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?
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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.)
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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.
Best Answer
I am going to take a slightly different approach to this.
Firstly, are you sure the pickup is magnetic? If so, then there are essentially two things you need to do:
Connect one side of the DC supply to the rooms shielding right where the cables pass into the room (Both legs of the supply cable must pass thru the same hole in the room shielding), this ensures that circulating ground currents due to the EMC cap in the supply stay outside the room.
All the internal wiring inside the room for stuff must be tightly twisted, this will ensure substantial field cancellation. Worst case use a group of four wires twisted around a common centroid (Used in microphone circuits and known as "Starquad" in that application), wiring is by paralleling opposite conductors, and it gets you maybe another 10 or 20dB of suppression.
If the pickup is E field rather then H, then screening the cables (and bonding the screen to the room shielding is an easy fix).
To answer the question asked, a linear supply built with a transformer having a grounded interwinding screen will be almost as quiet as a battery.
You might find that opening up your switcher and removing the cap across the isolation barrier, and then grounding one leg of the output to the room screening is useful, the supply will no longer meet EMC, but that may or may not matter to you.
Good luck, this stuff can be a bear to track down.