PCB Design Principles - Grounding

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PCB Design Principles - Grounding [Copy link]

1. Common impedance interference of ground wire

The ground wire on the circuit diagram represents the zero potential in the circuit and is used as a common reference point for other points in the circuit. In actual circuits, due to the existence of ground wire (copper film wire) impedance, common impedance interference will inevitably occur. Therefore, when wiring, points with ground wire symbols cannot be connected together at random, which may cause harmful coupling and affect the normal operation of the circuit.
　　2. How to connect the ground wire

Usually in an electronic system, the ground wire is divided into system ground, chassis ground (shielded ground), digital ground (logic ground) and analog ground. When connecting the ground wire, you should pay attention to the following points:
　　1) Correctly choose single-point grounding and multi-point grounding. In low-frequency circuits, the signal frequency is less than 1MHz, the inductance between the wiring and the components can be ignored, and the voltage drop generated on the resistance of the ground wire circuit has a greater impact on the circuit, so the single-point grounding method should be used. When the signal frequency is greater than 10MHz, the influence of the ground wire inductance is greater, so it is advisable to use the multi-point grounding method of nearby grounding. When the signal frequency is between 1~10MHz, if the single-point grounding method is used, the ground wire length should not exceed 1/20 of the wavelength, otherwise multi-point grounding should be used.
　　2) Separate digital ground and analog ground. There are both digital circuits and analog circuits on the circuit board. They should be separated as much as possible, and the ground wires cannot be mixed. They should be connected to the ground wire end of the power supply separately (it is best to connect the power supply end separately). Try to increase the area of the linear circuit. Generally, digital circuits have strong anti-interference ability. The noise tolerance of TTL circuits is 0.4~0.6V, and the noise tolerance of CMOS digital circuits is 0.3~0.45 times the power supply voltage. As long as there is microvolt noise in the analog circuit part, it is enough to make it work abnormally. Therefore, the two types of circuits should be laid out and wired separately.
　　3) Try to thicken the ground wire. If the ground wire is very thin, the ground potential will change with the change of current, causing interference to the signal of the electronic system, especially the analog circuit part. Therefore, the ground wire should be as wide as possible, generally greater than 3mm.
　　4) Make the ground wire form a closed loop. When there are only digital circuits on the circuit board, the ground wire should be looped, which can significantly improve the anti-interference ability. This is because when there are many integrated circuits on the circuit board, if the ground wire is very thin, it will cause a large ground potential difference, and the ring ground wire can reduce the ground resistance, thereby reducing the ground potential difference.
　　5) The grounding points of the same level circuit should be as close as possible, and the power filter capacitor of the circuit at this level should also be connected to the grounding point of this level.
　　6) The connection method of the total ground wire. The main ground wire must be connected in strict accordance with the order of high frequency, medium frequency, and low frequency, from weak current to strong current. It is best to use a large-area enclosing ground wire for the high-frequency part to ensure a good shielding effect.