In PCBs, the ground could take different forms; ground planes, which were indicated by patches or a whole layer of copper, or simply just an external jumper that could be connected to the negative port of an external supply source.
In general applications, the GND or ground symbol was to be indicated by the following symbol. The symbol could often be seen in circuit schematics: logical, electrical, etc.
The power ground is commonly used if any potential difference between the metal case (or power supply’s ground) and the real ground is present.
Signal ground indicates the ground which is used for a reference voltage. Implementing it as a comparator input, for example, would be useful to control the shape of the said input signal.
Besides the generic signal symbol shown in the picture above, specific signal GND symbols (i.e., digital and analog) could also be used to provide a better description of the system.
The discussion led us to one important question, “Are this all necessary for PCB construction?" The answer is no. If there were no special conditions that need to be taken into account, it’s all right to use the general ground symbol of GND for construction, allowing easier and cheaper production of the PCB.
Are ground planes always needed for PCB construction? Again, no. Usually, in power electronics applications, the inputs and outputs of the system are very often isolated, hence adding the ground plane layer would break the isolation. However, when constructing more complex PCBs with more than 2 layers, adding the ground plane would be very much useful since isolation could still be maintained.