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On Monday, May 21, 2018, 3:43:08 PM EDT, Fabio Trevisan <fabio.tr3visan@...> wrote:

Hello Nielsen
See my comments right after your points (upon which Harvey already answered... this is just my 2 cents).

On Sun, May 20, 2018 at 10:40 am, nielsentelecom@... wrote:

Harvey, thanks for your example. I do somewhat the same with a voltmeter in
respect to measuring voltage drop/gain rather than doing the math, I can get
higher resolution too since the decimal moves over.

Your mention of using a voltmeter, to measure voltages between 2 points captures the essence of taking a differential measurement (because in a "sorf-of" way, this is what your voltmeter is doing, measuring the voltage of the red lead "in relation" to the voltage in the black lead).
However, that usage you mentioned of a voltmeter is only possible because you're talking about a portable voltmeter that's isolated from the mains, isolated from earths ground and also isolated of your D.U.T.
in other words, the voltmeter is floating, and then it fits exactly in the same bill as using a floated scope (i.e.: a battery operated one).
Should you be using a bench voltmeter, which is powered from the mains and therefore grounded and you would need to make that same measurement between two nodes of the circuit where neither of them is ground on the D.U.T., you would face exactly the same problem as when trying to make that measurement with the scope.

Your high voltage idea in respect to exceeding limits; would it be possible to
use the 2 channels to measure a floating high voltage that exceeds the single
channel to scope chassis limit of say, 500V peak? Given a 1M channel
impedance,? do I end up with a 2M impedance? Could this be a method to cut
scope loading on the DUT in half?
And yes, this is an academic question, the examples help me to understand the
testing possibilities with my scope.
NielsenTelecom

The high voltage idea... I suppose you're talking about this part of Harvey's answer:

Harvey wrote...
Some people (and you have to be very careful with this) might put the UUT (Unit Under Test) to float,
connect the ground at say, the 500 volt supply, and then the scope inputs won't see 500 volts, but the moderate difference.
Unfortunately, this puts the REST of the UUT at 500 volts with respect to the scope frame, which is NOT a good idea.

Well, in that answer, he's only referring to an alternative to floating the oscilloscope, which is to float the D.U.T.
Since the signals you're measuring are hypothetically riding on top of a voltage (e.g. 500V) that would be forbidden to the scope, if you float the device and connect the scope's ground lead to that hypothetical 500V of the D.U.T. you would be bringing that 500V to the oscilloscope ground lead's potential (0V), turning any voltage on the D.U.T. that are around that ballpark of 500V closer to ground and measurable by the scope.
However, since the scope's ground lead is at 0V, and now you turned the DUT's 500V voltage into 0V, all voltages on the D.U.T that would be near 0V, will be now in the -500V ballpark (read... possibly ever y exposed metal part of the D.U.T. is now probably at -500V).

This "strategy is as much hazardous as "floating" a scope that's not meant to be operated floated (i.e. any non-battery operated scope, including your Tek or your Hitachi).

This strategy also, doesn't have any thing to do with the impedances seen by the D.U.T. as you suggested.

Indeed, what you suggest makes sense, but the rational is flawed because one normally cannot disregard the ground leads of each channel input.
The scope's inputs' impedance from the tip of the channel 1 probe to the tip of the channel 2 probe would be the sum of each channel's impedance (i.e. 2MOhm) if the ground leads would remain disconnected (from the U.U.T.s), and also disconnected from ground (which would take - again - the scope to be floating because, if it isn't floating, both channels' ground leads are grounded, via the scope's grounding wire to your facility ground).

So, academically...
1. if you use a floated oscilloscope (for safety's sake, a battery operated one)
2. If you don't connect EITHER of the inputs' ground leads to the D.U.T., but only connect both ground leads one to each other (I`m assuming here that, a battery operated scope can also be one where the two inputs' ground leads are isolated from each other, such as some Tek scopes meant for this kind of floated operation).
Then, academically,
1. The resultant input impedance would be 2MOhms
2. Possibly (depending on how the oscilloscope is wired internally, as this is an academic reasoning), it **could** withstand twice the maximum input voltage from the tip of CH1's probe to the tip of CH2's probe.

Rgrds,
Fabio