I operate an MEG system (extremely sensitive magnetic sensors for measuring brain activity) and I recently found that the DC power source that feeds into our magnetic shielded room (power used for moving the MEG/chair/bed – thus required), produces 60 Hz line noise detected by the sensors. I'll detail my understanding the sources of this noise, but my question basically is: what kind of supply or what specific features should I look for to avoid this noise?
Noise from currents in wires. The current supply is a typical cheap switched-mode floating DC supply, 24V 0.5A. I did a few tests to confirm that although the difference between + and – looks "clean" and constant, each wire has a large voltage oscillation w.r.t. ground. And this "voltage" seems to be the result of a current source, so even with no load, there will be an oscillation of charge density in the wires, enough to generate a magnetic field detected by our system. (In the kOhm to MOhm range between + and G, I get on the order of 0.1 mA peak current.)
Noise from ground currents. Unfortunately, if I tie the negative to ground, of course the oscillation w.r.t. ground is gone, and that mostly gets rid of the detected noise, but the "offending" current is not eliminated, instead going through the ground. That also causes some noise, probably primarily because our shielded room is itself grounded. Worth mentioning that this noise is much smaller.
Edit: Price is not a primary concern. I realize a well designed low-noise supply would likely be much more expensive.