PCB Technical - Ground interference and suppression in PCB layout design

1. Definition of ground wire

In PCB layout design, what is the ground wire? The definition of the ground wire that everyone learns in textbooks is: the ground wire is an equipotential body used as the reference point of the circuit potential. This definition does not conform to the actual situation. The potential on the actual ground wire is not constant. If you use a meter to measure the potential between the points on the ground line, you will find that the potentials of the points on the ground line may vary greatly. It is these potential differences that cause the abnormal operation of the circuit. The definition of a circuit as an equipotential body is only people's expectation of ground potential. HENRY gave a more realistic definition of the ground wire. He defined the ground wire as a low-impedance path for the signal to flow back to the source. This definition highlights the flow of current in the ground wire. According to this definition, it is easy to understand the cause of the potential difference in the ground wire. Because the impedance of the ground wire will never be zero, when a current passes through the finite impedance, a voltage drop will occur. Therefore, we should imagine the potential on the ground wire as waves in the sea, one after another.

2. The impedance of the ground wire

Speaking of the potential difference between the points on the ground wire caused by the impedance of the ground wire, which can cause the circuit to malfunction, many people find it incredible: when we use an ohmmeter to measure the resistance of the ground wire, the resistance of the ground wire is often in the milliohm level. How can such a large voltage drop occur when the current flows through such a small resistance, causing abnormal circuit operation.

3. Ground interference mechanism

3.1 Common impedance interference When two circuits share a section of ground wire, the ground potential of one circuit will be modulated by the working current of the other circuit due to the impedance of the ground wire. The signal in such a circuit will be coupled into another circuit, this coupling is called common impedance coupling.

In digital circuits, due to the higher frequency of the signal, the ground wire often presents a larger impedance. At this time, if there are different circuits sharing a section of ground, the problem of common impedance coupling may occur

4. Ground interference countermeasures

4.1 In the PCB design layout, ground loop countermeasures From the mechanism of ground loop interference, it can be known that as long as the current in the ground loop is reduced, the ground loop interference can be reduced. If the current in the ground loop can be completely eliminated, the problem of ground loop interference can be completely solved. Therefore, we propose the following solutions to ground loop interference.

A. Floating the equipment at one end. If the circuit at one end is floated, the ground loop will be cut off, so the ground loop current can be eliminated. But there are two issues that need to be paid attention to. One is that for safety reasons, circuits are often not allowed to float. At this time, consider grounding the device through an inductor. In this way, for 50Hz AC current equipment, the grounding impedance is very small, and for higher frequency interference signals, the equipment grounding impedance is larger, reducing the ground loop current. But doing so can only reduce the ground loop interference of high frequency interference. Another problem is that although the device is floating, there is still a parasitic capacitance between the device and the ground. This capacitance will provide a lower impedance at higher frequencies, so it cannot effectively reduce the high-frequency ground loop current.

B. Use a transformer to realize the connection between devices. Use a magnetic circuit to connect two devices to cut off the ground loop current. But it should be noted that the parasitic capacitance between the primary and secondary transformers can still provide a path for the higher frequency ground loop current, so the transformer isolation method has a poor suppression effect on the high frequency ground loop current. One way to improve the high-frequency isolation effect of the transformer is to install a shielding layer between the primary and secondary stages of the transformer. But it must be noted that the grounding end of the shielding layer of the isolation transformer must be at one end of the receiving circuit. Otherwise, not only cannot improve the high-frequency isolation effect, but also may make high-frequency coupling more serious. Therefore, the transformer should be installed on the side of the signal receiving device. A well-shielded transformer can provide effective isolation at frequencies below 1MHz.

C. Another way to cut off the ground loop using an optical isolator is to use light to realize signal transmission. This can be said to be the most ideal way to solve the problem of ground loop interference. There are two methods for optical connection, one is optocoupler device, and the other is connection with optical fiber. The parasitic capacitance of the optocoupler is generally 2pf, which can provide good isolation at very high frequencies. Optical fiber has almost no parasitic capacitance, but it is inferior to optocoupler devices in terms of installation, maintenance, and cost.

D. Using common mode chokes on the connecting cable is equivalent to increasing the impedance of the ground loop, so that under a certain ground voltage, the ground loop

The circuit current will decrease. But pay attention to control the parasitic capacitance of the common mode choke, otherwise the isolation effect on high frequency interference is very poor. The more turns of the common mode choke, the greater the parasitic capacitance and the worse the effect of high-frequency isolation.

4.2 Elimination of common impedance coupling There are two ways to eliminate common impedance coupling. One is to reduce the impedance of the common ground wire, so that the voltage on the common ground wire is also reduced, thereby controlling the common impedance coupling. Another method is to avoid the common ground wire of circuits that are easy to interfere with each other through proper grounding. Generally, avoid the common ground wire of strong and weak current circuits, and the common ground wire of digital circuits and analog circuits. As mentioned earlier, the core problem of reducing the impedance of the ground wire is to reduce the inductance of the ground wire. This includes using a flat conductor as a ground wire, and using multiple parallel conductors that are far apart as a ground wire. For printed circuit boards, laying a ground wire grid on a double-layer board can effectively reduce the ground wire impedance. In a multilayer board, a special layer of ground wire has a small impedance, but it will increase the PCB circuit board’s resistance. cost. The grounding method to avoid common impedance through proper grounding method is parallel single-point grounding. The disadvantage of parallel grounding is that there are too many grounded wires. Therefore, in practice, it is not necessary for all circuits to be connected in parallel with single-point grounding. For circuits with less mutual interference, single-point grounding in series can be used. For example, the circuits can be classified according to strong signals, weak signals, analog signals, digital signals, etc., and then use single-point grounding in series within similar circuits, and single-point grounding in parallel for circuits of different types.