1. What is a common-mode signal and what is a differential-mode signal?
Let's start with the two kinds of household equipment that everyone is more familiar with, one is called air circuit breaker, and the other is called leakage protector.
Air circuit breaker is commonly known as "air open", the main role is short circuit protection and overload protection, simply put, when L, N short circuit or the load is too heavy, resulting in excessive current I, so as to disconnect the switch, cut off the power supply, to protect the equipment. Its main function is to protect the line and equipment, and it cannot protect the human body from electric shock, because usually the normal working current (ampere level) of the equipment is far greater than the limit current (milliampere level) that the human body can withstand.
Figure 1 - Air circuit breaker
As a result, people have invented a device called "leakage protector", so how does the leakage protector protect the human body?
When the equipment is working normally, the current on the live wire and the neutral line should be one in and one out, equal in size and opposite in direction, but at this time, someone accidentally touches the live wire, and it is even more unfortunate that his bare feet are directly connected to the earth, then there must be a part of the current flowing to the earth through the human body. When the leakage protector finds I+≠I-, then it must know that a part of the current leaks to the earth through other means, and when the leakage current value exceeds a certain value (milliampere level), it will automatically disconnect the switch to prevent the human body from continuing to be electrocuted.
Figure 2 - Leakage protector
This is the basic principle of the two types of switches, and the concept of differential-mode signals and common-mode signals is embedded in these two switching models.
To put it simply, the air switch protects the differential mode signal not to exceed the standard, and the leakage switch protects the common mode signal not to exceed the standard.
2. Common mode signals and differential mode signals in electronic products
Now, we need to turn our attention away from the application in our lives and back to the design of electronic products. According to Ampere's loop law, the current always needs a complete loop, and the current has to pass through the complete loop back to the source.
Figure 3 - Current drive vs. return path
In this loop, there are two transmission paths of the signal, one is the drive path, and the current is from the driving end to the receiving end. That is, we draw a variety of signal networks on the schematic diagram, the other is to return the power supply from the receiving end, called the return path, look at the figure above, the signal transmission path, there are driving paths and return paths, on these two paths, the current is equal, and the direction is opposite.
The differential mode current is the normal working current of the product, and the useful signal it transmits is the current that the equipment really needs.
GND in the schematic diagram below means the common end, which can also be said to be "land", but this "land" is not a "land" in the real sense. It is a hypothetical "ground" for the purpose of application, and for a power supply, it is the negative pole of a power supply.
Figure 4 – Common side in the circuit diagram
Let's talk about common-mode signals.
There is also a kind of current in the circuit, the common mode current, which is like a dogskin plaster, like a shadow of a shadow, just like the leakage current in the leakage protector, which is a useless signal and a scourge. Let's take a look at the common mode current, first look at the high-frequency distribution parameter effect, the high-frequency distribution parameter effect, in any circuit, any two conductors will form a distributed capacitance.
Figure 5 - Common mode and differential mode circuits
As long as it is a conductor, there is a distributed capacitance, so it is inevitable that the working circuit will form a distributed capacitance between the surrounding conductors, such as the chassis, the horizontal reference grounding plate, the I/O cable, and even the human body will form a distributed capacitance, and the distributed capacitance exists at any time, but the size is different. These distributed capacitors provide a path for the AC signal, so that a portion of the current escapes from the differential mode loop and returns to the signal source through the capacitor.
As shown in Figure 5 above, the green reflow path is the current loop required for the normal operation of the product, which is predictable and easy to perceive because they are equal in size and opposite in direction, which is called differential mode current. But another due to the existence of the equivalent distributed capacitance, the reflow path, they are unpredictable, not pre-designed, difficult to measure, they pass through the entire path on the circuit board, including the differential mode drive path and return path, through the distributed capacitance, and finally flow back to the power supply side, because they are in the same direction, so it is called a common mode signal.
Summary: Common mode is a subsidiary product of differential mode, which should not appear in the first place, and has nothing to do with the work of our circuit, that is, the common mode current is the current that is not needed when the product is working, and it is a useless interference. Like the leakage current in a leakage protector, we don't want it, so we need to think about how to suppress the generation of common-mode signals as much as possible when designing the circuit.
3. Three ways to generate common-mode interference
There are at least two transmission lines that transmit power or signals between two devices, one to one and one to the loop, as shown in Figure 6
Figure 6 - Schematic diagram of signal transmission
There are generally three types of common mode interference that may occur during the transmission of power or signal.
First, the external electromagnetic field induces a voltage on the transmission line 1 and the transmission line 2 at the same time, and the induced voltage generates an induced current and propagates on the transmission line. This method is caused by third-party interference sources, such as the equipment in a strong magnetic field environment, which is why the product is designed for electromagnetic compatibility.
Figure 7 - Common-mode interference generated by third-party interferers
Second: the grounding points of device 1 and device 2 GND1 and GND2 are not the same, resulting in a voltage difference between the actual two devices, resulting in the induced current propagating on the two transmission lines, in this way as the reason for the damage of the PHY chip mentioned in the previous article - why is the PHY chip always broken inexplicably? As shown in Figure 8:
Figure 8 - Common-mode interference due to ground potential differences
The third type: there is a potential difference between the transmission line 1 and the transmission line 2 and the ground, that is, the two wires do the way and the ground wire does the way, so that there will also be a common mode current on the cable, as shown in Figure 9. There is no third-party interference source in this method, but the high-frequency signal belonging to the device itself is transmitted to the ground through a non-designed path, which is also difficult to troubleshoot.
Figure 9 - High-frequency signals from the device's own products are transmitted to the ground
4. How to reduce common-mode interference
The following methods are commonly used to reduce the effects of common-mode interference:
1. Shielding and insulation: Use shielded cables or shielding boxes to prevent external interference signals from entering the signal lines or circuits.
2. Differential signal: Differential signal transmission is used, where the signal is composed of two opposing signal lines, and the common-mode interference signal will have the same effect on both lines, so that it can be canceled out at the receiving end.
3. Ground design: Optimize the ground wire and grounding system to reduce ground noise and common mode interference.
In order to bypass the common mode current to ground, the ground plane of the printed circuit board can be split close to the connector as a "noise-free" input/output ground, and in order to avoid contamination of the input/output ground, only the decoupling capacitor of the input/output line and the shield of the external cable can be connected to the "noise-free" ground, and the electrical induction of the decoupling loop is as small as possible. In this way, the common-mode current of the printed circuit board carried by the input/output lines is bypassed to ground by the decoupling capacitor, and external interference is also bypassed to ground by the decoupling capacitor before it reaches the component area, thus protecting the normal operation of the internal components.
4. Use a common-mode inductor to increase the common-mode impedance, thereby reducing the common-mode current, such as the common-mode inductor L in the figure below.
Figure 10 - Common mode inductance
The two wires are wound in the same direction on the ferrite magnetic ring to form a common-mode choke, and the DC and low-frequency time-difference mode currents can pass through, but for the high-frequency common-mode current, the impedance is very large, and it can be seen from the equivalent circuit of the common-mode radiation model that the increase of the common-mode impedance leads to the decrease of the common-mode radiation current, so that the common-mode radiation field strength is suppressed.