Sometimes it feels like a ground-loop is inevitable. Please follow along the following thought-experiment. It would be wonderful if you could provide solutions to the issue described.
Imagine a board where one cannot use a ground plane. There are some chips on the board:
A digital or analog signal flows from Chip 2 to Chip 3. Suppose the trace cannot go straight to the other chip, but needs to bend around some other circuitry on the board.
Now consider the current flow when the signal moves from Chip 2 to Chip 3:
This looks bad. The surface enclosed by the current flow is very large. If it is a digital signal, it will surely inject lots of noise into the circuitry soldered on that surface. And if it is an analog signal, it will probably absorb noise from the circuitry on that surface.
Trying to be smart, I come up with the following solution. Let's add a return path for the signal:
Huray, it looks like the signal is very well protected now. The enclosed surface is much smaller. It won't emit magnetic fields, nor absorb them.
But wait a minute… do you see that? Do you see the ground loop I just created? Look again:
Yikes! What now? Did I just improve the situation, or did I actually make it worse?