High Voltage 10-100MHz Differential Probe Investigated

[Mechatrommer]

Original Spec:
a) ±330V input (using simple resistors voltage divider)
b) 100MHz bandwidth (using proper opamps)
c) simple input protection using clamp/limiting diodes
d) built-in fixed attenuator (so not to hassle around with input and many attenuators)
e) from jellybeans components (%1 resistor etc, need 0.01% matching? use trimpots!)
thats all as far i remember

Current Spec:
1) ±500V input (still using simple resistors voltage divider, because its... "simple")
2) 20-100MHz (it depends on my "engineuering competency" up to this point)
3) still with simple input protection using clamp/limiting diodes (the study has not been furthered to this stage, "good enough" is assumed)
4) noise:
   50-100mVpp (effective noise at 1/100X setting), currently at ~200mVpp achieved
   5-10mVpp (effective noise at 1/10X setting), currently at ~20mVpp achieved
   0.5-1mVpp (effective noise at 1X setting), currently at 2mVpp achieved
5) CMRR:
   >60dB @ 1MHz and slower freq (preferably 90dB and more but... thats my tools' limitation i believe) currently at 45dB @ 1MHz, i believe i can reach nearer to opamp's CMRR limit.
   ~40dB @ 10MHz (currently at 25dB and not sure at higher freq, as mystical things creeped in)
6) psu from unregulated bipolar ±20V, regulated to ±12V
thats it! i think for now...

note: the input can "easily" be upgraded to ±1KV but for later...

as i said, this one is not finalized, and infact i still continue to refine this esp in several areas:
1) input attenuation, matching and freq compensation.
2) bandwidth specification (combo'ed with above)
3) CMRR (opamp selection)
4) noise level (opamp selection, combo'ed with (2))

(msg edited)

[Mechatrommer]

1st pardon me for my non-standard way of explaining things, i'm no ee "engineuers" and i "aint no A?tium", and i think i hate to redraw, this typing alone is a tremendous effort i try to achieve. so i just post what i have done, if its not clear, please ask. i drew things in diptrace (non-standard my own library symbols) in "dog's breakfast sphagetty way" but i hope it will be self explanatory with its marking (i tried to be as standard as i can ) i also have tina-spice schematics, but i think those are from older version, i didnt update/redraw in latest revision, too tired with all the changes. i'll post when i see fits. and also pardon me for black style background, my eyes work better with that in diptrace.

first we start with the "well known" 3 opamps instrumentation amplifier, here (wiki image) for those who didnt know...

there's 2 opamps type, why 3? because its matched high impedance noninverting input, thats good, so thats the choosen one, right in the first place. the latest version will use gain 2X ins-amp (originally gain 10X)

and then, no opamp can take ±500V mains line direct, not even any high speed bjt/fet i'm aware of (only big type ones), even if there is, designing raw discreete transistors will complicate things astronomically for me, so i stayed with opamps as the simplest building block. so in order the opamp to eat the ±500V, it needs attenuator, resistors voltage divider is the only thing i can understand easily for now. again from wiki...


since we are dealing with high frequency input, some compensation using capacitors will be needed, something like this, in combo with the above...

(i'll use 1/100X divider in the latest revision, combo'ed with gain 2X in ins-amp, resulting in 1/50X to the scope input unterminated, and 1/100X if terminated, beauty! )

thats it! simple isnt it? the rest is ofcourse... psu, and the supporting components (bypass caps, compensation networks, bnc or sma connectors and a bunch of coaxes and wires and cheapo smd hooks etc... and "very" good pcb layout to run it with, never excel with that yet) and of course, some tweaking with generator,scope and the circuit.

so all those basic things can be summarized in block diagram as below (diptrace)...

input=attenuator or voltage divider.
cmrr+differential = the mentioned 3 opamp ins-amp, i divided them into 2 parts
power = is power supply.
whats not shown is the output which is just a simple 50ohm terminated coax cable. and the image shown is for ver H above, but its been like that from the early stage, no change on this part/concept.

now lets take a look one by one at the block. we start first at psu, its nothing just a standard datasheet circuit using bipolar LM317/LM337 (jellybeans?) regulator. its the latest rev H. in earlier version, the regulator resides in the wallwart (or my bench psu), but now its integrated in the probe (due to some long story reason). here (diptrace ver H)...

the device will take unregulated bipolar supply from wallwart through LA_BATT tab there, regulated by the LM317/337 and feed the circuit (the other blocks) through the V+,gnd,V- connector there, ±12V supply to circuit, the opamps will be expected to output ±10V to the scope. simple! can be overclocked to ±15V supply to circuit if you wish for ±600V input, beauty!

and then the next block is the input rev G. here...

the important components are Ri+, Rd+ and series trimpot for matching and its negative counterparts just a mirror of them (divider is 1Mohm and 10Kohm, ie 1/100X), i leave the compensation part for you (Cci, Cod, Cadj etc), i provided more compensation network in case i need it.

in the latest input rev H, theres 3 network, LF compensation, MF and high freq (HF), probably i'm not going to need all, just in case. here's the half-completed rev H input (for 1X setting). 100Kohm input impedance (Rt+, Rt-), too low for ±5V input probing? yea, talking about the wrong (not so good) opamp buffer here (THS3092 1Mohm input). larger than 100Kohm terminator/pull down will output garbage nasty offset at the output. even with this 100Kohm, but thanks to the "drifty" null offset circuit (below)


next... cmrr+differential block. they form the 3 opamps ins-amp config mentioned above, with supporting circuit, compensation, null offset from zeners, trimpot and buffer to zero the output offset. this should be pretty simple. i leave the detailed explanantion to you to avoid "boringness", any questions just ask ins-amp buffer is 2x opamp THS3092, and the differential opamp is THS3091. currently achieved BW is 20-50MHz (due to not so good input matching and compensation, reflections and other mystical things) so quite a waste for 100++MHz BW opamps. re-selection will be in order if required.




to the 4th post... the result...

[Mechatrommer]

the following is the latest "real circuit" ver.G thats what i can show you the result with (currently working on rev H, no real circuit or pcb layout yet).

1) the "not so good" diy circuit.




2) some probing test i made earlier on offline cheapo smps circuit with rev F circuit. the probing is between the hi-freq transformer primary high side leads. the low side? there's mosfet switching going on, but not shown/care.






3) the DUT to probe test with. i will plan for bigger circuit when i finish this project (if i manage to get around it), namely using PC PSU transformer, which with its small size capable of supplying 12V 15A, which is not available in 50/60Hz linear supply transformer, at that lightweightness, not even very close.




4) the noise floor of the device if probe input disconnected (150-250mVpp) at 100X scope attenuation setting, too bad (a shame?) non engineering achievement from rev.F (blue trace)


5) last but not least, i used to enjoy looking at differential signal between my FG output (sine) and its synch output (square). at 3-6Vpp, the output signal looks cleaner, acceptable and nice at the 100X setting, so i truely believe there is a hope since there is a "visible" proof . yellow = fg output, blue = diff.probe output (sine-square)




to the 5th post... the conclusion...

[Mechatrommer]

so what do you think guys? is this workable to you? (it is for me ) is it really worth it? what other explanations should i include here? constructive comments and better suggestions are very welcomed. since i made this open, i'm hoping and look forward to working together as community so we can build a better future and "save the world" . and more importantly... "redefining industry" at hobby level, i hope thats not so offensive a statement for some people otherwise... there is not much point of making this open isnt it? until then.

[T4P]

so what do you think guys? is this workable to you? (it is for me ) is it really worth it? what other explanations should i include here? constructive comments and better suggestions are very welcomed. since i made this open, i'm hoping and look forward to working together as community so we can build a better future and "save the world" . and more importantly... "redefining industry" at hobby level, i hope thats not so offensive a statement for some people otherwise... there is not much point of making this open isnt it? until then.

It's really useful, mate. Keep on going, i'll give you the morale boost 

[NiHaoMike]

You can probably get a large improvement in CMRR by powering it from batteries and using RF modulated isolation.

[NiHaoMike]

using RF modulated isolation

whats that animal? example please? thanks for advice. i did try with batteries earlier, i cant see any substantial difference.

You take the signal from the amplifier, modulate a RF carrier with it, send the signal across a RF transformer, then demodulate the RF.

[Mechatrommer]

i think i'll end this masaccre at ver 1H (rev2) for now. i've ammended input front end, separated attenuator decision is cancelled, back to built-in. so here's the latest ver.1H (rev2), the rest are the same...


pcb layout, top with silk...


pcb layout, top without silk...


pcb layout, bottom (mirrored for easier tracing with top layer)...


pcb layout, top 3d...


pcb layout, bottom 3d (mirrored for easier tracing with top layer)...


pcb layout, top 3d explained...


the pcb size (and mounting holes) is fixed based on project enclosure that i have. i hope i'll hear some CnC before making this into reality (esp on high freq components placement and signal path/coupling)...

edit: via stitches not shown yet, but solved. no problem there

[T4P]

That's really very nice. Good job mecha!

[Fox]

Very nice project.

To improve  the cmrr you should use a single Instrumentation opamp not three opamp in separate packages, search for Instrumentation amps or ADC Driver opamps.
The reason for not so good cmrr with the three opamp version is, that the cmrr is strongly dependent from the ratio of your resistors (precision and temperature wise).
For the best cmrr, the input dividers should be as stable as you can get them (thick film hybrid resistors for example).

Here al link to the "Op Amp Applications Handbook" from Analog Devices, i highly recommend it and it's free.
http://www.analog.com/library/analogDialogue/archives/39-05/op_amp_applications_handbook.html


The Gainbandwithproduct of your Opamp should be much higher than 200MHz ( 100MHZ signal Bandwith, Gain of 2) i suggest at least 350MHz.
For your input divider something like this should be suitable.
http://giciman.com/notes/7.shtml

[Mechatrommer]

thanks for the advice fox. i think i have many (including the one you linked) "generic" opamp application app-note already.

To improve  the cmrr you should use a single Instrumentation opamp not three opamp in separate packages, search for Instrumentation amps

i thought i have been an "accomplished" at this (bolded quote), the problem is, premade ins-amp will go max to few MHz (1-2MHz iirc), but please enlighten me! as my net is very slow i cannot search the whole world very quickly.

or ADC Driver opamps

talk about overkill app here? afaik, "digital" is not needed in this simple application. pls prove me wrong

that the cmrr is strongly dependent from the ratio of your resistors (precision and temperature wise).
For the best cmrr, the input dividers should be as stable as you can get them (thick film hybrid resistors for example).

200% agree. as for now, i just use cheapo resistors and trimmers to match (acceptable result! considering those crappies , but alas drifty maybe due to high tempco). if i got the circuit right, its just a matter of replacing with high quality components.

The Gainbandwithproduct of your Opamp should be much higher than 200MHz ( 100MHZ signal Bandwith, Gain of 2) i suggest at least 350MHz.

for voltage feedback opamp, i agree with you. i have current feedback opamp here (THS3092) that will be happy doing 100MHz at gain of 10 (no vfb GBW/gain law here). and/but i'm thinking to replace it with LM6172 vfb (GBW 100MHz) in my stock, at gain of 2, i think its good for 20-30MHz, i think i'll lower the spec from 100 to 20MHz, since mystical things like reflection is enough to make my head scratching (with my limited tools). i'll finalized the circuit/pcb with 20-30MHz spec first, and in case i need to increase the BW, it just a matter of replacing the opamp with higher one or cfb/cfa.

for the 3 discreete opamps ins-amp setup:
-the 2 input buffer (with gain) opamps... utmost importance is lowest noise, high imput impedance and thats it!
-for the difference (3rd) opamp... utmost importance is, highest CMRR and thats it! (so even my best resistors matching, it will not be limited by lower CMRR opamp)

of course all of them must be capable of the said BW and slewrate (2pi.BW.V) 7-10Volt and supply from the psu, which as for now, it ±12V, but i will be glad to move down to ±5V if there's better combo (which i'm not yet figure out) for the above mentioned opamps requirement.

For your input divider something like this should be suitable.
http://giciman.com/notes/7.shtml

good read! thanks!

[SeanB]

For some nice old design notes try this old Tek plugin...

http://w140.com/tekwiki/wiki/5A19

I like it, nice and easy to use, in the scope of course........ That nice good old Tek build and design.

[Mechatrommer]

thanks sean! good reference, frontend divider and compensation, and but err... discrete transistors design ... thanks.

[SeanB]

This was designed in the days when Bob Peace was still young, and was busy working the kinks out of the 741 and it's relatives. The scope design is old, dating from the early 1960's.

[Mechatrommer]

yes old but i guess resistor never die

[alm]

I'm not aware of any published schematics for recent diff probe designs. The most recent I'm aware of is in the Tek P6046 service manual (designed in late 60s, produced until late 90s, something like 10k:1 CMRR at 1MHz), but this is also a discrete transistor design. Nobody in their right mind would design a circuit like this with discrete transistors these days, controlling thermal offset drift is much easier when they are on the same die. Most of the relatively low bandwidth (<= 200 MHz or so) HV diff probes use video line drivers as amplifier.

CMRR at 1kV might also be a problem. Most resistors have a voltage coefficient. Even if this effect is less than 1% at 1kV of common mode voltage, it will reduce the CMRR to a little over 40 dB. Same applies to mismatches and changes in the compensation capacitors.

[Mechatrommer]

10k:1 CMRR

yup that is... pretty much impossible for me (to do or to measure).

CMRR at 1kV might also be a problem. Most resistors have a voltage coefficient. Even if this effect is less than 1% at 1kV of common mode voltage, it will reduce the CMRR to a little over 40 dB. Same applies to mismatches and changes in the compensation capacitors.

i believe the keyword for that is "component quality", or maybe some fancy technique, like tempco cancellation dave talked about (for capacitor) i still having difficulty finding sources for -ve and +ve tempco resistor and its characteristic stuffs. so i decided i just work it out with jellybean components. thats one of the limitation here

[hlavac]

I'm a bit worried about the safety of this. Track clearances on the high voltage inputs - I would expect a minimum of 5mm required clearance to be safe, probably more (there are standards for that).

Also common resistors have a breakdown voltage across them of around 250V, you would need to put multiple of them in series or use some special high voltage ones...

[Mechatrommer]

yes i'm aware of that. and i dont have access to the "exact" standard sigh (i aint engineuer). i did experiment with multiple series input resistor (the through hole one), compensation and things, my earlier pcb got more clearance, and later got rid of that phobialism (lesser clearance and only one "power" resistor/divider). thats why i tried to limit it to ±500V (1KV differential), it means at any instant, each probe will sense no more than 500V magnitude, i'm not going to venture in 1KV "magnitude" risking dialectric breakdown. and "THAT" 500V magnitude is considered "MAXED", ie things like short spike and transient. it doent mean you have to probe steady 500V. from my wattage calculation, probing steady at 500V is using 100% of the resistor wattage capacity (0.25W), ie it will get hot and thats not good. probing at (hobby level) 110-240Vrms will use less or equal to quarter of the capacity iirc (experiment showed no heat build up so far, not sure about resistance value and drift). yes, it will come down to component quality (beside the arrangement and standard).

anyway, please prove me wrong and i'll be happy if i can get the link to the "bylaw standard". and i agree, this thing need to be highlighted and we should aware of, thanks for highlighting. if i have the standard, i'll be happy to ammend my pcb (if i got the room) but since the intention is at hobby level (no serious high voltage high power stuff), so yeah as sad as it is, standard is pretty much at the bottom of my list right now. even the sma input connectors i used will not up to that 5mm clearance compliance (if it is). its not that i want to ignore it, but the thing is i would like to make it simpler (irreducibly complex) and concentrate on signal path (at greater than 10-20MHz, things got nasty without coax/sma connection) so the word dilemma or compromise is the term.

[T4P]

Ain't SMA pretty small compared to coax ?

[Mechatrommer]

Ain't SMA pretty small compared to coax ?

SMA is a "connector" / "adapter", coax is a "cable". there's big coax small coax, each with its own characteristic impedance 50ohm 75ohm and "what not". connector have SMA, BNC, N and "what have you". you dont put big coax into small connector vice versa. be carefull when buying SMA, there's SMA to RG718, there's SMA to RG58, different! same with BNC. go search wiki, there's many type than i (even wiki) can list.

[T4P]

Ain't SMA pretty small compared to coax ?

SMA is a "connector" / "adapter", coax is a "cable". there's big coax small coax, each with its own characteristic impedance 50ohm 75ohm and "what not". connector have SMA, BNC, N and "what have you". you dont put big coax into small connector vice versa. be carefull when buying SMA, there's SMA to RG718, there's SMA to RG58, different! same with BNC. go search wiki, there's many type than i (even wiki) can list.

Oops i mean BNC, sorry about that

[hlavac]

According to this, for 500V you need absolute minimum of 2.5mm, preferably 3mm of clearance.

[Mechatrommer]

According to this, for 500V you need absolute minimum of 2.5mm, preferably 3mm of clearance.

great! thanks alot pal! will make it as reference.

[Lightages]

I like this project Mechatrommer. I think most people would like a differential probe for many things if they weren't so damned expensive. As you see, one of the reasons they might be so expensive is the many considerations in design and safety for a commercial product. If you do continue to develop this, I would be happy to buy a kit or circuit boards from you so you can profit from your work.  Please continue....

[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.

[tivoi]

im try HV diff probe with ltc6269 and ths3201

[flyglas]

Hi tivoi,

do you have some more informations about your design?
Bandwith, CMRR, etc.

Cheers,
flyglas