WIFI and/or Bluetooth - I think it was. I did find the source but now I forget, but pretty sure it was WIFI. Switch off all radios - routers, laptops, desktops, phones, repeaters, taplets etc anything that transmits OR receives WIFI.
Heh.. yep... Any strong enough RF source can mess with the gear, thru getting rectified/detected by diode junctions in the gear and making AF noise.
One time at a company designing a implantable defibrillator, with sensitive ECG inputs... we had a noise problem. Strange, intermittent, hard to track down. But we had to, this was a implantable medical device. Whole R&D group are involved. Finally, after a couple of weeks, I discovered it was the RF from the cell phone in my pocket, being picked up by the HV differential probe on the scope, NOT our device, and only when you were standing very close! A phone, even when not on a call, will blip out a "here I am" every so often so the cell system knows about it.
I'd experiment with looking at your measurement system's baseline noise, but with all the normal leads, etc. I would say you probably don't have to turn off all your wireless stuff, but the nearer it is the more likely it is to interfere. So, yes the wifi in the test laptop would be more likely to cause issues, but the house router 20-30ft away possibly not. Don't put your phone down on the test bench. ;-) And, just about always, there is no RX only wifi devices, they all talk back and forth, so watch your wireless plugs and other less expected things.
With the E1DA ADC, I'm seeing all kinds of cray sheet. I see a rolling HF noise floor if I have the cheap LED garage lights turned on, above the measurements. I have to turn my lights off!
Now, there's a lousy/badly filtered SMPS! Good thing you're not a Ham or SWL... some of those cheap products are terrible RF noise generators. But if you can save 5-10% of the unit cost by skipping decent filtering, and fake a Gov cert label (since Gove checking is spotty) think of the money you would make! And, I've heard of some lights being pretty quiet... problem is you pretty much have to test them yourself.
Anyway, good work on trying the supplies and measuring the results. Its the only way to really know.. well, its better than just hearing hum and hiss or not. And, can teach you what needs to be done to improve it even more.
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