Low cost HV differential probe (PS, new user, hi guys) - Page 1
Forum section: electronics / projects

author: benandrews
author_rank: Newbie
timestamp: July 15, 2014, 09:06:44 pm
content: OK, so I've come to realize I'm going to need a differential probe for SMPS design if I ever want to measure more than one voltage and not melt my scope.You will have to bear with me because I've had no formal training and know just enough to be dangerous. I was hoping I could ask you good people to critique my schematics, and maybe throw some knowledge my way.First things first, a design brief....-3db Bandwidth: 100MHz, or 10MHz at a pinchVoltage range: +/- 1KV from groundSimple to power (i.e. usb 5V. I stole this idea, more on that later)seperate inputs for 10x , 100x etc, therefore no switch to accidentaly hitDC coupled inputsNot sure on acceptable input impedance yetI've looked at some DIY projects that a few other people have put together, so I've got a few ideas on which direction to set off on.http://xellers.wordpress.com/electronics/1ghz-active-differential-probe/1Ghz dual gate Fet input differential by Daniel Kramnik. They didn't achieve the original spec of 1Ghz bandwidth and had problems with PS noise. But this looks like a very cool project and quite an achievement for a weekend hackathon.Maybe the dual input fets could be used withhttp://prosje.be/CO/Schemas/slides/DiffProbe.htmlThis one looks OK, I'm not sure on the bandwidth though. I think impedance could be an issue at higher frequencies.I like the idea of a monolithic instrumentation amp (High impedance, good CMRR) but I've not found anything with decent bandwidth.I've done some ltspice simulations, and it looks like the LTC6246 could work for input buffers, unity gain stable, -3db @ 120MHz. Not sure what noise would be like. If anyone has suggestions for alternatives I'd love to hear your ideas.Power supply wise, I think I'll try a switched cap inverter like the MAX1720, followed by LDO on pos and neg rails. I dont know how close i can put the PSU to the amplification on the board though.I'll post some schematics soon once I've cleaned them up.Any ideas for over voltage protection? best i can think of is somehow clamping the voltage divider with zenners.I'll be printing a case on my Reprap and lining it with foil tape for shielding, any good resources on shielding / enclosure design?I probably sound like i know more than I do, so if anyone does feel kind enough to chip in, please treat me like an idiot.Cheers.P.S.Sorry if this should have gone in the beginners forum

author: poorchava
author_rank: Super Contributor
timestamp: July 15, 2014, 10:44:57 pm
content: Aren't differential HV probes supposed to be galvanically isolated from the scope itself? Will the probe inputs survive several hundred volts of common mode voltage vs scope ground?

author: benandrews
author_rank: Newbie
timestamp: July 15, 2014, 10:57:48 pm
content: I suppose they are, I wouldn't know where to begin completely isolating the inputs whilst being able to measure differential dc.I'm hoping that i can just attenuate two signals referenced to ground, so that neither input buffer gets hit with higher than say 5V.For my purposes, I'd only need to get readings +/- 1kv from ground. Essentially similar to using CH1 - CH2 on the scope with 100x pasive probes, except with a differential amp thus only using one channel.I guess there are very good reasons why comercially available probes are so expensive

author: poorchava
author_rank: Super Contributor
timestamp: July 15, 2014, 11:39:48 pm
content: Another thing is that high-speed differential probes and HV differential probes are 2 different things.When dealing with high speeds, voltage levels are rather low (several volts tops) and common mode voltages are rather low too.HV on the other hand usually means you're dealing with mains at least, so isolation is always a good idea. If your probe had negative input at ground potential and you put that on a live wire... Fireworks. But high voltages don't typically pair up with high frequencies or rise times, so it would probably be feasible to modulate some high speed carrier with data, pass that over isolation barrier (optocoupler, transformer or one of those giantic magnetoresistivity gizmos which are surprisingly obtainable - for a few Euros) and then demodulate into signal. Maybe some sort of voltage-frequency converter?Both high speed and high voltage are extremly rare because in such conditions dv/dt values become really scary. 1ns rise time of 1V signal is already dv/dt=1kV/us and 1ns translates to ~340MHz bandwidth. 1kV/us isn't that bad, there are opamps that can do much more than that. But to get eg 1GHz at 100V...you'd need something that can do almost 300kV/us. I wouldn't wan't to be anywhere near a source that can produce such signals.

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 16, 2014, 11:13:27 am
content: taming down HV to LV HF is no problem we do it all the time, nevermind fancy galvanic isolation transformer. the problem with this particular stuff is matching input1 and input2. having said that, your problem is not in the nice monolithic chip or selecting the lowest CMRR thereof, but on those passives components (matched dividers) and pcb layout (parasitics) that need to be taken care of. and of course some black magic art.

author: mij59
author_rank: Frequent Contributor
timestamp: July 16, 2014, 11:45:23 am
content: Differential probes aren't low cost.The fet probe is not for high voltage measurement, it's a low voltage high frequency probe.You may have a look at the Testec site, or search on the forum for differential probe.http://www.testec.de/251.0.html?L=1

author: robrenz
author_rank: Super Contributor
timestamp: July 16, 2014, 06:03:22 pm
content: This is pretty much ideal(shown in this post)

author: Richard Head
author_rank: Frequent Contributor
timestamp: July 17, 2014, 11:29:39 am
content: I've been designing switchmodes for years and only ever used a short wire measurement technique with the scope probe.

author: David Hess
author_rank: Super Contributor
timestamp: July 17, 2014, 04:26:18 pm
content: I have considered doing something like this with a considerably simplified design based on the Tektronix 7A13. A differential to single ended conversion would need to be added to drive an external oscilloscope.By including AC and DC CMRR adjustments, it could be used with a standard pair of x10 oscilloscope probes which will help keep the input impedance at the probe tips high. Keep in mind that the input loading at the probe tips combined with different source impedances will degrade the CMRR no matter what you do so keeping the input impedance at the probe tips high is important.

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 17, 2014, 04:30:07 pm
content: Quote from: Richard Head on July 17, 2014, 11:29:39 amyes but it will be nice to have 2 cheap diff probes on 2 channel scope right? maybe you can share how to manage things like saturated core on unknown smps transformer, the right freq for the particular load? because in some smps i can see changing freq for different load, whats going on in the smps chip/circuit is a mystery. i'm talking 240Vac to 5/12V smps, not 12V to 40V SMPS or vice versa.