"tm"

You can do it anyway you want. Just that many good scopes have a way to
add
the inverted second channel and can be used to look at a signal that is
raised above ground.

** The problem with that idea is the very limited common mode range
available with that scope function.

The buffers on most ( analogue) scopes become overloaded with input signals
that exceed full screen hight by a factor of 2 or 3 times.

Say you want to examine a 1 volt p-p signal and you set the scope input
atten to 0.5 volts per division - then the largest undistorted input
allowable is gonna be about 0.5 x 8 x 3 = 12 volts p-p.

Not much use with the ACTUAL common mode signal is several hundred volts p-p
from rectified AC supply.


No isolation transformer is needed and it is IMO, safer.

** Fraid that is just not true when dealing with off-line SMPS.



..... Phil