Interference and Suppression of PCB Ground Wire

Interference and Suppression of PCB Ground Wire
In PCB design, especially in high frequency circuit, some irregularities and anomalies caused by ground wire interference are often encountered This paper analyzes the causes of ground wire interference, introduces three types of ground wire interference in detail, and puts forward solutions according to practical experience These anti-jamming methods have achieved good results in practical application, and some systems can operate successfully in the field The printed circuit board is an important component used to support circuit components and provide electrical connections between circuit components and devices There must be a certain impedance in the conduction process The inductance component in the wire will affect the transmission of voltage signal, and the resistance component will affect the transmission of current signal The influence of inductance is especially serious in high frequency circuits Attention must be paid to and the influence of grounding impedance must be eliminated
Printed circuit board

1. The reason for the interference
Resistance and impedance are two different concepts.  Resistance refers to the resistance of the conductor to the current in the DC state, and impedance refers to the resistance of the conductor to the current in the AC state, which is mainly caused by the inductance of the conductor Because the ground wire always has impedance, when measuring the ground wire with a multimeter, the resistance of the ground wire is usually mm Ω) Take a 10 cm long metal wire with a width of 1.5 mm and a thickness of 50 mm. The impedance can be calculated R=ÏL/s(Î ©),  Where L is the length of the wire (m), s is the cross sectional area of the wire (mm2), Ï is the resistivity Ï=0.02, so the resistance of this wire is about 0.026 Î ©.  When a wire is far away from other wires and its length is much greater than its width, the self inductance coefficient of the wire is 0.8mH/m, so the inductance of a 10cm long wire is 0.08mH Then calculate the induced reactance of the conductor by the following formula: XL=2 Ï fL. In the following formula, f is the frequency (Hz) of the signal passing through the wire, and L is the self induction of the wire per unit length (H) Therefore, in the actual circuit, the induced reactance value of the wire at low frequency and high frequency is calculated respectively. The signal causing electromagnetic interference is usually pulse signal, which contains rich high-frequency components. Therefore, it will produce a lot of noise on the ground wire Voltage From the above formula calculation, it can be seen that the wire resistance is greater than the wire inductance in low-frequency signal transmission For digital circuits, the operating frequency of the circuit is very high. In high-frequency signals, the lead inductance is much greater than the lead resistance Therefore, the grounding impedance has a great influence on digital circuits This is why a large voltage drop occurs when the current flows through a small resistor, resulting in abnormal circuit operation
2. Ground wire interference mechanism
2.1 Ground loop interference
Ground loop interference is a relatively common interference phenomenon that often occurs between devices that are connected by long cables and are far apart.  The main reason of electromagnetic interference caused by ground wire is the impedance of ground wire When the current flows through the ground wire, the ground wire will generate voltage, which is ground wire noise Driven by this voltage, the grounding circuit current will be generated, causing interference to the grounding circuit Grounding circuit interference: the grounding voltage is formed due to the different grounding potentials of the two equipment Driven by this voltage, the current flows between the loops formed by "device 1", interconnection cable, device 2, "And the ground". Because the circuit is unbalanced and the current on each wire is different, this will generate differential mode voltage, which will interfere with the circuit. Because the grounding circuit interference is caused by the grounding circuit current, it is sometimes found that when the grounding wire of the equipment is disconnected, the interference disappears, because the grounding circuit is cut off when the grounding wire is disconnected. This phenomenon often occurs in low-frequency interference occasions. When the interference frequency is high, it is not important whether the grounding wire is disconnected
2.2 Common Impedance Interference
In digital circuits, due to the high signal frequency, the ground wire usually presents large impedance At this time, when multiple circuits share a section of ground wire, due to the impedance of the ground wire, the ground potential of one circuit will be modulated by the working current of the other circuit. In this case, the signal in one circuit will be coupled to the other circuit, which is called common impedance coupling The solution to common impedance coupling is to reduce the impedance of the common ground wire, or use single point grounding to completely eliminate the common impedance, which is an interference phenomenon Figure 2 is a simple circuit with four gates Assuming that the output level of grid 1 changes from high to low, the parallel capacity in the circuit (values there is a filter capacitor at the input of gate 2) will discharge to the ground wire through gate 1. Due to the impedance of the ground wire, the discharge current will be the peak voltage generated on the ground wire If the output of grid 3 is low at this time, the peak voltage will be transmitted to the output of grid 3 and the input of grid 4 If the amplitude of the peak voltage exceeds the noise threshold of grid 4, gate 4 will fail
2.3 Ground loop electromagnetic coupling interference
The "ground loop" will surround a certain area.  According to the law of electromagnetic induction, if there is a changing magnetic field in the area around the coil, the circuit will generate induced current, causing interference The space magnetic field changes everywhere, so the larger the enclosed area, the more serious the interference
3. Methods to solve the ground wire interference
3.1 Solve the ground loop interference
There are three basic ideas to solve the ground loop interference: one is to reduce the interference of the ground wire, so as to reduce the interference voltage The second method is to change the grounding structure, connect the grounding wire of one host shell to the other host shell, and ground through the other chassis. This is the concept of single point grounding The third is to increase the impedance of the grounding loop, so as to reduce the current of the grounding loop When the impedance is infinite, the grounding loop is actually cut off, that is, the grounding loop is eliminated Therefore, the following solutions are proposed to solve the grounding circuit interference
1) Float the device on one side
If one side of the circuit is left floating, the grounding circuit is cut off, thus eliminating the grounding circuit current But there are two problems that need attention First, for safety reasons, circuits are not allowed to float In this case, consider grounding the equipment through inductors With this kind of pipeline, the grounding impedance of 50 Hz AC current equipment is very small. For interference signals with high frequency, the grounding impedance of equipment is large, reducing the grounding loop current But this can only reduce the ground loop interference of high frequency interference Another problem is that although the device is floating, there is still parasitic capacitance between the device and the ground This capacitor provides lower impedance at higher frequencies, which cannot effectively reduce the current of high-frequency grounding loop
2) Use a transformer
The basic method to solve the ground loop interference is to cut off the ground loop.  The isolation transformer is used for this purpose. The signal transmission between the two devices is realized through magnetic field coupling to avoid direct power connection At this time, the interference voltage on the ground wire appears between the primary and secondary of the transformer, not at the input end of the circuit One way to improve the high frequency isolation effect of the transformer is to set a mask layer between the primary and secondary of the transformer However, it must be noted that the grounding terminal of the isolation transformer shield must be located at the receiving end of the circuit Otherwise, it can not improve the high frequency isolation effect, but also make the high frequency coupling more serious Therefore, the transformer should be installed on the side of the signal receiving device The transformer isolation method has some disadvantages. It can not transmit direct current. It is bulky and expensive Due to the parasitic capacitance between the primary and secondary of the transformer, the high frequency isolation effect is not very good
3) Use Optical Isolation Elements
Transmitting signals with light is an ideal solution to ground loop problems.  As shown in Figure 3, the parasitic capacitance of the optocoupler device is about 2 pF, so it can be isolated at very high frequencies If optical fiber is used, there is no parasitic capacitance problem, and ideal isolation effect can be obtained However, the use of optical fiber will bring about other problems, such as: the demand for higher power, the demand for more peripheral equipment, the linearity and dynamic range of optical connection cannot meet the requirements of analog signals, and the installation and maintenance of optical cables are more complex Pay attention to the use of optical isolation components
4) Use a common mode choke
The ground wire voltage is actually a common mode voltage The common mode choke used on the connecting cable is equivalent to the impedance of the newly added grounding circuit, which reduces the current of the grounding circuit under the action of a certain grounding voltage However, attention should be paid to controlling the parasitic capacitance of the common mode choke, otherwise, the isolation effect of high-frequency interference is poor The more turns of the common mode choke, the greater the parasitic capacitance and the worse the high frequency isolation
5) Suppression of ground loop interference by balanced circuit
A balanced circuit is defined as two conductors and the circuit they connect to have the same impedance with respect to a ground wire or other reference object.  It is difficult to balance at high frequency, but the actual circuit will have many parasitic factors, such as parasitic capacitance, inductance, etc These parameters play a greater role in the circuit impedance at higher frequencies Because of the uncertainty of these parasitic parameters, the impedance of the circuit is also uncertain Therefore, at high frequency, the circuit balance is usually poor, which means that the balance circuit has poor suppression effect on current interference of high frequency grounding loop

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