One of my previous question revealed that a variable voltage test device's power supply which is an AC/DC adapter, and is SMPS type which causes common-mode voltages. A solution given by tying the supply ground to earth ground. This works only if the AIGND of the data-acquisition device is earthed.
Anyway below is about my question:
Now I found another workaround which I cannot explain the reasons. To begin with the issue was due to the parasitic leakage of the SMPS adapter which causes common-mode voltages to appear all the way at the device terminals wrt earth. I show this in Figure 1 below:
Now if I tie the ground of the device to the AIGND of the differential input data-acquisition channel as in Figure 2 below(green line), I still have 50Hz noise:
But if I tie the ground of the power adapter to the AIGND of the differential input data-acquisition channel as in Figure 3 above(green line), 50Hz common-mode noise completely disappears.
Even more strangely is that: when I make a continuity test between the adapter and device ground(GNDA and GNDB in Figure), they are already connected to each other.
So theoretically tying from GNDA or GNDB to AIGND shouldn't be different.
My questions are:
1. Why does the common-mode voltages disappear when I connect the adapter ground to AIGND?
2. The continuity test tells adapter and device ground already connected. Bu then how would you explain the difference between the results of Figure 2 and Figure 3?
3. Can I safely use this method in this type of situations? I mean tying the power supply GND to AIGND.
I mean I have a setup when the switch above is open. And when the switch is closed the common mode noise-disappears even though there is continuity between the GNDA and GNDB. Can this technique be used safely?