Strange analog circuit problem with grounding

[Mad ID]

Hello,
I have developed an FSK (HDX) RFID reader which works great when it is powered by a clean isolated lab power supply. The reader has a buck converted at the input. If I ground the lab power supply it works the same.

However, if I power it through an isolated low quality 12V wall adapter which is very very noisy my RFID reading range falls to 1cm. What I have noticed it that the noise itself is not a problems, it's the fact that it is isolated + noisy in combination!

When I ground the system the reading range goes to normal regardless of noise. If I try to probe my analog signals with a non-isolated scope, as soon as I touch the signal ground with scope ground problem is of course gone.

Please help, I want to understand the problem so that I can avoid it in the field.

Cheers. 

[Mad ID]

New info, ground potential difference between earth ground and crap power adapter is 85Vrms AC.

This is the problem definitely, still not sure why this would be a problem for RFID communication. So my reader antenna is changing potential to ground at 50Hz rate. Anyone?

[poorchava]

Have you torn apart the adapter? Maybe it's the parasitics in the adapter that cause this problem?