Electronics > Beginners

Low noise voltage opamp oscillating

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magic:
Now it's making sense :phew:

Yes, it's picking up the electric field of mains wiring. It doesn't matter how far the cables are, if they are all around you will experience their field.
The solution is to surround the whole input node (opamp, cables, resistor) with a grounded conductor. It may be your hand, or both feet for better coverage, or a metal case with holes for cables ;)

pwnell:

--- Quote from: magic on December 11, 2021, 06:32:11 pm ---The solution is to surround the whole input node (opamp, cables, resistor) with a grounded conductor. It may be your hand, or both feet for better coverage, or a metal case with holes for cables ;)

--- End quote ---

Problem is - whenever I bring anything grounded to mains ground close to the circuit it just makes things way worse.  I used a large metal dome, connected to mains ground, and placed it over the circuit, PSU and cables - it was about 5 times worse than just leaving it in open air.

It is the best when I do not probe it with my oscilloscope, but instead use my macbook air unplugged from the wall, and a Scarlett 2i2 USB audio interface powered via USB-C.  Then I find minimal AC interference as long as I do not come close to the circuit.

magic:
The shield should be grounded to the circuit's ground.
Ideally, you would have a star ground point near the first opamp and connect to it:
- the first opamp's gain resistors
- the second opamp's gain network
- the shield
- ground of the output connector

But you can probably get away with a lazier ground layout, as long as everything is contained on that one board and no big currents flow through the circuit.
One thing that won't work very well is grounding the shield to some random mains socket. Too much potential difference, even if it's just a fraction of a volt.

Here's how you do shielding around such things:
http://www.vk2zay.net/article/251
Except that the guy should have run a coax cable between his two cans instead of a pair of bare wires, or put everything in one can.
But he only cares about DC so maybe it's good enough for him as long as the opamp doesn't clip due to picked up noise.
(Or as long as he doesn't notice that it clips and just accepts a wrong result and never finds out it was wrong >:D)

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