Beginner scope usage- probing differential and common mode voltage

[Buildingthings]

I am using my new Siglent 1104x to troubleshoot CANbus signals. I have watched the video on how to not blow up your oscilloscope, and I think that I am safe.

I would like to probe the differential voltage of a CANbus signal pair, and measure the common voltage offset. Abnormalities can help identify faulty devices or faulty connections to the network.

I am planning:

•   channel one to High signal, channel two to Low signal, (no ground) to measure differential voltage.
•   Next the common voltage- do I want to connect the probe grounds to the network ground? Both probes?

*I am powering the network and devices through an isolated ground power supply, so no mains ground reference.

Thanks for any help- newbie here.

[bdunham7]

What point do you want to measure the common mode voltage from?  Whatever that point is, put both your ground clips at that point together (for best signal) or just use one ground clip (for maximum protection against errors).  Then you can use the math function--common mode is (CH1 + CH2)/2 and differential mode is CH2-CH1. 

[Doctorandus_P]

In situations like this I use my scope mostly for verifying signal integrity, and thus probe the individual wires (referenced by ground). I don't look at the data at all, but just signal levels, ringing, abnormal levels which indicate faults.
When you want to look at the data itself, it's usually better to measure on the "other side" of your transceiver IC.

[David Hess]

If the oscilloscope and device under test share earth ground, then the probe grounds should only be tied together.  If the oscilloscope and device under test do not share earth ground, then the probe grounds should be tied together and to the single point ground on the device under test, although this may not be necessary.

[Buildingthings]

In situations like this I use my scope mostly for verifying signal integrity, and thus probe the individual wires (referenced by ground). I don't look at the data at all, but just signal levels, ringing, abnormal levels which indicate faults.
When you want to look at the data itself, it's usually better to measure on the "other side" of your transceiver IC.

Yes I wish to do the same. I will research the ringing further, have never looked at that before.
For reading the CANbus data I have a gateway that connect bus to USB then an app that reads packets. I save that file as text, load to excell and search for specific PGNs of concern. I am working on an Excell script to calculate CANbus loading (packets sent per minute) and then to chart it.

[Buildingthings]

Thanks for the replies. I used an isolated ground power supply for the device powering the CANbus. I connected one probe ground to the device ground and measured common mode voltage, then both probe grounds to the same device ground. Same reading, so I suppose it does not matter.