There are two potential problems here:

1. The grounds need to be connected by a low-impedance path. If the grounds of circuits are connected with a high-impedance path, then there is a voltage drop between the ground pins in figure 3. This causes disruptions to all of the devices referenced to this ground and can degrade input noise margins.

2. The loop area formed by the current loop needs to be as small as possible. All loops act as antennas. Generally speaking, a greater loop area will increase the chances of the loop to radiate or the chance of the loop to have a larger susceptibility to the surrounding electromagnetic fields. It should be a goal of every PCB designer that the return current is able to travel directly underneath the signal trace, thus minimizing loop area.

 

Regarding the power/ground structures, the most important part of them is the ground structure, which acts as return path for the currents traveling through the signal tracks. The types of the ground structure are the following:

1. solid ground plane;
2. partial ground plane;
3. grid ground structure;
4. ground tracks structure