Electronics > Beginners
Nead an insulated probe for oscilloscope
JxR:
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.
FriedMule:
--- Quote from: JxR on June 05, 2019, 05:54:01 pm ---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.
--- End quote ---
Yes 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. :-)
bdunham7:
--- Quote from: FriedMule on June 05, 2019, 05:28:25 pm ---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.
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This 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.
Mr.B:
--- Quote from: FriedMule on June 05, 2019, 11:48:01 am ---But what about that 500X attenuation, ...
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Daves one is x10 and x100.
capt bullshot:
--- Quote from: FriedMule on June 05, 2019, 05:28:25 pm ---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 100MHz
I 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.
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There'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.
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