Definition of Ground Wire

A ground wire on a PCB board serves as the reference point of circuit potential, providing a low impedance path for signals to return to the source.

Impedance of the Ground Wire

When current flows through a ground wire, its impedance, not just resistance, plays a crucial role in circuit operation. The impedance of a ground wire can cause significant voltage drops, especially at high frequencies, affecting the performance of digital circuits.

Ground Wire Interference Mechanism

Ground Loop Interference

High-current conditions can induce voltage on the ground wire, leading to ground loop interference that affects connected devices. External electromagnetic fields can also contribute to this interference.

Common Impedance Interference

When two circuits share a ground wire, common impedance coupling can occur, allowing signals from one circuit to impact the other. High-frequency signals can exacerbate this interference, affecting the proper operation of digital circuits.

Ground Wire Interference Countermeasures

• Reducing current in the ground loop can help mitigate interference.
• Floating one end of the equipment or using transformers for inter-device connections are effective solutions to eliminate ground loop interference.

PCB Ground Loop Solutions

1. Isolation Techniques:

• Optical isolators: Optocouplers with low parasitic capacitance effectively cut ground loops by using light for signal transmission, providing superior high-frequency isolation.

2. Common Mode Choke Coils:

• These components increase ground loop impedance, reducing ground loop current. However, caution is required to manage parasitic capacitance that may affect high-frequency isolation.

3. Eliminating Common Impedance Coupling:

• By reducing common ground wire impedance or implementing single-point grounding, common impedance coupling can be eliminated. Segregating circuits prone to mutual interference and utilizing ground grids or distinct ground layers in PCBs can further minimize impedance.

Summary

Ground wire impedance is a significant factor in electromagnetic interference, leading to voltage generation and ground loop currents. Common impedance coupling occurs when circuits share a ground wire, causing signal transmission between them. Effective mitigation strategies involve isolating or reducing ground loop currents and using balanced circuits to minimize common impedance. Employing proper grounding techniques such as single-point grounding optimization can enhance PCB performance in addressing these challenges.