Differential Measurements With Digital Oscilloscope Using Single-Ended Probes

Thread Starter

reynolds087

Joined Jul 22, 2020
6
Hi,
Can anyone help me figure out what I am doing wrong when trying to measure audio signals using two channels of my oscilloscope. I really want to avoid buying a differential probe, and I am trying to use the math operations on my Rigol 1054Z to simply measure the difference in voltage between the two probes. If I connect the ground lead of a probe to the shield of an audio cable and put the probe on the signal wire, I can see the audio signal without problems. But I've tried using the math operations with A set to Channel 1 and B set to Channel 2, then A - B with the operation enabled, and I've tried inverting one channel or keeping both channels non-inverted but no matter what I just get a bunch of noise. And the noise is showing up as like 50v or more in some cases. Is it possible my scope is damaged or something?
 

bertus

Joined Apr 5, 2008
21,524
Hello,

When using two channels on a scope with the A - B function, both channels should have the same sensitivity.

Bertus
 

Thread Starter

reynolds087

Joined Jul 22, 2020
6
Thanks, Bertus. I have double-checked that the probe is set to 10x for both of them. That's on the probe itself (physical switch) as well as the setting for each channel on the scope. Is there anywhere else I may have to change the settings for the sensitivity?
 

tautech

Joined Oct 8, 2019
208
Differential measurements do not require use of the probe reference lead (Gnd lead) when the second probe is in fact the reference for the signal.
As a sanity check use the 1KHz probe comp output with one probe tip on the output and the other on the Gnd terminal and then use Maths A-B.
Be sure to remove reference leads from your probes if you are to work over a PCB when using A-B Maths mode.
 

Thread Starter

reynolds087

Joined Jul 22, 2020
6
Differential measurements do not require use of the probe reference lead (Gnd lead) when the second probe is in fact the reference for the signal.
As a sanity check use the 1KHz probe comp output with one probe tip on the output and the other on the Gnd terminal and then use Maths A-B.
Be sure to remove reference leads from your probes if you are to work over a PCB when using A-B Maths mode.
Thanks, tautech! I had the hardest time explaining to the tech at Rigol what I was trying to do, and she insisted that I need to connect the ground clip to the shield of the audio cable, but you are correct that I would like to remove the ground clips and connect one probe tip to the audio ground and measure across to the signal wire with the other probe tip and then apply the math operation. I will try with the probe comp output. I am assuming that is like a built in signal generator? I am not that familiar with the scope yet.
 

tautech

Joined Oct 8, 2019
208
Just remember every measurement requires a reference connection like when you use both leads of a DMM however as the scopes BNC terminals will/should be at mains earth potential you can't place a probe reference lead anywhere you like in a circuit as it can cause a short circuit to mains ground. This is called a ground loop and in high energy circuits sparks and magic smoke will escape !

Always before connecting reference leads take a moment to check a ground loop will not eventuate. Sometime use of a DMM can prove a reference point is at mains ground potential.
 
Older scopes needed the gound. One was set to A, the other to B inverted. Then ADD mode. A-B wasn't an option.
Ground of the scope probe was Earth and you had to have that reference.

Ground to earth maximums had to be respected.
 
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