The ground wire is also have impedance, when current flows through the ground wire, a voltage will be generated, this is a noise voltage, and the noise voltage is one of the interference sources that affect the stability of the system, which is not desirable. Therefore, the prerequisite for reducing ground noise is to reduce the impedance of the ground wire.
As we all know, the ground wire is the path for the current to return to the source. With the widespread application of large-scale integrated circuits and high frequency circuits, low-impedance ground wire design is particularly important in circuits. Here is a brief list of several commonly used grounding methods:
Single point grounding
Single point grounding, as the name implies, is to connect all loops in the circuit to a single, same reference potential point. As shown below.
Single point grounding can be divided into series grounding and parallel grounding. The way of single point grounding in series is simple, but there is a common ground wire, which leads to the existence of a common ground wire impedance. If a circuit with a large power difference is connected in series at this time, the mutual interference will be very serious. Parallel single point grounding can avoid the factor of common ground wire coupling, but each part of the circuit needs to lead the ground wire to the grounding point, which requires too many ground wires, which is not practical.
Therefore, in practical applications, a single point grounding method that combines series and parallel connections can be used. When drawing the PCB board, put the circuits that are not easy to interfere with each other on one layer, put the circuits that easily interfere with each other on different layers, and then connect the grounds of different layers in parallel. As shown below.
Single point grounding is in high-frequency circuits, because the ground wire is long and the impedance of the ground wire is an unavoidable factor, it is not applicable. What should we do? Multi point grounding will be introduced below.
Multi point grounding
When the frequency of the circuit is high, imagine how serious the high frequency signal will affect the peripheral circuit when it propagates along the ground wire, so all the circuits will should be connected to the ground nearby, the ground wire require shortest. At this time, the multi-point grounding will generate.
The purpose of multi point grounding is to reduce the impedance of the ground wire. In the high-frequency (under certain conditions) circuit, to reduce the impedance, there are two main considerations.
One is to reduce the resistance of the ground wire, and the other is to reduce the ground wire inductive impedance.
1. Reduce the resistance of the ground conductor. From the formula of the relationship between resistance and cross section, we know that the cross-sectional area of the ground conductor should be increased. However, in a high-frequency environment, there is a skin effect of high-frequency current, and high-frequency current will pass on the surface of the conductor, so simply increasing the cross-sectional area of the ground conductor is often not effective. You can consider plating the surface of the conductor with silver, because the conductivity of silver is better than other conductive materials, so it will reduce the conductor resistance.
2. The best way to reduce the inductive impedance of the ground wire is to increase the area of the ground wire.
In practical applications, the ground wire is short and the ground area is large, and the anti-interference effect will be better.
When I write here, some people may ask, what is a high-frequency circuit? Refer to Professor Yang Jishen’s book "Electromagnetic Compatibility EMC Technology", it is mentioned that usually 1MHZ or less is a low-frequency circuit, and single-point grounding can be used, and 10MHZ or more is a high-frequency circuit, multi point grounding can be used. At 1MHZ and 10MHZ, if the longest ground wire does not exceed 1/20 of the wavelength, it can be grounded at a single point, otherwise multiple points are grounded.
-How to do if there are both high-frequency signals and low-frequency signals in the circuit? -Mixed grounding would be a good choice!
Mixed grounding
The first structure in the above figure, assumes that it works in a low-frequency circuit, according to the capacitive reactance Zc = 1/2πfc, it can be known that the capacitive reactance is very large in a low-frequency environment, but very small in a high-frequency environment. Then the ground wire is disconnected at low frequencies and is close to conducting when subjected to high frequency interference. This connection method can effectively avoid the interference of the ground loop.
The second structure in the above figure, assumes that it works in a high-frequency circuit, according to the inductive reactance Zl = 2πfl, it can be seen that the inductive reactance is very small in a low-frequency environment, but very large in a high-frequency environment. Then the ground wire is similarly conductive at low frequencies, and disconnected when subjected to high frequency interference. This connection can effectively avoid the influence of ground loop current.
In summary, in practical applications, the circuit can effectively avoid interference signals by adopting a suitable grounding method according to the working environment and achieve the optimal effect of the circuit.
