50 Hz noise problem in isolated AC-DC power adapter

noisepower supply

I have a circuit which is powered using AC-DC 12V wall converter. One of the power supply adapter has ground pin (3 pins) in its input and the other doesn't have (2 pins). The ground pin in the first one is electrically connected to the output 0V. With the power supply without ground, there is more 50Hz noise in the signal from the circuit. But I have to use the isolated power supply (without ground).
How can I remove these 50Hz noise due to power supply? I cannot use notch filter inside the circuit. Therefore I want to make changes in the power supply itself.

Best Answer

Using switching supplies use one side of the input to conduct noise back to the line. That means that there is a fairly substantial capacitance between one of the mains input wires and the output ground. Often, but not always, the input pins are polarized so that the coupling is to the neutral line. Not sure what happens in your 50Hz country, but in North America one of the blades is made wider (for the neutral). If you have the socket wired wrong way 'round or plugged in the wrong way 'round (assuming your flavo[u]r of receptacles allows that).. you can get common mode AC at mains frequency on the output.

TL;DR: Try swapping the input wires around and see if the 'noise' mostly goes away.