EMC issues - grounding techniques and precautions for PCB engineers

    Single point grounding refers to the entire system where only one physical point is defined as the grounding reference point, and all other points that require grounding are connected to this point.

    Single point grounding is suitable for circuits with lower frequencies (below 1MHz). If the working frequency of the system is so high that the working wavelength can be compared to the length of the system grounding lead, the single point grounding method is problematic. When the length of the ground wire is close to 1/4 wavelength, it is like a transmission line with a short circuit at the end. The current and voltage of the ground wire are distributed as standing waves, and the ground wire becomes a radiation antenna and cannot function as a "ground".

    In order to reduce grounding impedance and avoid radiation, the length of the ground wire should be less than 1/20 wavelength. In the processing of power circuits, single point grounding can generally be considered. For PCBs that are widely used in digital circuits, due to their rich high-order harmonics, it is generally not recommended to use a single point grounding method.

    Multipoint grounding refers to the direct connection of each grounding point in the equipment to the nearest grounding plane, in order to minimize the length of the grounding lead.

    The multi-point grounding circuit has a simple structure and significantly reduces the high-frequency standing wave phenomenon that may occur on the grounding wire, making it suitable for occasions with high operating frequencies (>10MHZ). However, multi-point grounding may lead to the formation of many grounding loops inside the equipment, thereby reducing its resistance to external electromagnetic fields. In the case of multi-point grounding, attention should be paid to the issue of ground loops, especially when networking between different modules and equipment. Electromagnetic interference caused by ground wire circuit:

    The ideal ground wire should be a physical entity with zero potential and zero impedance. But the actual ground wire itself has both resistance and reactance components, and when a current passes through the ground wire, a voltage drop occurs. The ground wire will form a circuit with other connecting wires (signal, power line, etc.), and when the variable electromagnetic field is coupled to the circuit, an induced electromotive force is generated in the ground circuit, which is coupled to the load by the ground circuit, posing a potential EMI threat.