Grounding Technology in Circuit Design

1 Introduction

In circuit design, the importance of grounding is well known. Many problems in electronic products are actually closely related to the treatment of grounding, such as noise and interference problems in small signal systems, radiation spurious and stability problems in large signal systems, etc., all of which are caused by improper grounding. This article analyzes the commonly used grounding methods, hoping to avoid the losses we have suffered in this regard.

2. Generation of interference

Let's use an example to analyze how the ground wire interferes with the operation of the circuit.

System A, system B and system C have a common loop R. Assuming that system C is the interference source loop, the noise current I3 generates an interference voltage I3R, thereby affecting system A and system B.

Figure 1 Generation of interference in the circuit

In Figure 1, let E = 24V, R1 = R2 = 3Ω, R = 2Ω, and R3 is variable. When R3 = 3Ω, it is easy to find the total resistance of the circuit:

The total current is 8A, and the current flowing through R1 and R2 is 2.6A. When R3 = 6Ω, it can be calculated that the current flowing through R1 and R2 is 3A. This shows that the load change or current change of system C will affect system A and system B, causing the current flowing through R1 and R2 to change from 2.6A to 3A. The reason for this change is that the three branches have a common loop R, and the change of system C causes changes in system A and system B. If system C is the interference source, then system A and system B are interfered with.

3. Interference judgment

When encountering interference problems, the first thing to do is to find out the frequency of the interference, and the second is to determine where the interference source is. This can be measured using instruments such as a spectrum analyzer or oscilloscope. The methods for determining interference are:

① By detecting parameters such as ground impedance, loop current and ground voltage;

② The interference source can be measured with a spectrum analyzer. First, make a probe and weld a wire loop at the end of the coaxial cable, which is also called a near-field probe. The loop is connected to the coaxial cable, and the coaxial cable is connected to a preamplifier and then to the input of the spectrum analyzer. Using the spectrum analyzer as a receiver, the location and size of the main interference source in a specific frequency area can be found.

From actual tests and theoretical analysis, we can conclude that ground interference is mainly caused by ground loop interference and common impedance interference. The so-called ground loop interference is that the ground voltage causes the ground loop current. The current on each branch is different, so a differential mode voltage will be generated, causing interference to the circuit. In addition, since the equipment is in a strong electromagnetic field, the loop current is induced in the loop formed, which will also cause this situation. The so-called common impedance interference is that when two circuits share a ground line, under the action of the ground impedance, the ground potential of one circuit will be modulated by the working current of the other circuit, causing mutual interference.

4 Ground interference suppression

The commonly used method to suppress ground interference is to use a suitable grounding structure and take effective measures such as isolation, shielding, decoupling and filtering.

4.1 Grounding structure

The so-called grounding structure is the grounding method. Common grounding methods are:

① Each branch or equipment is not directly grounded, so all process loops are connected and grounded, that is, there is only one grounding point and no loop;

② Make the potential of the grounding points equal, reduce the impedance of the ground wire, and thus reduce the interference voltage;

③ Use signal isolation or an isolator in the loop to disconnect the loop without affecting normal signal transmission.

4.2 Signal Isolation

The isolation between signal ground lines is very important in practice. If the interference signal is mainly a high-frequency signal, the high frequency can be effectively isolated by using low-pass filtering technology, such as first-order or second-order resistance-capacitance filtering. However, if the potential between the ground lines of the two signals is not equal, only a signal isolator can be used to isolate the ground lines between the signals.

5. Grounding method

Choose a reasonable grounding method according to the actual working environment, working frequency, grounding structure and other requirements. Common grounding methods include: floating grounding, single-point grounding, multi-point grounding, etc.

5.1 Floating grounding

The circuit has no reference ground level, and the circuit itself has an independent zero potential. This grounding is called floating grounding. Due to its independence, it can avoid noise and electromagnetic interference with other circuits, but the electrostatic charge cannot be effectively released. Therefore, floating grounding is only suitable for some low-frequency electronic equipment and is often used in circuits without power transformers.

5.2 Single point grounding

Single-point grounding means concentrating the grounding wires of each circuit at one point for grounding. Their respective potentials are only related to their own grounding resistance and ground current, and do not interfere with each other. In practice, a common bus is often used to achieve single-point grounding. Under normal circumstances, they are only connected at the power supply point. Each circuit is grounded independently, which can ensure that each system has a unified ground potential and avoid the formation of common impedance by the ground wire. There is no loop in the entire grounding channel, which avoids the interference of ground loop current caused by the external magnetic field and improves the performance of the circuit.

5.3 Multi-point grounding

For the signal ground of high-frequency systems, the nearest grounding is often adopted, which is called multi-point grounding; at the same time, short and thick wires are used as connecting wires to reduce the ground impedance. The purpose is to prevent interference caused by ground inductance and capacitance, but due to the potential difference between the grounding points, it will cause large common mode noise and is not suitable for low-frequency operation. In a multi-level circuit with multi-point grounding, the grounding must be done in the order of the ground current flowing from the small signal unit to the large signal unit, otherwise it will cause interference.

5.4 Grounding in practical circuits

In actual work, we will encounter many complex situations, such as the grounding of A/D converters, the grounding of shielding lines, the grounding of PCB boards, etc. The specific processing methods are analyzed as follows.

5.4.1 Grounding of analog and digital circuits

The current of digital signals is relatively strong, and they are all high-level and low-level jumps, so there are large noises and current spikes on the digital ground; while the current of analog signals is relatively weak. Therefore, special attention should be paid to the correct connection of the ground wire in analog-to-digital and digital-to-analog conversion circuits, otherwise the interference will be very serious, thus affecting the accuracy of the conversion results. Independent analog ground and digital ground pins are provided on A/D and D/A chips. In circuit design, the analog ground and digital ground of all devices are usually connected separately, and then the analog ground and digital ground are connected only at one point.

In the design of PCB boards [10], the power and ground wires of analog and digital circuits should be widened as much as possible or separate power and wiring layers should be used to reduce the impedance of the power and wire loops and any interference voltage that may be in the power and ground loops. The analog and digital grounds of the independently working PCB can be grounded at a single point near the system ground point. If the PCB board is inserted into the motherboard, the power grounds of the analog and digital circuits of the motherboard should also be separated, and the analog and digital grounds should be grounded at the motherboard ground point. In high-frequency circuits, the ground lead also has a certain impedance. Whether it is single-point grounding or multi-point grounding, it must form a low-impedance loop to enter the real ground. A 25mm long PCB copper wire will have an inductance of about 15nH-20nH. With the presence of distributed capacitance, a resonant circuit will be formed between the ground plane and the chassis, which will produce transmission line effect and antenna effect when flowing through the ground wire. For the power ground wire in the PCB board, the power plane should be close to the ground plane and arranged below the ground plane. In this way, the capacitance between the two metal plates can be used as a smoothing capacitor for the power supply, and the ground plane also shields the radiation current distributed on the power plane. The power line and ground line of a single-sided or double-sided board should be as close as possible. The best way is to lay the power line on one side of the PCB board and the ground line on the other side, which will minimize the impedance of the power supply.

5.4.2 Grounding of the measurement platform

When using electronic instruments to measure circuits, especially in a test platform composed of multiple instruments, adverse consequences often occur due to improper grounding, shielding and protection. In the worst case, the instrument cannot work properly, and in the worst case, it causes damage to personnel and instruments. For example, leakage current, contact resistance and transition process are all interferences to the instrument.

Figure 2 shows a typical way of powering instruments and equipment from the mains.

Figure 2 Mains power supply system

In the figure, the power supply current flows through the live wire, the primary coil of the transformer and the neutral wire. To ensure that the casing and the earth are at the same potential, the casing of the instrument and equipment is connected to the earth. The purpose is to allow the leakage current after a fault to flow to the earth through the low-resistance grounding wire to protect the safety of personnel. The neutral wire is also connected to the earth at the power distribution point, but the neutral wire cannot be connected to the casing and must be separated. The purpose is to prevent the power supply current from returning to the ground through the grounding wire.

In fact, the grounding wire also has distributed resistance. Although it is very small, voltage will be generated, causing a slight potential difference between the various parts of the casing. When the low-potential line of the signal line is not separated from the grounding line, part of the current flowing into the grounding line passes through the low-potential line of the signal line, generating voltage on it and superimposing it on the signal, which can seriously damage the instrument. We once did not pay attention to this when building the test platform, causing the probe of the peak power meter to burn out.

The general grounding principle of the instrument system is shown in Figure 3. In the figure, the input signal is connected to the power ground point, so when measuring, the signal ground point at the input end must not be directly short-circuited with a point with a voltage difference with the earth. Therefore, in a grounding circuit without a power isolation transformer, if the AC power is directly introduced into the circuit, a loop is formed between the circuit and the earth, and a potential is formed between the circuit and the earth. If the power plug is inserted backwards, there will be a high potential difference between the neutral line and the earth, which will cause serious accidents. Therefore, before connecting the input point and starting to measure, it is necessary to check carefully.

Figure 3 Instrument input and power grounding

There are also instruments that are connected in a suspended manner, with the input end suspended from the ground. Under ideal conditions, as long as the voltage difference between the two input ends is within an acceptable range, any input end can be connected to any voltage.

In short, when building a system measurement platform, try to plug the power cables of each instrument into the same power socket. This can reduce the grounding resistance, that is, reduce the generation of interference. Plug high-power instruments and equipment into another power bus, use short signal connection lines, use low-resistance cables, and preferably coaxial cables. Never use instruments and equipment with the chassis ground wire connected to the ground end of an ungrounded power plug socket.

5.4.3 Shielding cover grounding

Circuits that are susceptible to electromagnetic radiation interference, such as various signal sources and amplifiers, usually require shielding covers. Since there is parasitic capacitance between the signal circuit and the shielding cover, the signal circuit ground wire must be connected to the shielding cover to eliminate the influence of the parasitic capacitance, and the shielding cover must be grounded to eliminate common-mode interference.

For the grounding of the cable shield, such as the coaxial cable used in closed-circuit television, the metal mesh outside is used to shield the signal. There are eight thin metal wires wound inside the cable, four of which play a shielding role to ensure the correct grounding of the digital signal. Usually, the single-point grounding method is often used for the grounding of the cable shield of low-frequency circuits, and the grounding point is generally the negative pole of the power supply. For the grounding of the shield of high-frequency circuits, multi-point grounding is often used. For multi-layer shielded cables, each shield layer is grounded at a single point, but the shield layers should be insulated from each other. When the system needs to suppress electromagnetic interference, the entire system should be shielded and the shield body should be connected to the system ground.

6 Conclusion

Different methods should be used for grounding design according to the actual situation. No matter which grounding method is used, the purpose is to achieve "zero" impedance, which can completely avoid the introduction of interference. Therefore, in different types of circuits, according to the characteristics of the circuit, the correct choice of single-point grounding and multi-point grounding should be made. If necessary, a mixed grounding method can also be used.

References

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About the Author

Yao Xiaoping (1960-), male, associate professor/senior engineer, applied electronic technology and testing technology.