Electronics > Beginners
Nead an insulated probe for oscilloscope
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macboy:
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.

SteveyG:
Take a look at a couple of my videos on the Micsig diff probes. They're good value
Nerull:
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.
bob91343:
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.
FriedMule:
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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