Ground in PCB design, another app note about grounds from Renesas. Link here (PDF)
Ground is supposed to be ideal. It should be a black hole for stray currents where the voltage is always zero. Unfortunately, those stray currents travel through some non-superconducting material, so small voltages arise. You may not notice small changes in ground potential, you may, instead, notice surplus noise or instability or other unwanted attributes in your system. We are going to discuss ground. Every circuit is unique and grounding paths are different for every device on your board and in your system. Therefore, we are starting with an intuitive approach to try and give you a feel for the paths currents choose to travel and how that affects the ideal assumption of ground being zero volts.