EMC: PCB Engineer Considerations and Grounding Tips

Division and convergence of the ground:

Grounding is one of the important means to suppress electromagnetic interference and improve the EMC performance of electronic equipment. Proper grounding not only improves the ability of the product to suppress electromagnetic interference, but also reduces the external EMI emissions of the product.

The meaning of grounding:

The "ground" of an electronic device usually has two meanings: one is "earth" (safely) and the other is "system reference ground" (signal ground). Grounding is the establishment of a low-resistance conductive path between the system and a potential reference plane. "Connecting to the earth" is based on the earth's potential, and the ground is used as the 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 ground is often due to the following considerations:

A. Improve the stability of the equipment circuit system;

B. Static discharge;

C. Provide security for the staff.

Grounding purpose:

A. Safety considerations, that is, protective grounding;

B. Provide a stable zero potential reference point (signal ground or system ground) for the signal voltage;

C. Shield grounding.

Note to PCB engineers:

Single point grounding:

Single point grounding is the entire system, only one physical point is defined as the ground reference point, and other points that need to be grounded are connected to this point.

Single point grounding is suitable for circuits with lower frequencies (below 1 MHZ). If the operating frequency of the system is so high that the working wavelength is comparable to the length of the system ground lead, the single-point grounding method is problematic. When the length of the local line 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 line are distributed in standing wave, and the ground line becomes a radiating antenna, but it cannot function as a "ground". .

In order to reduce the grounding impedance and avoid radiation, the length of the ground wire should be less than 1/20 wavelength. In the processing of the power circuit, a single point grounding can generally be considered. For PCBs with a large number of digital circuits, because of their high level of harmonics, single-point grounding is generally not recommended.

Multi-point grounding:

Multi-point grounding means that each grounding point in the equipment is directly connected to the ground plane closest to it so that the length of the grounding lead is the shortest.

The multi-point grounding circuit has a simple structure, and the high-frequency standing wave phenomenon that may occur on the grounding line is significantly reduced, and is suitable for applications with a high operating frequency (>10 MHz). However, multi-point grounding may cause many ground loops to form inside the device, thus reducing the ability of the device to resist external electromagnetic fields. In the case of multi-point grounding, pay attention to the ground loop problem, especially when networking between different modules and devices. Electromagnetic interference caused by ground loop:

The ideal ground should be a zero-potential, zero-impedance physical entity. However, the actual ground wire itself has both a resistive component and a reactance component. When a current passes through the ground wire, a voltage drop is generated. The ground line forms a loop with other connections (signals, power lines, etc.). When the electromagnetic field is coupled to the circuit, an induced electromotive force is generated in the ground loop and coupled to the load by the ground loop, posing a potential EMI threat.

Floating:

Floating ground refers to a grounding method in which the grounding system of the equipment is electrically insulated from the earth.

Due to some weaknesses of the floating surface itself, it is not suitable for general large systems, and its grounding method is rarely used.

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 regarded as a high frequency circuit, and vice versa, it is regarded as a low frequency circuit. According to the rule of thumb, for circuits below 1MHZ, single-point grounding is preferred; for above 10MHZ, multi-point grounding is used.

good. For frequencies in between, as long as the length L of the longest transmission line is less than /20, a single point ground can be used to avoid common impedance coupling.

The general selection principles for grounding are as follows:

(1) Low frequency circuit (<1mhz), it is recommended to use single point grounding;

(2) High frequency circuit (>10MHZ), it is recommended to use multi-point grounding;

(3) High and low frequency hybrid circuits, mixed grounding.