Table of contents
1. Reduce the ground loop area
2. Single-point grounding and multi-point grounding
3. Impedance coupling on ground
4. Reduce the impact of ground loops
Signal Ground Design
Today we will talk about the grounding problem in circuit design.
In the electrical system, grounding involves two concepts, one is safety grounding, and the other is signal grounding; the former is generally used in strong electric equipment, and the shell is grounded to prevent people from getting an electric shock; the latter is the return path of signals in the circuit. Here we mainly talk about signal grounding , and I will write another special topic about safety grounding.
1. Reduce the ground loop area
First, let's talk about the concept of the current loop.
See the figure below, assuming that there is a circuit on the top layer of the printed board, the source sends out a signal, and the load is connected through a wire, while the ground plane is on the bottom layer of the printed board, and the circuit is connected to the ground plane of the bottom layer through vias A and B:
The current I of the signal is from the source terminal, through B to ground, from the ground plane to A to the load, and finally back to the negative terminal of the source. The path traveled by the current is a complete loop. It is essentially a circular path through which electricity flows.
We understand the phenomenon of electromagnetic induction: If a changing magnetic field passes through the area enclosed by a toroidal coil, a current will be induced in the coil. When the surrounding circuit is working, changing electric and magnetic fields are unavoidable, and the current induced in the loop is harmful interference. Therefore, reducing the loop area of the circuit as much as possible can reduce this part of interference .
In a low frequency circuit, as shown in the diagram below, current will flow along the path of least resistance:
Therefore, we only need to make the area enclosed by the entire loop as small as possible when wiring (the position of the via hole can be moved, and the position of the top-layer device can also be moved to reduce the length of the trace).
At high frequencies, the situation is different. As shown in the figure below, the current will flow along the path of least inductance, which is the path of the adjacent layer directly opposite the top layer wire:
At this time, as long as the ground plane is adjacent to the signal layer and is complete , it is the minimum area of the loop. What we need to do is not to cut off the formation on the path of its current flow.
2. Single-point grounding and multi-point grounding
We know that the ideal ground plane is a good conductor with equal potential everywhere and no impedance. But in reality, it has resistance and reactance. When the current in the circuit is larger, the resistance will cause a significant voltage drop, making the level of each ground point different; when the frequency of the signal in the circuit is higher, the conductor as the ground plane will have a more obvious inductance effect, resulting in a larger of inductive resistance. This leads us to the first question of our grounding design: single-point grounding and multi-point grounding.
The following figure shows the form of single-point grounding and multi-point grounding:
As can be seen from the figure, multi-point grounding means that each circuit unit is directly connected to the ground plane, while single-point grounding means that the grounds of each circuit unit are first connected together, and then connected to the ground plane through one point. So what is the difference between single-point grounding and multi-point grounding, and where are they used?
We know that the ground wire is also a wire, which can be equivalent to the following form: resistance and inductance in series.
The higher the frequency of the signal, the greater the inductive reactance generated by the inductance L. When the line length is an odd multiple of 1/4 wavelength, a more serious antenna effect will be formed, and the grounding impedance will be very high. Therefore, in this case, to connect the ground of the circuit to the ground plane (the shorter the ground wire, the smaller the equivalent L), it is suitable to use multi-point grounding. When the signal frequency is higher than 100kHz, or when digital circuits are involved, it is recommended that the circuit module be grounded nearby to minimize the impedance of the ground . The length of the ground wire is generally less than 1/20 wavelength.
In the low-frequency signal circuit, in order to reduce the ground loop conveniently, it is more recommended to use single-point grounding.
When there are both high frequency and low frequency in the circuit, a mixed grounding method can be used. The following figure is a way to achieve mixed grounding. The low frequency circuit is grounded through a wire, and the signal of the high frequency circuit can be coupled to the ground through a capacitor nearby:
3. Impedance coupling on ground
In the single-point grounding system in series form, the problem of impedance coupling may occur, as shown in the following figure:
The three circuits 1, 2, and 3 are grounded at a single point in series, and the resistances of each section of the grounding line are R1, R2, and R3.
At this time, the potential of point A is Va=(I1+I2+I3)*R1,
And the potential of point C is Vc=Va+(I2+I3)*R2 + I3*R3,
It can be seen that the potentials of points A and C are different, that is to say, in this system, the potentials of the actual grounding points of each circuit are different!
It is also easy to see that the potential of the ground point of circuit 3 will be affected by circuit 1, that is to say, the potential of the ground point of the circuit will be affected by other circuits! This problem can be solved by changing the connection method, and replacing it with a parallel single-point grounding method, which can remove the potential influence of other circuits on this circuit.
Another example of common impedance coupling is shown in the figure below. It is easy to see that when there is a common ground line, the signals on the load RL1 and load RL2 are no longer separate VS1 or VS2, and are generated by the impedance Zg of the common ground line. coupling:
This coupling needs to be solved by minimizing ground impedance.
4. Reduce the impact of ground loops
The concept of the ground loop is shown in the figure below:
Assuming that there are two systems that are far apart to transmit signals, and the two systems are grounded nearby, and the internal circuits are not designed to float, then the ground and the signal lines of the system form a large loop.
There may be a variety of interference sources in this system: the voltage difference VG generated by the different potentials of the two grounding points may change with the current change in the ground when the surrounding electrical appliances are running; the ground loop is easily disturbed by the external changing magnetic field; There are multiple return paths for the signal at low frequencies...
At this time, because the two systems are far apart, the method of reducing ground impedance is not suitable; and if the signal is a high-frequency signal, it cannot be solved by single-point grounding. At this point there are several solutions:
A) Optocouplers and transformers are used to isolate the transmitted signal, and the ground loop is cut off;
B) The common mode choke coil is used in series on the transmission path. Since the common mode choke coil has no effect on the differential mode signal, it only isolates the common mode signal, which can remove the common mode interference generated by the ground loop;
C) Using differential transmission , the receiving end only recognizes differential signals and suppresses common mode signals.
Original link: https://blog.csdn.net/little_grapes/article/details/129253222