Recently I blogged about a mains powered isolated dual low noise power supply I had built.
scottiebabe suggested that for some purposes it would be easier and cheaper to use a USB isolator.
These can be bought quite cheaply from Amazon and very cheaply from Aliexpress so I bought an Amazon version:
I made it a little load adapter (100R approx 50mA).
And I measured then noise, first as delivered and then with a low ESR 22uF 25V polymer electrolytic shunt capacitor.
No additional capacitor
22uF low ESR shunt capacitor.
And the original linear mains powered supply:
Just as you would expect the little switching supply in the USB adapter produces a lot of HF noise.
Even with the extra capacitor the RMS noise is 20x worse and at an inconvenient 80kHz . The broad bandwidth of the noise will not be welcome in measuring systems. The biggest spectral peak is at -65dBV (at 80kHz) compared with -95dB (at 100Hz) for the linear mains driven supply.
To get a dual supply you would need to use two USB isolators which would add noise and possible LF beating effects.
The little USB isolators look quite handy for several purposes but wont win prizes a low noise isolated power supplies.
I shall try one with ARM JTAG debugging dongles because these often cause nasty earth loop problems when you need to connect a scope and the debugger to the same test thing.
MK
