Ground and power planes: follow the return current
Every signal current comes back through ground. Keeping the return path short and unbroken is all there is to ground design.
Where the return current flows
At low frequency it takes the path of least resistance; at high frequency it returns through the ground directly under the signal trace. The larger the loop formed by the signal trace and its return current, the more noise it emits and picks up.
Principles
- On a 4-layer board, make layer 2 an unbroken ground plane (signal / GND / power / signal).
- On a 2-layer board, fill the bottom layer with ground as far as possible and keep signals on the top.
- Do not cut the ground under a fast signal with other traces. Where it is cut, the return current takes a long detour.
Analog ground and digital ground
Physically splitting ground and joining it at a single point can ruin the return path if done badly. The current recommendation is to keep one ground and separate the areas by part placement, so digital and power return currents do not pass through the analog area.
What is special about a motor board
The high-current return of an inverter carries tens of amps. So that this current does not cross the MCU and current-sense ground, lay a separate wide path for power ground and connect it to signal ground at one point near the shunt.