PCB ground design can be considered the foundation of all PCB designs, and in electronic devices, grounding is an important method of controlling interference. Most interference problems can be solved if grounding and shielding are used in the right combination.
What is the ground?
Electronic equipment ground usually has two meanings.
- One is the earth (safety ground), grounding is to the earth’s potential as the reference, and the earth as the zero potential, the electronic equipment metal shell, the circuit reference point, and the earth connected.
- The other is the system reference ground (signal ground), grounding is the establishment of a low-resistance conductive path between the system and a potential reference surface.
The purpose of grounding
- Grounding can make all the unit circuits in our circuit system have a common reference 0 potential, that is, there is no potential difference between each circuit, to ensure that the circuit system can work stably;
- To prevent external electromagnetic interference. For example, the chassis grounding; for transient interference (ESD) provides a discharge channel; can also make due to electrostatic induction and the accumulation of a large number of charges on the chassis through the earth discharge; if the circuit has the use of shielding cover or circuit shield, choose the appropriate grounding, you can get a better shielding effect!
- To ensure safe work. When the occurrence of lightning (Surge) of electromagnetic induction, can avoid damage to electronic equipment.
Classification of grounding
If you do not consider safety grounding, only from the perspective of the circuit reference point, grounding can be divided into suspended ground, single-point grounding, multi-point grounding, and mixed grounding.
- The purpose of the suspended ground is to isolate the circuit or equipment from a common ground or a common conductor that may cause a loop current. The disadvantage of this type of grounding is that the equipment is not directly connected to the earth, easy to produce electrostatic accumulation, and eventually occurs with a strong discharge current of electrostatic breakdown phenomenon. Usually, take the approach is to connect the equipment and the earth into a resistance value of a large resistance to eliminate the accumulation of static electricity.
- Single-point grounding refers to a circuit in which only one physical point is defined as the grounding reference point.
- Multi-point grounding is a system in which each grounding point is connected to the nearest ground plane, in order to make the shortest length of grounding lead. It is the only practical grounding method for high-frequency signal circuits.
- Mixed grounding is the combination of single-point grounding and multi-point grounding, the applicable operating frequency range is generally 500kHz ~ 30MHz.
The computer control system, is roughly divided into the following types of ground, analog ground, digital ground, signal ground, system ground, AC ground, and protection ground.
PCB ground design guide
The PCB ground design should pay attention to the following points.
The correct choice of single-point grounding and multi-point grounding
In the low-frequency circuit, the signal operating frequency is less than 1MHz, and its wiring and inductance between the devices have less impact, while the grounding circuit to form a loop current on the interference impact is greater, and thus should be used single-point grounding.
When the signal operating frequency is greater than 10MHz, the ground impedance becomes very large, at this time should try to reduce the ground impedance, which should be used near the multi-point grounding. When the operating frequency is between 1 and 10MHz, if a single point of grounding, the ground length shall not exceed 1/20 of the wavelength, otherwise, a multi-point grounding method should be used.
Separate the digital circuit from the analog circuit
The circuit board has both high-speed logic circuits and linear circuits, they should be separated as far as possible, while the ground of both does not mix, respectively, with the power supply side of the ground connection. Try to increase the ground area of the linear circuit.
Try to thicken the ground wire
If the ground wire is very thin, the ground potential changes with the change of current, resulting in the timing of electronic equipment signal level instability, anti-noise performance becomes bad. Therefore, the grounding wire should be as thick as possible, so that it can pass through three is located in the allowable current of the printed circuit board. If possible, the width of the grounding wire should be greater than 3mm.
Form the ground wire into a closed-loop
When designing a ground system for a printed circuit board consisting only of digital circuits, making the ground wire a closed loop can significantly improve noise immunity.
The reason for this is that: the printed circuit board has many integrated circuit components, especially in the case of more power-consuming components, due to the limitations of the thickness of the ground wire, will produce a large potential difference in the ground junction, causing a decrease in noise immunity, if the grounding structure into a loop, it will reduce the potential difference value and improve the noise immunity of electronic equipment.
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