High Voltage 10-100MHz Differential Probe Investigated

[Mechatrommer]

As you see, one of the reasons they might be so expensive is the many considerations in design and safety for a commercial product

the thousands dollar tek or agilent can go up to GHz range, with astounding cmrr and spec, certainly not within the class...

ps: to be fair as i cannot promise anything...
there's competetive probe around at $200++. i've been eyeing on that model, Pintek... just recently enter ebay... http://www.ebay.com.my/itm/New-Professional-Differential-probes-DC-25Mhz-ADP25-Max-Voltage-1300V-1000V-RMS-/380411931457?pt=LH_DefaultDomain_0&hash=item5892516b41 those pincer they used is not widely available and they sell it at high price+shipping. i'm not so sure if i can hit quarter the price of BOM. if anyone in great need, thats the link. i believe that will be "china" approved consumer product.

the spec is better (i'm striving for that as benchmark) i'm not sure how in real life it will be. reducing my BW to 20MHz will make this project less competetive. whats the hope left is we can tweak this project to boost its potential, but that will need time. i'm kinda tired this time and would like to finalize this version, use it in another project and see how it performs. selling this as kit or such is kinda shame for me, since there still much to improve, and for this particular project is not easy and not cheap.

[T4P]

i WILL definitely buy it from you once you get it done 

[Lightages]

Have a look at the Pintek page on differential probes:
http://www.pintek.com.tw/

I wonder what the cost is of the 100MHz one. I have not found any prices yet.

If you can make a 20MHz or better differential probe for under $100 as a kit or partial kit I think you would have something there.

[Mechatrommer]

globalmediapro... http://www.globalmediapro.com/do/search/?q=Pintek+probe
all sort of their (pintek) range. i tried to order the pincer from them. the shipping is well above the price of 8 pincers that i tried to order. and they only take bank transfer... sad. based in poland or somesort.

[Mechatrommer]

one person has made a teardown. cant recall the site. google for diff probe, pintek or such. let me re-post his image here...

[casinada]

Looking for an accessible Differential probe myself. They're expensive.
http://www.sapphire.com.tw/
It seems that they make probes for LeCroy and Agilent.

[Mechatrommer]

i think i'm closer. BW probably around 10-50MHz, currently doing 25-30MHz testing. anomalies are gone now, but cmrr is still not a wonder, that part still needs improvement and some black magic perharps. the last time anomalies due to crappy psu used and some resonanse here and there, i guess this is the reason i got bald spot top back region skull now. checking my archive, the earliest file was created 14th Feb 2012 (ver 1a) this thread was created during ver 1g or 1h or so, its now ver 1m2. so its 2-3 weeks to its 1 year celeb, the most historical and difficult project i've messed around. i'm still 50-50 if i need to publish the schematics or not. still the cheapest similar classed diff probe (high-voltage) i can find around is Pintek brand one somewhere $200+ in ebay, i'm aiming at $100 material + labour cost or less, then only then its worth an effort. cheers, me? still... %-B ing

[casinada]

Don't give up, It's your project and a learning experience. Of course you can buy something already made, people try to design Power supplies, DC Loads, component testers every day.
On the other hand Pintek prices are coming down?
http://www.globalmediapro.com/dp/A01LT0/Pintek-DP-25-Differential-Probe-25MHz-1000V/
$189
And Some Pictures of the Nice probes it comes with:
http://www.ebay.com/itm/New-Pro-Differential-probes-DC-25Mhz-Max-Voltage-1300V-Tektronix-OEM-Taiwan-CAT3-/290711961811?_trksid=p2047675.m1851&_trkparms=aid%3D222002%26algo%3DSIC.FIT%26ao%3D1%26asc%3D163%26meid%3D5132151774567772912%26pid%3D100005%26prg%3D1088%26rk%3D1%26sd%3D360574664698%26
The probes were reviewed here no long ago.

It looks that the founder of Sappire Instruments started just like you designing a differential probe so you never know.....

  "Sapphire Instruments is a privately owned Taiwanese corporation. It was founded in 1988 with the vision to provide testing instruments of high quality and high performance. Our sole shareholder and general manager, Mr. Jeff Hsu, who was a former assistant professor in a university, developed the first handheld active differential probe in the world. Our company has developed a series of active differential probes and established our status as a professional producer in this field. Recently, we are a worldwide supplier of active differential probes......"

So keep us posted on your progress and remember us when you're rich and famous

[jram112]

Any update on this probe? I am very interested in this as are/were others. Any chance you are sharing or selling the latest design?

[Mechatrommer]

Any update on this probe? I am very interested in this as are/were others. Any chance you are sharing or selling the latest design?

if you ask me... i've laid out some long term vision and mission to my life about a year ago, i must work things in order so i have some meaning to my life. sadly this diff probe project is going down the list and will be in the drawer for quite sometime, probably years (similar to my all and every ee projects in the list). i have my pcb ver1 made by itead, i've spent alot of time on it and money to buy parts, its in the stage of "working or functioning", but the spec is mediocre compared to eevblog's standard and the ready made unit around. to give you some clue, i love electronics and want to make many things, but during the journey i hit the wall on pcb schematic capture & sim software "free version" limitation. the diptrace (that i love so much in term of interactiveness) charges quite significant for full version i cant justify for my hobby work, and it has its own weaknesses too (i know we have a list of free and OSHW eda somewhere, so i'm aware of them, suggestion is not necessary here, they are all not suite my need). so on the top list emerged a new task... to build my own schema+pcb+sim eda software before anything, but i know the task is daunting, it will take years to complete. i've started on the surface at the end of last year or so, but since the complexity, i think i will need rest from time to time i cant spent my whole time on it my wife will get angry, my kids will be ignored, i have another "home improvement" list that is also on hold everything must be completed sigh. so i made my first rest/pause on pcb eda project at the early of this year, and that "rest" continues until today, i think i will need to collect more guts before continuing. pretty much now, i'm in idle or hibernation state in ee arena... thats the sad story answer most of you dont want to hear.

but i still believe this diff probe project is a "worth it" project, promising as an open source or community project or cheap alternative to ready made, i wish i can continue but sadly i cant. from my experience of doing this, comparing to even the cheapest unit around, i believe the cost of parts is not so much, the most cost lies in the R&D of it, knowledge and experience needed to accomplish and specify it, thats where we need to work on. the recent LeCroy AP031 Diff Probe teardown made by andyb indicates similar topology (i believe) as the OP http://bardagjy.com/?p=1664 with some additional feature, refinement and compliance of course. so its stiffen my belief that this is doable. i wish i could share my current (latest) design, but i assume its still in incomplete state, and most importantly i'm a very poor at documenting and designing circuit in standard "engineer approved" way. anyway, since my limitation, i also would like to pass this project to the community so i will not keep this project dangling with me. so FWIW attached is my diptrace file (schematics and pcb) file. be warned that i used my own "proprietary" convention and "style" to design it, so dont complain much on that. you may ask what the parts used, but the rest of the circuit chronology, i encourage you to study it, the basic idea is already explained at the beginning of this thread. so here they are, i wish you good luck on your journey. i hope the attached files will help you significantly instead of making your life miserable. i have many other report files, but i dont think i want to attach them here since it even more nastier and will make your and my life more miserable, trust me.

and for those of you who expect a ready made ready to buy item, then just ebay or google it and buy whats available that suites you most, i'm sure it will be your well worth of investment and time saving from DIYing. have a good day and good luck

Regards,
Shafri.

[robrenz]

Mecha,  I am glad to hear you are doing well.  I need to apply more of what you are doing.

[Alexei.Polkhanov]

Here is photos of Agilent/HP diff probe (I think this model was sold for like 20 years). It is 200MHz probe, and can capture differential signal +-35V with external attenuator attached at the input - It comes with 3 one for AC coupling plus 1:10 and 1:100, 1:10 is one I ever used. So it is not high voltage although it is capable of handling HV its intended for dynamic range in order of +- 5-10V (CAN bus is most likely use).

You can look-up all details including some pretty detailed schematics in user/service manual on Agilent website. Just search for 1141A diff prob (page 59 for schematics). It comes with external power supply/configuration box where you can adjust offsets. Bulky, but works pretty well. Notice the custom ceramic box on top side - that is the "secret" I guess and it gets hot pretty fast and stays that way.

Manual: http://www.home.agilent.com/agilent/facet.jspx?k=1141A&neighborhood=ETM&kt=1&cc=CA&lc=eng&homesearch=Search&searchbtn=Search

I would not be discouraged by fact that building it costs more than buying off the shelf. If I can build a prototype of diff probe with specs comparable to brand name alternatives I would be extremely happy and proud. I think you can always sell this kind of experience for lot more if you use it building some small niche product or if you just put it on your resume. (bring the prototype to interview - at some places they love that).

[Marco]

Here is photos of Agilent/HP diff probe (I think this model was sold for like 20 years). It is 200MHz probe, and can capture differential signal +-35V with external attenuator attached at the input - It comes with 3 one for AC coupling plus 1:10 and 1:100, 1:10 is one I ever used. So it is not high voltage although it is capable of handling HV its intended for dynamic range in order of +- 5-10V (CAN bus is most likely use).

Ehh, it can't handle a signal superimposed on mains (on this side of the ocean) so I wouldn't say it's HV capable.

I wouldn't be surprised if these work fundamentally different from the instrumentation opamp approach BTW. I think they are more likely to have a difference amplifier similar to this.

You can look-up all details including some pretty detailed schematics in user/service manual on Agilent website. Just search for 1141A diff prob (page 59 for schematics).

Hmm, I'm being very contrary today ... I wouldn't call these detailed schematics either

[Alexei.Polkhanov]

Ehh, it can't handle a signal superimposed on mains (on this side of the ocean) so I wouldn't say it's HV capable.

I never said it is HV probe - I said that max differential swing is only 30V for this probe. I think HV version would be exactly same except x500, x1000, x2000 attenuation added in front.

I noticed that on Pintek probe photos it has x500, x1000 - I assume it is for attenuation. Also besides attenuation and input isolation part HV probe should be cheaper and easier to make because you only need 15-20 MHz bandwith. I mean how often do you work with 2Kv signals that have 500MHz bandwidth unless you work on nuclear detonators or something like that?  Am I wrong?

[sorin]

Any news, after 2 years?

[Mechatrommer]

Any news, after 2 years?

yes there are. the old pcbs are still lying around. i bought CAT3 or something big and quality rating uni-t clip probe, few high quality resistors and caps from element14 that i never used or tested, just before temporarily closing the project file. i supposed to start revision 3 iirc back then but i havent got the chance. the closing are due to lost of enthusiasm maybe or thinking that i need to reorder new pcbs ver3. but thinking the extra cost and the wasted 10-20 wrong pcbs, maybe lead to lost enthusiasm, i also think i was short in term of knowledge and experience. i always think about it, i believe one day i will re-open the case... but, until the time is right... the basic circuit are already laid down, some newer discussion threads emerged, that can also be used as lessons for rev3. anybody want to follow this venture may do so on their own, this is quite easy until you try to match both input... fwiw.

[moffy]

I enjoyed reading of your project. Thank you for sharing. Just a comment about the low CMRR, I don't think you can use multiturn pots to adjust the resistances because they are a bit more complex than just a resistor. I think you would have to bite the bullet and use at least 0.1% if you want 60db. Also, what diodes did you use for the input clamping? 1N914/1N4148 can have a deal of capacitance that is non linear. I'm still looking for a good clamping diode.

[Dago]

I made something very similar (inspired by Mechatrommers topic!) http://www.dgkelectronics.com/high-voltage-differential-probe/ https://www.eevblog.com/forum/projects/my-high-voltage-differential-probe-design/

[Mechatrommer]

I don't think you can use multiturn pots to adjust the resistances because they are a bit more complex than just a resistor.

i'm guessing the inside is just a simple linear resistive rail with some worm gearing or such. i'm not sure never made a tear down of it. but i like it because we can get high resolution with "multiturn". thanks for suggestion, another point to be investigated...

[LA7SJA]

Can we sponsor your project in any way?

Johan-Fredrik

[Mechatrommer]

Can we sponsor your project in any way?

me? unfortunately its not a matter of money alone. its a combination of money, time, knowledge, enthusiasm and need. i believe noname brand diff probes have coming out from time to time, more affordable, like pintek unit. if you care to sponsor me, i thank you for your concern, but i believe the money is better off buying already made one. try pushing something like this to become sellable, or open source that can satifsy everybody is quite a heavy stuff for me... i cannot satisfy for myself, how can i satify everybody? pls dont count on me, sorry.

[moffy]

I don't think you can use multiturn pots to adjust the resistances because they are a bit more complex than just a resistor.

i'm guessing the inside is just a simple linear resistive rail with some worm gearing or such. i'm not sure never made a tear down of it. but i like it because we can get high resolution with "multiturn". thanks for suggestion, another point to be investigated...

In the Bournes book: https://www.bourns.com/pdfs/onlinepotentiometerhandbook.pdf, page 174, carbon type is recommended up to 1MHz, cermet up to 250kHz, and wirewound up to 1kHz.

[dospepe]

Since it seems that it will not continue with the project, you could share the list of component values, so that others could prove their design ?.
Thanks in advance.

[Mechatrommer]

Since it seems that it will not continue with the project, you could share the list of component values, so that others could prove their design ?.
Thanks in advance.

i have a plan to continue this. all parts are still safe in the shelf... values for components are a bit of challenge to publish, last time istr there are 5 configurations of components values for input volt divider and which one perform better is not known yet.. i also tried few opamp series and gain settings. so everythings are unfinalized. whats certain and unlikely to be changed are already posted in this thread.. mainly 3opamps config. even the position of trimpots shown will likely to be changed in the future. and maybe the front input divider will get separated from the main opamp input, so this is unfinished business...

[dospepe]

The truth is that I think a very interesting thread, and it would be a pity that all this work was lost; that we can not afford to spend 200 or 300$ in a probe we were very hopeful with your project.
Greetings.