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Grounding interference and suppression

Release time:

2014-11-28 00:00

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1 . Definition of ground wire

What is a ground wire? The definition of the ground wire that everyone learns in textbooks is: the ground wire is an equipotential body that serves as the reference point of the circuit potential. This definition is not in line with the actual situation. The potential on the actual ground is not constant. If you use a meter to measure the potential between the points on the ground, you will find that the potential of each point on the ground may vary greatly. It is these potential differences that cause the abnormal operation of the circuit. The definition of a circuit being an equipotential body is only one's expectation of the ground potential.

HENRY gave a more realistic definition of the ground line, he defined the ground line 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 is never zero, when a current flows through a finite impedance, a voltage drop occurs.

2 . Ground impedance

When it comes to the potential difference between the points on the ground wire caused by the impedance of the ground wire can cause the malfunction of the circuit, 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 at the milliohm level , How can such a large voltage drop be generated when the current flows through such a small resistance, resulting in abnormal operation of the circuit.

To clarify this problem, we must first distinguish between two different concepts of resistance and impedance of the wire. Resistance refers to the resistance of the wire to the current in the DC state, while impedance refers to the resistance of the wire to the current in the AC state. This impedance is mainly caused by the inductance of the wire. Any wire has inductance. When the frequency is high, the impedance of the wire is much greater than the DC resistance. The data given in Table 1 illustrates this problem. In actual circuits, the signal that causes electromagnetic interference is often a pulse signal, and the pulse signal contains rich high-frequency components, so a large voltage will be generated on the ground. For digital circuits, the operating frequency of the circuit is very high, so the impact of ground wire impedance on digital circuits is relatively serious .

Table 1 Wire Impedance ( Ω )

If the impedance at 10Hz is approximately considered as DC resistance, it can be seen that when the frequency reaches 10MHz , for a 1- meter-long wire, its impedance is 1000 to 100,000 times the DC resistance . So for radio frequency current, when the current flows through the ground, the voltage drop is very large.

It can also be seen from the table that increasing the diameter of the wire is very effective in reducing the DC resistance, but it has a limited effect on reducing the AC impedance. But in electromagnetic compatibility, people are most concerned about AC impedance. In order to reduce the AC impedance, an effective way is to connect multiple wires in parallel. When two wires are connected in parallel, the total inductance L is:

L = ( L1 + M ) / 2

In the formula, L1 is the inductance of a single wire, and M is the mutual inductance between two wires. It can be seen from the formula that when two wires are far apart, the mutual inductance between them is very small, and the total inductance is equivalent to half of the inductance of a single wire. Therefore, we can reduce the ground impedance through multiple ground wires. But it should be noted that the distance between multiple wires cannot be too close.

3 . Ground Interference Mechanism

3.1 Ground loop interference

Figure 1 is a circuit with two grounds. Due to the existence of the impedance of the ground wire, when the current flows through the ground wire, a voltage will be generated on the ground wire. When the current is large, this voltage can be large. For example, when a high-power electrical appliance is started nearby, a strong current will flow through the ground wire. This current creates a current in the connecting cable between the two devices. Due to the unbalanced nature of the circuit, the current on each wire is different, so a differential mode voltage is generated, which affects the circuit. Since this interference is generated by the loop current formed by the cable and the ground wire, it is called ground loop interference. The current in the ground loop can also be induced by the external electromagnetic field.

Figure 1 Ground loop interference

3.2 Public Impedance Interference

When two circuits share a ground wire, the ground potential of one circuit will be modulated by the operating 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, as shown in Figure 2 .

Figure 2 Common Impedance Coupling

In digital circuits, due to the high frequency of the signal, the ground wire often presents a relatively large impedance. At this time, if different circuits share a ground wire, the problem of common impedance coupling may occur. The example in Figure 3 illustrates a disturbance phenomenon.

Figure 3 is a simple circuit composed of four gate circuits. Assuming that the output level of gate 1 changes from high to low, the parasitic capacitance in the circuit (sometimes there is a filter capacitor at the input end of gate 2 ) will discharge to the ground through gate 1 , and the discharge current will be at A spike voltage is generated on the ground. If the output of gate 3 is low at this time, the spike voltage will be transmitted to the output of gate 3 and the input of gate 4. If the magnitude of the spike voltage exceeds the noise of gate 4 Threshold, it will cause door 4 malfunction.

Figure 3 Circuit malfunction caused by ground wire impedance

4 . Ground Interference Countermeasures

4.1 Ground loop countermeasures

It can be seen from the mechanism of ground loop interference 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. Float the device at one end

If you float one end of the circuit, you cut off the ground loop, thus eliminating the ground loop current. However, there are two issues that need attention. One is that for safety reasons, the circuit is often not allowed to float. At this time, you can consider grounding the device through an inductor. In this way, the grounding impedance of the equipment for the 50Hz AC current is very small, but for the interference signal with a higher frequency, the grounding impedance of the equipment is relatively large, which reduces 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

Connecting two devices using a magnetic circuit cuts off ground loop currents. However, it should be noted that the parasitic capacitance between the primary and secondary stages of the transformer can still provide a path for the high-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 set a shielding layer between the primary and secondary stages of the transformer. However, it must be noted that the grounding end of the shielding layer of the isolation transformer must be at the receiving end of the circuit. Otherwise, not only cannot improve the high-frequency isolation effect, but also may make the high-frequency coupling more serious. Therefore, the transformer should be installed on one side of the signal receiving equipment.

A well-shielded transformer can provide effective isolation at frequencies below 1MHz .

C. Use an optoisolator

Another way to break ground loops is to transmit signals using light. This can be said to be the most ideal way to solve the problem of ground loop interference. There are two ways to connect with light, one is an optocoupler device, and the other is to connect with an 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 a common mode choke

Using a common mode choke coil on the connecting cable is equivalent to increasing the impedance of the ground loop, so that under a certain ground voltage, the ground loop current will decrease. But attention should be paid to controlling the parasitic capacitance of the common mode choke coil, otherwise the isolation effect on high-frequency interference is very poor. The more turns of the common mode choke coil, the larger the parasitic capacitance and the worse the effect of high frequency isolation.

4.2 Eliminate common impedance coupling

There are two ways to eliminate the common impedance coupling, one is to reduce the impedance of the common ground part, so that the voltage on the common ground is also reduced, thereby controlling the common impedance coupling. Another method is to prevent circuits that are prone to mutual interference from sharing the ground wire through appropriate grounding methods. Generally, strong current circuits and weak current circuits should be avoided from sharing ground wires, and digital circuits and analog circuits should share ground wires.

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 flat conductors as ground wires and using multiple parallel conductors spaced apart as ground wires. For printed circuit boards, laying the ground grid on the double-layer board can effectively reduce the impedance of the ground wire. Although the ground wire in the multi-layer board has a small impedance, it will increase the cost of the circuit board. .

A grounding method that avoids common impedance through proper grounding is a parallel single-point ground, as shown in Figure 4 . The disadvantage of parallel grounding is that there are too many grounding wires. Therefore, in practice, it is not necessary for all circuits to be connected in parallel to single-point grounding. For circuits with less mutual interference, series single-point grounding can be used. For example, circuits can be classified according to strong signal, weak signal, analog signal, digital signal, etc., and then use series single-point grounding in similar circuits, and parallel single-point grounding for different types of circuits, as shown in Figure 5 .

Figure 4 Parallel single point grounding

Figure 5 series-parallel hybrid single-point grounding

5 . summary

The main reason for the electromagnetic interference caused by the ground wire is the impedance of the ground wire. When the current flows through the ground wire, a voltage will be generated on the ground wire, which is the ground wire noise. Driven by this voltage, a ground loop current will be generated, forming ground loop interference. When two circuits share a ground wire, a common impedance coupling is formed.

The methods to solve the ground loop interference include cutting off the ground loop, increasing the impedance of the ground loop, and using a balanced circuit. The way to solve the common impedance coupling is to reduce the impedance of the common ground part, or use parallel single-point grounding to completely eliminate the common impedance.

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