That's really good progress news

Earthing is mentioned in the interface artice. ie a PC is grounded
to mains earth and they advise caution about measuring circuits
like switch-mode supplies. In your case you say you can see what
looks like a sine wave, without using a ground connection to the
circuit. Possibilities are that the PS ground for your circuit is the
same as the PC and what you see is the actual wave, or that the
ground for you PS is not grounded andd you could be looking at
induced mains hum. The big clue will be the frequency of the sine
wave. If your PIC or whatever isn't putting out 50Hz or 60Hz but
that's what you see, then dot dot dot dot dot [ join them ;-) ]

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