Single ended inputs VS Differential oscilloscope inputs

[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]

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.

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]

Exactly!A differential input is actually two inputs,both referenced to a common point(usually earth).

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

[Kiriakos-GR]

Ok, it looks that my first question was a bit easy ... 

Lets raise the challenge to Level 2   


Lets say that we have  :
Point (a) is at +7.0volts, & point (b) is at -3.0volts DC

And now lets say , that I am using my smart DMM with Peak to Peak detection.
And I get this +7.0volts & -3.0volts , at the Peak ( Min / Max ).

And now the question :

Can we describe this specific instrument ,  as capable to measure Differential input voltages  ?



 

[tnt]

Actually a "differential probe" for oscilloscope can provides _exactly_ that AFAIK.

I say 'can' because some types of probe are high performance ones (like multi GHz bandwidth, see Tektronix P7250)
Some other have much more limited bandwidth but do 'floating' measurements like a multimeter. You can measure difference between any two points even referenced against anything. (There is obviously a limit just because of dielectric limits but it can be 1 kV for some probes). See Agilent N2791A for example.

[Cj1corbystarlet]

In relation to Question 2,

Check the technical documentation for the DMM and see if it mentions a internal reference point in the DMM for D.C measurements.

[vk6zgo]

I really should revise this stuff before I rush into print.

After I finished  the last posting,I got to thinking about Differential inputs, as we used them on VDAs to reject hum.
That lead to  thinking about audio line amps. The old ones had transformer inputs,but the new ones have--you guessed it !
Differential inputs !

I Googled, & found this:http://www.tek.com/Measurement/App_Notes/DiffOscPrimer/51W_10540_1.pdf-

I think I was thinking about Quasi-Differential input using a dual input plugin like the 7A18.

So ignore my previous posting,the Differential input can look at two voltages,unrelated to their earth reference.

A DMM is floating so it doesn't need differential input.

VK6ZGO

[Kiriakos-GR]

Lets raise the challenge to Level 3   

( A DMM is floating so it doesn't need differential input )


Can I use an DMM instead of one Oscilloscope with Differential inputs,
and get the same voltages ?

If yes , what this DMM must have on it , so to be capable for the task ?  

[Mechatrommer]

Can I use an DMM instead of one Oscilloscope with Differential inputs,
and get the same voltages ?

no! we have discussed this many times. dmm reads average. dso reads by sampling.

[Kiriakos-GR]

Lets raise the challenge to Level 4

Is the Peak function found on the expensive multimeters ,
an operation that uses sampling ?

Yes / No .

Give your Vote .

[Mechatrommer]

i dont know, i never own one but i believe, the answer is no.

[tekfan]

Lets raise the challenge to Level 4

Is the Peak function found on the expensive multimeters ,
an operation that uses sampling ?

Yes / No .

Give your Vote .

Definetly yes. In dedicated AC voltmeters you can select either average response, True RMS, calibrated RMS and peak (either positive or negative), transient hold. Sometimes you even have peak to peak option.
It doesn't use sampling techniques but it rather integrates the input voltage across a capacitor. This way it can store a fast transient. Sound and vibration measuring amplifiers are really just very advanced AC voltmeters.

[qno]

I have peak detect multimeters that use a positive and negative peak detector made with an opamp.
With a sample rate of 5 conversions/second the peak detect function wouldn't be very efficient.

Newer multimeters may have a sample rate of thousands of samples/second.
Here the peak detect may be done with software.

[Mechatrommer]

Is the Peak function found on the expensive multimeters ,
an operation that uses sampling ?

Definetly yes... It doesn't use sampling techniques but it rather integrates...

then the answer is no.

[Kiriakos-GR]

Lets raise the challenge to Level 5

We have AC this time , and actually an three phases system ( R S T ) . 
phase to phase 380V

We use an oscilloscope with Differential inputs,
and we try to measure the voltage of R+S

What we get as measurement ?
a)  380V

b) the difference in voltage, between the two phases ( In case that there is one)
Example:  phase R to ground = 220  / Phase S to ground = 219   The difference could be just 1Volt.

c) something other = explain your answer.       

[jahonen]

Lets raise the challenge to Level 5

We have AC this time , and actually an three phases system ( R S T ) . 
phase to phase 380V

We use an oscilloscope with Differential inputs,
and we try to measure the voltage of R+S

What we get as measurement ?
a)  380V

b) the difference in voltage, between the two phases ( In case that there is one)
Example:  phase R to ground = 220  / Phase S to ground = 219   The difference could be just 1Volt.

c) something other = explain your answer.       

My answer is a, 380 volts if we can agree that it means RMS volts. Or if reading peak-to-peak (sinusoidal RMS times sqrt(2)*2) using a scope, then it is 1075 volts.

Regards,
Janne

[vk6zgo]

Ok, it looks that my first question was a bit easy ... 

Lets raise the challenge to Level 2   


Lets say that we have  :
Point (a) is at +7.0volts, & point (b) is at -3.0volts DC

And now lets say , that I am using my smart DMM with Peak to Peak detection.
And I get this +7.0volts & -3.0volts , at the Peak ( Min / Max ).

And now the question :

Can we describe this specific instrument ,  as capable to measure Differential input voltages  ?

 

Well,as I was wrong in my understanding of the operation of a true Differential input device,the Oscilloscope would not show separate
+7volt & -3volt deflections,but it would only show a single 10 volt deflection.
I think the DMM would just show 10 volts.I doubt the Peak (Min/Max) would do anything because it is a DC voltage.
As I said in an earlier posting ,the DMM doesn't need Differential inputs,because it is floating.

VK6ZGO

[vk6zgo]

Lets raise the challenge to Level 3   

( A DMM is floating so it doesn't need differential input )


Can I use an DMM instead of one Oscilloscope with Differential inputs,
and get the same voltages ?

If yes , what this DMM must have on it , so to be capable for the task ?  

Yes,for the DC set up as above.

A qualified "No" for AC ( The DMM is seeing the same voltages,but is calibrated in RMS)----You can obviously calculate RMS from the P-P sinewave display on the Oscilloscope,& vice versa.
 Does your DMM display peak of sine waves?
VK6ZGO

[vk6zgo]

Lets raise the challenge to Level 5

We have AC this time , and actually an three phases system ( R S T ) . 
phase to phase 380V

We use an oscilloscope with Differential inputs,
and we try to measure the voltage of R+S

What we get as measurement ?
a)  380V

b) the difference in voltage, between the two phases ( In case that there is one)
Example:  phase R to ground = 220  / Phase S to ground = 219   The difference could be just 1Volt.

c) something other = explain your answer.       

(c) As 380 volts phase to phase is an RMS value, the Oscilloscope will show  2(sq root 2 xRMS) or as Janne says:1075 volts p-p


VK6ZGO

[Kiriakos-GR]

Does your DMM display peak of sine waves?

Yes .

Even so I just did an phase to phase measurement.
I got 390V  
And when I use the P-P  i get  -569 / 569 AC

When I do that at 220 , I get P-P -310/ 310 AC




.

[jahonen]

Does your DMM display peak of sine waves?

Yes .

Even so I just did an phase to phase measurement.
I got 390V  
And when I use the P-P  i get  -569 / 569 AC

When I do that at 220 , I get P-P -310/ 310 AC




.

So your DMM shows positive and negative peaks, to get the P-P voltage mentioned, calculate it as 569-(-569) V =1138 V.

Regards,
Janne

[Kiriakos-GR]

So your DMM shows positive and negative peaks

Regards,
Janne

Yes the Fluke 28II hides some scientific power in it. 

Funny enough the orange Agilent , hides even more.
It can measure and the length of the waves in microseconds.