Grounding problem? Wave attenuates when I plug in USB.

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KingNothing
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Hi all. I have a nice little PIC-based system built onto a breadboard. It has ADC and USB capabilities. I wish to connect a function generator to one of the ADC input pins of the PIC to feed it a sine wave. The board is powered by the USB. However, whenever the three grounds are united (board ground, USB ground, function generator ground), the signal gets severely attenuated to where is it barely recognizable.

Can anyone help explain this?
 

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When you plug in the USB plug, the signal vanishes?

But the circuit is being supplied with power by the USB.

So, what is powering it before you plug in the USB?

Maybe you need to explain your circuit and layout a bit more.