Improper grounding is the single most common source of EMI. This video explains that there are basically two safe ways to ground a system. Everything else is just varying shades of grey that may or may not work. If you have multiple signals with the letters GND in them you will almost certainly have either EMI or signal noise issues. There is only one system common signal we call GND or Ground. If you have any type of significant inductance in the path called GND you will probably have issues of varying severity. Designing systems with a proper Ground topology means that you must visualize where the currents flow and the amount of inductance between those locations. This is too complicated for text alone. Watch the video and we can talk about it in class.