This is abit a hypothetical question. Imagine a signal chain where a AC-DC power supply is powering a transducer and the transducer output is coupled to a data acquisition input by a BNC cable.
Or I can illustrate the chain like:
AC mains->PSU->Transducer—————->DAQ.
And the DAQ is single ended as well. I tried to simplify this by the drawing below:
Now Vsig represents the transducer; Vcm represents the common-mode voltage.
X and Y are the output terminals of the transducer.
Rs is the output impedance, Rw is the BNC resistance.
A and B are the terminals of the coax cable right at the DAQ side where signal is BNC coupled to the DAQ.
Rin is the input impedance of the data acquisition board.
Now assuming inside the dotted line is a black box to us and we don't know initially what we measure as noise is CM interference originated or not.
And our aim to make a quick test to verify if the noise is originated due to a CM interference.
After that premise my two questions are as follows:
1-) If I short the terminal Y with a wire to the power supply ground, and if in that case the noise disappears does that reveal the existence of Vcm?
2-) If I only have access to A and B terminals, is there any way to reveal the existence of Vcm? Can this be done by a multimeter between the terminal B and DAQ ground(when the BNC to DAQ is decoupled)?
Best Answer
In some cases, \$V_{cm}\$ is an AC voltage produced by current in a ground loop (in orange, below).
It cannot be shorted out.
If this ground loop can be broken, no current can flow around the loop, so no voltage is developed across Rw. One way out: disconnect (float) \$V_{sig}\$ so that Y doesn't connect to ground.
In some cases, the signal source cannot be floated, and is unavoidably grounded. And it is likely that the DAQ is grounded as well. So the loop cannot be easily broken.
Adding extra ground wires likely increases loop area or loop current. \$V_{cm}\$ gets worse, or at least doesn't decrease. A very annoying problem to solve, especially in complex measurement systems where lots of equipment is grounded.