrect probing techniques can lead you down the garden path. All the way to hell!!