Meaning
Unintended magnetic flux storage within a circuit layout occurs when a signal current and its return current flow through a large loop area. High parasitic loop inductance degrades signal transitions and increases radiated emissions by acting as an efficient antenna for switching noise. This parameter is highly dependent on the layout and stackup of the printed circuit board.
Physical Mechanism
The size of the loop area is the primary factor that determines the amount of inductance introduced into the signal path. When a trace is routed far from its reference plane, the return current must flow through a longer path to complete the circuit. This separation increases the loop area and the associated inductance.
Placing a solid ground plane directly beneath the signal trace minimizes this loop. It also helps to shield the signal from external electromagnetic noise.
Noise Generation
High inductance in the return path causes voltage spikes and ringing during fast switching events. These spikes can exceed the voltage ratings of sensitive components and introduce noise into other circuits. This ringing is a common source of radiated emissions that can cause a device to fail electromagnetic compatibility tests.
Minimizing the loop area is necessary to keep these noise levels low.
Design Minimization
Using decoupling capacitors placed close to the power pins of active components is the most effective way to reduce the loop area for high-frequency currents. These capacitors provide a local, low-inductance source of charge during switching. This layout practice prevents the noise from spreading into the larger power distribution network.
