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Single ended inputs VS Differential oscilloscope inputs

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Kiriakos-GR:
I had just read an article about Single ended inputs & Differential oscilloscope inputs.

And one new silly question came up ..

Can we describe the DMM as an device with Differential inputs, just because its not connected with earth ground ?

Mechatrommer:
yes

Leo Bodnar:
No, because "differential" assumes presence of reference level elsewhere.

Differential inputs marked as e.g. IN+ and IN- means they are both referred to ground level.
They also tend to be symmetrical in impedance, etc.

Multimeter is at best floating single ended device.

vk6zgo:

--- Quote from: Leo Bodnar on May 22, 2011, 01:46:24 pm ---No, because "differential" assumes presence of reference level elsewhere.

Differential inputs marked as e.g. IN+ and IN- means they are both referred to ground level.
They also tend to be symmetrical in impedance, etc.

Multimeter is at best floating single ended device.

--- End quote ---

Exactly!A differential input is actually two inputs,both referenced to a common point(usually earth).
Either of the 2 inputs can individually display the voltage of a test point with respect to the common ,whereas a DMM if connected between two test points can only show the difference between them.

Sometimes it's not that important,but a DMM across two test points will give the same reading  of 10.0 volts when:-
 
(1) Point(a) is at+5.0 volts, & point(b) is at -5.0volts

or

(2)Point (a) is at +7.0volts, & point (b) is at -3.0volts

Obviously,if you are trying to adjust (a) & (b) for equal & opposite voltages this won't do it!

With an Oscilloscope with Differential inputs,you can connect both inputs to common,adjust for a line mid screen height(Centre zero).
Adjust the voltages for equal number of divisions either side of the Centre zero line.
This will give equal but opposite polarity voltages with respect to common.(Useful for some Op amps)
If both voltages are the same polarity,you need to switch one channel to invert.

You could do the same adjustment by measuring  in turn between each point & common,swapping after each adjustment.or use two meters,but it is not as quick accurate,or convenient.

These things are very easy to demonstrate with an Oscillocope in front of me,but a bit hard to describe.

VK6ZGO

Mechatrommer:

--- Quote from: vk6zgo on May 23, 2011, 05:58:14 am ---Exactly!A differential input is actually two inputs,both referenced to a common point(usually earth).

--- End quote ---
well, if the DUT is some KV floated, it could zap the differential amplifier inside, which is not the point of differential input.

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