With a switching DC wall adapter, when I measure AC rms voltage from either of the output wires to earth ground, what am I actually measuring and is this a controllable spec of a wall adapter?
I am dealing with what must surely be an AC ground loop issue that causes 60 Hz hum on telecom equipment. I have many 9 volt switch DC wall adapters. Some work fine (meaning the 60 Hz is not measurable) while others are quite loud.
I have found that the ones that work I measure less than <10 VACrms from wall adapters output to earth ground. The ones that do not work measure >50 VACrms from either of the output wires to earth ground.
On a scope the signal is 60 Hz and a very distorted sine wave.
When trying to source a new adapter is this something I can find in the datasheet? What controls this in an isolated switching power adapter?