Grounding tips and PCB engineer considerations

EMC Problem When working on the layout, you must also pay attention to EMC suppression! It’s incredibly challenging to strike the right balance, as distributed capacitance is always present. How do we handle grounding properly? PCB design inherently involves many considerations, and different environments demand different approaches. While I’m not a PCB engineer, I recognize that my experience is somewhat limited. Ground Segregation and Tandem Grounding is one of the crucial methods to suppress electromagnetic interference and enhance the EMC performance of electronic equipment. Proper grounding not only strengthens the product's ability to counteract electromagnetic interference but also minimizes external EMI emissions. The Meaning of Grounding In an electronic device, the term “ground” typically carries two meanings: one being “earth” (for safety purposes), and the other being “system reference ground” (or signal ground). Grounding establishes a low-resistance conductive path between the system and a potential reference plane. “Connecting to the earth” relies on the earth’s potential, using the ground as a zero potential to connect the metal casing of the electronic device and the circuit reference point to the earth. Connecting the ground plane to the earth is often motivated by several considerations: A. To improve the stability of the equipment circuit system; B. To prevent static discharge; C. To ensure the safety of personnel. The Purpose of Grounding A. Safety considerations, i.e., protective grounding; B. Providing a stable zero potential reference point (signal ground or system ground) for signal voltages; C. Shielding grounding. Basic Grounding Methods There are three fundamental grounding methods in electronic equipment: single-point grounding, multi-point grounding, and floating. Single-Point Grounding Single-point grounding defines the entire system with only one physical point acting as the ground reference point, to which all other grounding points are connected. Single-point grounding is suitable for circuits operating at lower frequencies (below 1 MHz). However, if the system's operating frequency is so high that the operating wavelength becomes comparable to the length of the system's ground lead, the single-point grounding method becomes problematic. When the length of the local line approaches 1/4 wavelength, it behaves like a transmission line with a short circuit at the end. The current and voltage distribution along the ground line forms standing waves, turning the ground line into a radiating antenna, which defeats its purpose as a “ground.” To minimize grounding impedance and avoid radiation, the length of the ground wire should be less than 1/20 wavelength. In the context of power circuits, single-point grounding can generally be considered. For PCBs with numerous digital circuits, due to their high harmonic levels, single-point grounding is typically not recommended. Multi-Point Grounding Multi-point grounding involves each grounding point in the equipment being directly connected to the nearest ground plane, ensuring the shortest possible grounding lead. The multi-point grounding circuit has a simpler structure, and the high-frequency standing wave phenomenon on the grounding line is significantly reduced, making it suitable for applications with higher operating frequencies ("10 MHz"). However, multi-point grounding may create numerous ground loops within the device, thereby reducing its resistance to external electromagnetic fields. In cases of multi-point grounding, the issue of ground loops must be carefully managed, particularly when networking between different modules and devices. Electromagnetic interference caused by ground loops: The ideal ground should be a zero-potential, zero-impedance physical entity. However, the actual ground wire itself has both a resistive and reactive component. When current flows through the ground wire, a voltage drop occurs. The ground line forms a loop with other connections (signals, power lines, etc.). When electromagnetic fields couple to the circuit, an induced electromotive force is generated in the ground loop and coupled to the load via the ground loop, posing a potential EMI threat. Floating Floating is a grounding method where the grounding system of the equipment is electrically insulated from the earth. Due to certain inherent limitations of the floating method, it is not suitable for general large-scale systems, and its application is rare. General Selection Principles for Grounding Methods For a given device or system, at the highest frequency of interest (corresponding to the wavelength), when the length L of the transmission line is >, it is considered a high-frequency circuit, whereas below this threshold, it is treated as a low-frequency circuit. As a rule of thumb, for circuits below 1 MHz, single-point grounding is preferred; for above 10 MHz, multi-point grounding is preferred. For frequencies in between, as long as the length L of the longest transmission line is less than /20, single-point grounding can be used to avoid common impedance coupling. General Selection Principles for Grounding (1) For low-frequency circuits (1 MHz), it is recommended to use single-point grounding; (2) For high-frequency circuits ("10 MHz"), it is recommended to use multi-point grounding; (3) For hybrid high and low-frequency circuits, mixed grounding is advised. To summarize, the choice of grounding method depends heavily on the specific application and the frequency range involved. Understanding these nuances is essential for achieving optimal EMC performance in your designs.

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