Kost, I'm actually not completely sure where the USB cable shielding is supposed to be connected. I thought that the shield should be connected to the GND, but through a resistance and filtering. However, it's not something that I know with any certainty. I will say that if the scope connection causes the signal to clean up, it could be that you have some sort of grounding issue. With the scope connection to earth ground, you may have removed a ground loop voltage that caused the noise that you see. Joseph Wu
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