EEBoard Analog Input GND connection

[dclock]

Hello,

I've just started to use the EEBoard and I'm having some troubles with the ground connection of the Scope.
The first thing I wanted to do was to use the scope functionality to observe a +-5mV signal on an audio board.
While I take the measurement I monitor the output audio signal with a pair of headphones

I connect a 1x probe between the GND pin and DC pin of Scope.
The moment I connect probe GND to the DUT GND a clicking noise comes out of the headphones.

Is the GND pin introducing noise in the audio board, or loading it in some way?

As an alternative solution I tried connecting the probe GND to AN1 and the probe signal to AN2.
Then In WaveForms I subtract AN1 and AN2 and it all works fine.
The connection to the DUT doesn't introduce noise anymore and I'm able to observe the audio signal.

Is there a recommended way to Scope an external board?

 

Thanks and Regards,

Dario