Author Topic: Nead an insulated probe for oscilloscope  (Read 5030 times)

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Offline FriedMuleTopic starter

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Nead an insulated probe for oscilloscope
« on: June 05, 2019, 12:39:12 am »
Hi I am just looking for a "floating" probe where the ground lead is not grounded trough my 350MHz oscilloscope.
I do not need high voltage measurements, but only the possibility to measure between two points that are not at ground voltage.
Hope someone please can help.
« Last Edit: June 05, 2019, 12:41:36 am by FriedMule »
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Offline ArthurDent

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Re: Nead an insulated probe for oscilloscope
« Reply #1 on: June 05, 2019, 12:49:34 am »
Do a search in the forums. This topic is discussed about once a week.
 

Offline rstofer

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Re: Nead an insulated probe for oscilloscope
« Reply #2 on: June 05, 2019, 01:40:19 am »
Hint:  search for 'differential probe'
 

Offline Brumby

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Re: Nead an insulated probe for oscilloscope
« Reply #3 on: June 05, 2019, 01:45:18 am »
Hint:  search for 'differential probe'

This    ^ ^ ^
 

Offline Mr.B

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Re: Nead an insulated probe for oscilloscope
« Reply #4 on: June 05, 2019, 01:48:16 am »
Buy one of Daves.
Thats what I did... Does not have to be high voltage to use as a differential probe.
https://www.eevblog.com/product/hvp70/
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Offline FriedMuleTopic starter

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Re: Nead an insulated probe for oscilloscope
« Reply #5 on: June 05, 2019, 10:27:25 am »
A differential probe looks like they costs as much as a beginner scope and often have 50X or even 500X attenuation.
I think what ia am looking for may be some DIY I maybe can connect my passive probes to?

I have tried to search but I can't seem to find something, I am looking for, is not to technical to understand or not for higher power / attenuation.
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Online tautech

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Re: Nead an insulated probe for oscilloscope
« Reply #6 on: June 05, 2019, 10:47:48 am »
A differential probe looks like they costs as much as a beginner scope and often have 50X or even 500X attenuation.
Yep and that's just the cheap ones !

Quote
I think what ia am looking for may be some DIY I maybe can connect my passive probes to?
Nope, don't do that if you value your safety or scope.

Quote
I have tried to search but I can't seem to find something, I am looking for, is not to technical to understand or not for higher power / attenuation.
Look for the DP-25, a Micsig one or get Daves one.

If you think just buying a scope was the end of cost, think again !
Really want a fright then look for a price on a 100 MHz current probe.  :P

Then venture into RF and have another fit at the cost of getting set up.  :scared:

Buy wise, once and have it for life.  :)
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Online mikerj

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Re: Nead an insulated probe for oscilloscope
« Reply #7 on: June 05, 2019, 11:14:20 am »
Commercial differential probes are expensive, but even DIY versions will not be particularly cheap.  Good DC precision, good common mode performance and high bandwith require careful design and trimming.

I recently needed a x1 differential probe for low-ish voltage use and an EEVBlog member pointed me to the Pintek model in the link below which is quite reasonably priced compared to many, though still not cheap compared to the cost of a entry level DSO.

https://www.globalmediapro.com/dp/A25XX7/Pintek-DP-30HS-Differential-Probe-30MHz-65V/
 

Offline FriedMuleTopic starter

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Re: Nead an insulated probe for oscilloscope
« Reply #8 on: June 05, 2019, 11:48:01 am »
Thanks a lot!
Am I understanding right if I am saying that I can either build a crappy probe that will be of no use anyway, or dig deep in my pocket once for all and get a decent one?

But what about that 500X attenuation, would that not eliminate the 2uV noise I may look for in an circuit?
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Online AndersJ

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Re: Nead an insulated probe for oscilloscope
« Reply #9 on: June 05, 2019, 01:01:18 pm »
You will most likely never see 2 uV signals on a oscilloscope,
regardless of which probe you use.
"It should work"
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Offline macboy

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Re: Nead an insulated probe for oscilloscope
« Reply #10 on: June 05, 2019, 01:44:20 pm »
You mean "Isolated" not "Insulated". A differential probe almost the same as an isolated probe, but not really. Each input is still referenced to ground, and has a specified impedance to ground (as well as to the other input), and is limited to be some voltage +/- w.r.t ground. An internal differential amplifier rejects the common mode signal, providing the difference at the output, so the reference point need not be ground. High voltage differential probes commonly handle several hundred volts or more. Low voltage, high bandwidth differential probes may be specified for signals of say +/- 4 V,. and can be damaged by as little as 7 volts (to ground) on either input. These would be used to directly view LVDS and similar high speed digital signals, or may be used with an optional input attenuator to extend the useful range.

You said you have a 350 MHz oscilloscope but said nothing about the bandwidth of the signals you want to probe. For low bandwidth, you should be able to easily build up a simple instrumentation amplifier type of circuit. The specifications (bandwidth, common mode rejection, gain, DC accuracy, slew rate, etc) will depend on the type of op-amps and passives that you use and the physical construction. For low voltage and audio bandwidth (or even a few MHz), the job is easy. If you want 10's or 100's of MHz, you are up for a challenge. High voltage (beyond op amp rails) is another can of worms which adds not only difficulty in the design, but demands a firm grasp of the safety issues and of the consequences of getting it wrong.

 

Offline SteveyG

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Re: Nead an insulated probe for oscilloscope
« Reply #11 on: June 05, 2019, 02:02:25 pm »
Take a look at a couple of my videos on the Micsig diff probes. They're good value
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Offline Nerull

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Re: Nead an insulated probe for oscilloscope
« Reply #12 on: June 05, 2019, 03:50:44 pm »
A poor mans diff probe is to use two channels simultaneously and a math function. If you're worried about accuracy this won't be nearly as good as a diff probe, though.
 
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Offline bob91343

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Re: Nead an insulated probe for oscilloscope
« Reply #13 on: June 05, 2019, 04:20:27 pm »
It's also possible to use a dual trace oscilloscope in differential mode with two channels, invert one and add.  This won't have such great bandwidth or common mode rejection but can be useful.

Just don't try using an isolation transformer.  You could experience some jolts while touching the chassis.
 

Offline FriedMuleTopic starter

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Re: Nead an insulated probe for oscilloscope
« Reply #14 on: June 05, 2019, 05:28:25 pm »
Again thanks, I can see that my lack of knowledge about the English and electronic is a bit of a problem here so I'll try to explain what I want.
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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Offline JxR

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Re: Nead an insulated probe for oscilloscope
« Reply #15 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.
« Last Edit: June 05, 2019, 05:59:08 pm by JxR »
 

Offline FriedMuleTopic starter

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Re: Nead an insulated probe for oscilloscope
« Reply #16 on: June 05, 2019, 06:44:07 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.
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. :-)
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Offline bdunham7

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Re: Nead an insulated probe for oscilloscope
« Reply #17 on: June 05, 2019, 07:23:32 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.

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.

A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline Mr.B

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Re: Nead an insulated probe for oscilloscope
« Reply #18 on: June 05, 2019, 07:51:59 pm »
But what about that 500X attenuation, ...

Daves one is x10 and x100.
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Offline capt bullshot

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Re: Nead an insulated probe for oscilloscope
« Reply #19 on: June 05, 2019, 09:36:15 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.

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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Offline Electro Detective

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Re: Nead an insulated probe for oscilloscope
« Reply #20 on: June 05, 2019, 11:33:37 pm »

A poor mans diff probe is to use two channels simultaneously and a math function.

If you're worried about accuracy this won't be nearly as good as a diff probe, though.


No contest on that  :-+

BUT.. with a bit of fiddling about getting both channel gains to match  :horse: and 'square as square can be' 10x probes calibration
one can get a lot of poor persons non critical tasks done in dual channel diff/invert/ADD mode,
with no wallet biting probe croc clips fitted or BANG potential earth/ground BNC/scope chassis connections necessary.
 
I've had good enough results doing this with 15 to 200mhz scopes, and on mains*** with extreme caution  :scared:

OP may get some serious bandwidth doing this with a 350mhz killa scope  8)

Best to practice this stuff*** on very low AC and or DC voltages FIRST before every session,
and take physical notes of the setup for repeatability and or future use,
before diving in and over-driving the probes and or channels into oblivion with higher voltages,

..whilst the scope screen displays no signal, due to misadjusted or bumped controls  =  :-//  :-BROKE  :-[  = $ $ $ $  :palm:


EDIT:

It would be handy to have some sort of knockup external box or easy internal mod (switchable) that can invert the signal on channel 4 on a four channel DSO,

so an impoverished user  :'(  can have two poor persons differential channels using four calibrated x10 probes

or is this pushing imagination and hope beyond DIY (+cheap n easy) feasabilty?   :-// :-[

..assuming this comment made any sense  ;D
« Last Edit: June 05, 2019, 11:48:00 pm by Electro Detective »
 

Offline FriedMuleTopic starter

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Re: Nead an insulated probe for oscilloscope
« Reply #21 on: June 06, 2019, 12:30:33 am »
great replyes! :-)

Lets say I am using probe 1 to measure input ripple before the full bridge rectifier and the smoothing of the DC 5V input and put probe 2 in the other end, to measure how much ripple there are left. If I do understand it correctly, this will give some strange result, due to the common ground connector?
How would you do that?
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Online tautech

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Re: Nead an insulated probe for oscilloscope
« Reply #22 on: June 06, 2019, 12:54:14 am »
great replyes! :-)

Lets say I am using probe 1 to measure input ripple before the full bridge rectifier and the smoothing of the DC 5V input and put probe 2 in the other end, to measure how much ripple there are left. If I do understand it correctly, this will give some strange result, due to the common ground connector?
How would you do that?
Why ?
What could you possibly learn from looking at the LV AC ripple from the transformer. We all know it will be at mains frequency.
What matters is the ripple at the PSU output under various states of load and for which you'd just connect across the output terminals (after checking 0V doesn't create ground loop issues with a DMM) and use a 1x probe for the additional sensitivity it offers.
For a low impedance source like a PSU rail the capacitive loading of a 1x probe is never an issue.
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Offline Electro Detective

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Re: Nead an insulated probe for oscilloscope
« Reply #23 on: June 06, 2019, 12:57:09 am »

OP may wanna look at both while he tweaks   :-/O   :-//
 

Online David Hess

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Re: Nead an insulated probe for oscilloscope
« Reply #24 on: June 06, 2019, 03:41:36 am »
Am I understanding right if I am saying that I can either build a crappy probe that will be of no use anyway, or dig deep in my pocket once for all and get a decent one?

That pretty much sums it up.  The cheapest way if you have the time is to build a specialized probe into the circuit at the measurement point.

The old way was to use an isolation amplifier between the probe and oscilloscope:

http://w140.com/tekwiki/wiki/A6902

Quote
But what about that 500X attenuation, would that not eliminate the 2uV noise I may look for in an circuit?

A high voltage differential probe will not be useful in that case because of its input attenuation but there may be other ways if that is your requirement.  An isolated probe will not measure 2 microvolts noise either.

An active differential probe might be able to resolve 2 microvolts under the right circumstances.

You will most likely never see 2 uV signals on a oscilloscope, regardless of which probe you use.

A Tektronix 7A22 differential amplifier can resolve 2 microvolts over a range of +/-1 volt but only at a bandwidth significantly lower than its rated 1 MHz.  Doing better than this even 40 years later would be a significant challenge but it could be done.
« Last Edit: June 06, 2019, 03:44:34 am by David Hess »
 


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