Single ended inputs   VS   Differential oscilloscope inputs - Page 1
Forum section: electronics / beginners

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: May 22, 2011, 10:55:51 am
content: 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 ?

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 22, 2011, 01:34:08 pm
content: yes

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: May 22, 2011, 01:46:24 pm
content: 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.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 23, 2011, 05:58:14 am
content: Quote from: Leo Bodnar on May 22, 2011, 01:46:24 pmExactly!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 -5.0voltsor(2)Point (a) is at +7.0volts, & point (b) is at -3.0voltsObviously,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

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 23, 2011, 07:34:04 am
content: Quote from: vk6zgo on May 23, 2011, 05:58:14 amwell, if the DUT is some KV floated, it could zap the differential amplifier inside, which is not the point of differential input.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: May 23, 2011, 08:44:12 am
content: Ok, it looks that my first question was a bit easy ...Lets raise the challenge toLevel 2Lets say that we have :Point (a) is at +7.0volts, & point (b) is at -3.0voltsDCAnd 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 ?

author: tnt
author_rank: Regular Contributor
timestamp: May 23, 2011, 09:45:52 am
content: 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.

author: Cj1corbystarlet
author_rank: Contributor
timestamp: May 23, 2011, 10:19:56 am
content: 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.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 23, 2011, 12:21:16 pm
content: 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.Soignore my previous posting,the Differential inputcanlook at two voltages,unrelated to their earth reference.A DMM is floating so it doesn't need differential input.VK6ZGO

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: May 23, 2011, 01:02:05 pm
content: Lets raise the challenge toLevel 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 ?

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 23, 2011, 01:15:28 pm
content: Quote from: Kiriakos-GR on May 23, 2011, 01:02:05 pmno! we have discussed this many times. dmm reads average. dso reads by sampling.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: May 23, 2011, 01:44:48 pm
content: Lets raise the challenge toLevel 4Is the Peak function found on the expensive multimeters ,an operation that uses sampling ?Yes / No .Give your Vote .

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 23, 2011, 01:56:16 pm
content: i dont know, i never own onebut i believe, the answer is no.

author: tekfan
author_rank: Frequent Contributor
timestamp: May 23, 2011, 05:36:57 pm
content: Quote from: Kiriakos-GR on May 23, 2011, 01:44:48 pmDefinetly 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.

author: qno
author_rank: Frequent Contributor
timestamp: May 23, 2011, 05:57:12 pm
content: 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.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 23, 2011, 06:59:21 pm
content: Quote from: tekfan on May 23, 2011, 05:36:57 pmthen the answer isno.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: May 24, 2011, 12:00:54 am
content: Lets raise the challenge toLevel 5We have AC this time , and actually an three phases system ( R S T ) .phase to phase 380VWe use an oscilloscope with Differential inputs,and we try to measure the voltage of R+SWhat we get as measurement ?a) 380Vb) 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.

author: jahonen
author_rank: Super Contributor
timestamp: May 24, 2011, 07:42:31 am
content: Quote from: Kiriakos-GR on May 24, 2011, 12:00:54 amMy 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

author: vk6zgo
author_rank: Super Contributor
timestamp: May 25, 2011, 12:42:06 pm
content: Quote from: Kiriakos-GR on May 23, 2011, 08:44:12 amWell,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

author: vk6zgo
author_rank: Super Contributor
timestamp: May 25, 2011, 12:49:53 pm
content: Quote from: Kiriakos-GR on May 23, 2011, 01:02:05 pmYes,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

author: vk6zgo
author_rank: Super Contributor
timestamp: May 25, 2011, 01:08:01 pm
content: Quote from: Kiriakos-GR on May 24, 2011, 12:00:54 am(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-pVK6ZGO

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: May 25, 2011, 02:47:19 pm
content: Quote from: vk6zgo on May 25, 2011, 12:49:53 pmYes .Even so I just did an phase to phase measurement.I got 390VAnd when I use the P-P i get -569 / 569 ACWhen I do that at 220 , I get P-P -310/ 310 AC.

author: jahonen
author_rank: Super Contributor
timestamp: May 25, 2011, 03:17:15 pm
content: Quote from: Kiriakos-GR on May 25, 2011, 02:47:19 pmSo your DMM shows positive and negative peaks, to get the P-P voltage mentioned, calculate it as 569-(-569) V =1138 V.Regards,Janne

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: May 25, 2011, 07:40:12 pm
content: Quote from: jahonen on May 25, 2011, 03:17:15 pmYes 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.