Oscilliscope Measurement Questions - Single vs Differential Measurements - Page 1
Forum section: electronics / beginners

author: Plasmateur
author_rank: Regular Contributor
timestamp: April 17, 2017, 02:53:35 am
content: Attached is the circuit schematic of my measurements.I'm trying to understand why I am getting different voltages readings in example 1 and example 2 in the attached picture. Ultimately I want to make a current measurement across the resistor that is closest to the battery. I have additional resistors added in the circuit as to not draw a large amount of current which will ultimately effect the measurement I am trying to make in another similar application.Ex.1) I am only using one probe.Without the probe ground being used, I do not see any voltage on my scope. When using the probe ground I see a value of 3.2 VoltsEx.2) I am using two probes.Unlike the first example, I am able to see a voltage when using both probes without using either probe's ground. Why is this?The difference in the voltage readings on each probe is 4.8 volts, which is different than the reading in the first example.Does this mean the current going through the resistor closest to the battery is different in between the first and second example?Ex.3) I am only using one probe.The schematic for example 3 shows I have moved where the probe's ground is connected. I receive a reading of 4.8 volts, just like example 2. Is this effectively the same measurement as example 2?

author: ebclr
author_rank: Super Contributor
timestamp: April 17, 2017, 03:18:47 am
content: You must understand that the scope its self is a simplistic way a 1M ohm resistor between input and ground when you use the differential you will have 1 mega on each side, while with only 1 channel only 1 resistor.The instrument is always part of the circuit, and if you want to measure thing on the same order of magnitude of the input impedance of the instrument, thing start to be tricky and need to "see out of the box"

author: tautech
author_rank: Super Contributor
timestamp: April 17, 2017, 04:49:20 am
content: Quote from: Plasmateur on April 17, 2017, 02:53:35 amThen the resistor as you put it becomes a resistive shunt and to measure the current through it is by measuring the voltage across it. At DC levels this is best done with a DMM not a scope.QuoteBecause there is no return path for the current.I may add if the DUT is not ground referenced there is no chance of creating a ground loop and when this is so disregard thinking of probe ground and think of the lead as for signalreference.This little mindshift in thinking and referring to the probe lead as a reference lead will help you understand oscilloscope measurement methodologies much better.With all your examples you have connected the probe and oscilloscope in a way thatcompletesa circuit and it then becomes an element of the circuit which it should not.Hope that helps.

author: Plasmateur
author_rank: Regular Contributor
timestamp: April 17, 2017, 05:23:01 am
content: Quote from: tautech on April 17, 2017, 04:49:20 amIts starting to help. Thank you for taking the time to respond.I'm still left slightly perplexed. I've seen in several research papers people using two oscilloscope channels in order to make a differential measurement to determine a current. (I'm trying to acquire a current measurement of a periodic signal on the order of 1MHz)I attached new examples in my reply to you.In example 4, my scope channel acquires a signal across the resistor of 9.6 voltsMaking a differential measurement in example 5, I again acquire 9.6 voltsBoth examples yield the same measured current.Going back to my original post and examples, it appears there should be some corrective factor that I can determine through circuit analysis theory in order to make the current measurement I am attempting to make in example 2 shown in the original post.

author: tautech
author_rank: Super Contributor
timestamp: April 17, 2017, 08:12:40 am
content: Quote from: Plasmateur on April 17, 2017, 05:23:01 amSee small correction above.Yes, you have two differential voltage measurements of the battery voltage only.Now add another 1M \$\Omega\$resistor in series and do a differential measurement across that. Why 2 ? One represents the circuit load and the other represents the current shunt that will have a voltage drop across it that represents the current value.These are unfortunate values that coincide with the probe resistance but in a real measurement the shunt resistance will be much lower, max of an Ohm or two.QuoteThere is NO corrective factor as the probes and connection methodology has an influence on the measurement value.I add this statement again for your OP examples:With all your examples you have connected the probe and oscilloscope in a way thatcompletesa circuit and it then becomes an element of the circuit which it should not.Keep in mind ANY measurement in some way alters the measurement value itself.

author: danadak
author_rank: Super Contributor
timestamp: April 17, 2017, 11:07:34 am
content: This might help -http://materias.fi.uba.ar/6644/info/osciloscopios/avanzado/Diferential%20oscilloscope%20measurements.pdfRegards, Dana.