Electronics > Beginners
Differential measurements using isolated oscilloscope
Aners:
--- Quote from: David Hess on December 12, 2019, 07:07:32 pm ---There are two major considerations:
1. Differential probes have high input impedance on both connections but with common mode rejection limited both inherently and because of mismatched source impedances.
2. Isolated probes have low input impedance on the common side, chiefly because of common mode capacitance to ground, but essentially unlimited common mode rejection.
The result is that at higher frequencies where the common mode rejection of the differential probe falls, an isolated probe has an advantage in accuracy. But an isolated probe can only be used where one of the connection points has a low impedance capable of driving its common mode capacitance to ground which can be 100 picofarads or more.
Note that common high voltage differential probes can have horrendous common mode rejection after a time because of a poor choice of substrate for the high impedance dividers and there are no provisions for the user to adjust it. Isolated probes do not have this issue so might be considered more reliable.
--- End quote ---
I am still a little new to some of this stuff but I use it daily and I feel that I need to better my understanding.
Could you give me an example of a meansurment with two high impedance signals?
David Hess:
--- Quote from: Aners on December 13, 2019, 09:19:45 am ---Could you give me an example of a measurement with two high impedance signals?
--- End quote ---
The signals only have to be high impedance compared to the probe impedance. And since an isolated probe is shunted by 100 picofarads or more to ground on its common side, that is not all that much. Even a low impedance high speed balanced transmission line would qualify in that case.
High side gate drive circuits are the common measurement case however their output is very low impedance so isolated probes work well for them; they are almost the ideal case. But if you are measuring the difference between two nodes inside the driver itself, there could be problems with an isolated probe.
Aners:
--- Quote from: David Hess on December 13, 2019, 12:43:58 pm ---The signals only have to be high impedance compared to the probe impedance. And since an isolated probe is shunted by 100 picofarads or more to ground on its common side, that is not all that much. Even a low impedance high speed balanced transmission line would qualify in that case.
High side gate drive circuits are the common measurement case however their output is very low impedance so isolated probes work well for them; they are almost the ideal case. But if you are measuring the difference between two nodes inside the driver itself, there could be problems with an isolated probe.
--- End quote ---
Ok, when you say it is shunted by 100pF what do you mean then? Because I have been looking at simplified probe schematics for single ended probes like the one below, and I dont see what you mean.
GeorgeOfTheJungle:
--- Quote from: tggzzz on December 12, 2019, 10:35:38 pm ---Sigh.
The OP, who is also new to this forum, would be well advised to read, learn, and inwardly digest this: https://www.eevblog.com/forum/projects/isolation-transformer-for-scope/msg2259465/#msg2259465 Especially the anecdote including "This guy was WELL AWARE of the danger and took a lot of precautions to prevent shock . . . Bottom Line: He died anyway."
--- End quote ---
(Never Ask A Question If You Don’t Know The Answer...)
@tggzzz, have you never ever floated your 'scope? Everybody else: raise your hand if you've never done it.
tggzzz:
--- Quote from: GeorgeOfTheJungle on December 13, 2019, 02:19:11 pm ---
--- Quote from: tggzzz on December 12, 2019, 10:35:38 pm ---Sigh.
The OP, who is also new to this forum, would be well advised to read, learn, and inwardly digest this: https://www.eevblog.com/forum/projects/isolation-transformer-for-scope/msg2259465/#msg2259465 Especially the anecdote including "This guy was WELL AWARE of the danger and took a lot of precautions to prevent shock . . . Bottom Line: He died anyway."
--- End quote ---
(Never Ask A Question If You Don’t Know The Answer...)
@tggzzz, have you never ever floated your 'scope? Everybody else: raise your hand if you've never done it.
--- End quote ---
No, I have never floated a scope. I've used my skill and imagination to find ways not to float a scope.
How many people have you watched die in front of you from avoidable accidents? That changes your perspective.
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