When designing a product, it is most economical to consider grounding during design. A well-designed grounding system protects against radiation and system susceptibility not only from the PCB, but also from a system perspective.

Important areas of concern regarding grounding systems include:
① The area of the current loop is reduced or minimized by careful placement of high-frequency components.

② When partitioning the PCB or system, separate high-bandwidth high-frequency circuits from low-frequency circuits.

③ When designing the PCB or system, the interference current does not affect other circuits through the common ground loop.

④ Carefully select the ground point to minimize the loop current, ground impedance and transfer impedance of the circuit.

⑤ Consider the current through the ground system as noise injected into or out of the circuit.

⑥ Connect very sensitive (low noise tolerant) circuits to a stable ground reference.

First of all, let's understand the classification and related definitions of grounding. According to the different functions of grounding, the "ground" of the equipment is divided into the following three categories:

Work place

The working ground is a reference potential provided for the normal operation of the circuit. The reference potential can be set as a certain point, a certain segment or a certain block in the circuit system. When the reference potential is not connected to the ground, it is regarded as a relative zero potential. This relative zero potential will change with the change of the external electromagnetic field, resulting in unstable operation of the circuit system. When the reference potential is connected to the ground, the reference potential is regarded as the zero potential of the ground and will not change with the change of the external electromagnetic field.

According to the nature of the circuit, the working ground is divided into different types, such as DC ground, AC ground, digital ground, analog ground, signal ground, power ground, power ground, etc. The above-mentioned different grounds should be set separately.

Here we focus on signal ground and power ground:

The signal ground is the common reference ground for the zero potential of the sensors and signal sources of various physical quantities. A good definition of signal ground is a low impedance path for the signal to flow back to the source. This definition highlights the flow of current. When current flows through a finite impedance, a voltage drop is bound to occur, so this definition reflects the potential on the actual ground wire.

The power ground is the zero-potential common reference ground of the load circuit or the power drive circuit. Because the current and voltage of the load circuit or power drive circuit are relatively strong, the interference on the power ground wire is relatively large. Therefore, the power ground must be set separately from other weak current grounds.

safely

Safety grounding means connecting the chassis to the earth. One is to prevent the accumulation of electric charge on the casing, resulting in electrostatic discharge and endangering equipment and personal safety; the other is to promote the protective action of the power supply and cut off the power supply when the insulation of the equipment is damaged and the casing is charged, so as to protect the safety of the staff.

Sometimes, lightning rods are set up to prevent lightning strikes to prevent equipment and personal safety from being endangered when lightning strikes. For safety reasons, it should be directly connected to the ground.

Grounding required for electromagnetic compatibility

In addition to equipment work and safety protection, sometimes designers need to take some measures for equipment, such as shielding, filtering, etc., which also need to be grounded, so the following ground is sometimes used.

(1) Shield grounding: necessary isolation and shielding must be performed between circuits due to parasitic capacitance, and the ground wire for these isolated and shielded metals must be provided. Shielding and grounding should be used together to achieve the shielding effect. The shield grounding mainly includes the grounding of the shielding cover of the circuit, the grounding of the shielding layer of the cable and the grounding of the shielding body of the system.

(2) Filter grounding: The filter generally includes bypass capacitors from signal lines and power lines to the ground, which are provided to the grounding of these bypass capacitors. If they are not grounded, these bypass capacitors will be in a suspended state and will not work. The role of bypass.

(3) Noise and interference suppression: The control of internal noise and external interference requires that many points on equipment and systems be connected to ground, thereby providing the lowest impedance channel for interference signals.

(4) Electrostatic grounding: non-conductive conductor devices are grounded to discharge static electricity and prevent radio waves from being re-emitted.

According to the above grounding classification, we found that there are actually two main functions of grounding, that is, to provide safety and signal "zero potential". Signal ground" two. The safety ground is for the electrical safety of the equipment, and is generally connected to the ground to ensure that the grounded equipment is at the same potential as the ground; the signal ground is for the circuit to work normally, not necessarily connected to the ground, but can be any position defined as a potential reference point .

8.1.3 Grounding method

There is not a very systematic theory or model for grounding, and people can only rely on their past experience or experience from books when considering grounding. There are many methods for grounding, and which method to use depends on the structure and function of the system. Grounding methods can be divided into three types: single-point grounding, multi-point grounding and mixed grounding. Among them, single-point grounding can be divided into series single-point grounding and parallel single-point grounding, as shown in the figure.

Signal grounding method

single point ground

Single-point ground connection means that in the design of the product, the ground line is connected to a single reference point, which is often referenced to the earth, as shown in the figure.

Single-point grounding requires that each circuit be grounded only once and at the same point, with a safety ground stud to prevent currents from two different subsystems (with different reference levels) from going through the same return path as RF currents , resulting in common impedance coupling.

In order to prevent power frequency and other stray currents from causing interference on the signal ground wire, the signal ground wire should be insulated from the power ground wire and the chassis ground wire. And it is only connected to the power ground, the chassis ground and the safety grounding bolt of the grounding wire connected to the ground (except the floating type).

single point ground

The low operating frequency (<1MHz) adopts a single-point grounding type, which means that the influence of the distributed transmission impedance is extremely small.

There are two types of single-point grounding, one is series single-point grounding, and the other is parallel single-point grounding. As shown in Figure 8-3.

Classification of single point grounding

Interference of single-point grounding in series:

It can be seen from the formula that the potentials of points A, B and C are affected by the working current of the circuit and vary with the ground current of each circuit. In particular, the potential at point C is very unstable.

The series grounding is a series chain structure. This structure allows the common impedance coupling between the ground references of each subsystem, which will cause serious common mode coupling noise. At the same time, due to the influence of the distributed capacitance to the ground, parallel resonance will occur. Greatly increases the impedance of the ground wire.

Although this grounding method has great problems, it is the most common in practice because it is very simple. If there are multiple circuits of different power levels, then this grounding technique cannot be used, because high-power circuits generate large return-to-ground currents, which will affect low-power devices and circuits. If this grounding method must be adopted, the most sensitive circuit must be set directly at the power input position (point A), where the potential is the most stable, and as far away as possible from low-power devices and circuits. In addition, from the amplifier situation discussed earlier, the power output stage should be placed at point A, and the preamplifier should be placed at points B and C.

The solution to this problem is to connect a single point ground in parallel. Parallel single-point grounding means that the grounds of all devices are directly connected to the ground junction. However, parallel single-point grounding requires more wires, and because each current return path may have different impedances, the ground noise voltage is aggravated. When multiple printed circuit boards are connected together in this parallel fashion, the product fails radiation inspection. When using this layout, the designer should keep the inductance values on each path back to ground approximately the same (hard to do in practice), so that the effect on circuit operation does not appear to be multi-resonant.

In the actual situation, these two single-point grounding methods can be flexibly used. For example, as shown in Figure 8-4, the circuits can be grouped according to their characteristics. The circuits are placed in different groups. Single-point grounding in series is used in the group, and single-point grounding in parallel is used for grounding in different groups. In this way, the problem of common impedance coupling is solved, and the problem of too many ground wires is avoided. But never allow circuits with widely different power levels or circuits with widely different noise levels to share a ground lead.

Hybrid scheme of single-point grounding in parallel and series

With single-point grounding techniques, in addition to RF radiated coupling, crosstalk can also occur, depending on the physical separation between the current return paths. The extent to which crosstalk exists depends on the frequency range of the return signal, with high frequency components radiating more than low frequency components.

Single-point grounding technology is common in audio circuits, analog equipment, power frequency and DC power systems, and plastic-encapsulated products. While single-point grounding techniques are typically employed at low frequencies, it is sometimes used in high-frequency circuits or systems, and is feasible when designers are aware of all the inductance-related issues that exist in different grounding structures.

multi-point grounding

Multi-point grounding refers to the different points where the ground wires of all circuits are connected to the common ground wire. Generally, the circuit is grounded nearby.

As shown in the figure, the circuit inside the device takes the chassis as the reference point, and the chassis of each device takes the ground as the reference point. This ground structure can provide lower ground impedance. The disadvantage of this grounding method is that various ground loops are formed, causing ground loop interference, which will adversely affect the circuits with lower frequencies simultaneously used in the equipment.

multi-point grounding

High operating frequency (>30MHz) adopts multi-point grounding type (that is, in this circuit system, a grounding plate is used to replace the respective ground loops of each part of the circuit). In order to reduce the inductance, the length of the ground wire is required to be as short as possible. In systems with very high frequencies, the ground wire is usually controlled within a few millimeters. The length of each ground wire is less than 1/20 of the signal wavelength.

Common Impedance Coupling Problems with Multipoint Grounding

Common impedance coupling problems are easy to occur when grounding at multiple points. In low frequency occasions, this problem can be solved by grounding a single point. But at high frequency, it can only be solved by reducing the ground wire impedance (reducing the common impedance). Silver plating on the surface of the conductor can reduce the resistance of the conductor, as shown in the figure.

A general rule of thumb is that for frequencies below 1MHz, a single point ground is preferred. Assuming that the signal is a signal with a long rising edge and a low frequency, the frequency is between 1MHz and 10MHz. At this time, only when the longest trace or ground lead length is less than 1/20 of the wavelength, a single-point grounding can be used, and The wire length of each trace should be taken into account. Multiple grounds can reduce inductance between the noise generating circuit and the 0V reference because there are many parallel RF current loops, and even if there are many parallel grounds on the 0V reference, it is still possible that between the two ground leads create a ground loop. These ground loops are susceptible to inducing ESD magnetic field energy or to EMI radiation.

Hybrid ground

Hybrid grounding is a comprehensive application that combines single-point grounding and multi-point grounding, including both single-point grounding and multi-point grounding characteristics. Generally, it is based on single-point grounding and then grounding at multiple points through some inductors or capacitors (as shown in Figure 8-7). It uses the characteristics of inductors and capacitors that have different impedances at different frequencies, so that the grounding system is It has different grounding structures at different frequencies, and is mainly suitable for circuit systems that work at mixed frequencies. For example, in the hybrid grounding strategy of capacitive coupling, at low frequency, for DC, the capacitor is open, which is equivalent to single-point grounding, while at high frequency, the low impedance characteristic of the capacitor to the AC signal is used, and the capacitor is turned on. , the whole circuit behaves as multi-point grounding.

Hybrid ground

The frequency is between 1MHz and 10MHz, and mixed grounding is used.

When many interconnected devices are bulky (the physical size of the devices and the connecting cables are large compared to the wavelength of any interfering signals present), there is the potential for interference through the action of the enclosure and cables. When this happens, the path of the disturbance current usually exists in the ground return of the system.

Hybrid grounding systems exhibit different grounding structures at different frequencies.

Hybrid Grounding Example

Figure a is a system working at low frequency, and the system is connected to ground at a single point in series. But this system is exposed to high-frequency strong electric fields, so the shielded cable needs to be grounded at both ends. Therefore, for the low-frequency signal transmitted in the cable, the system is grounded at a single point, while for the high-frequency interference signal induced in the cable shield, the system is grounded at multiple points. The capacity of the grounding capacitor is generally below 10nF, depending on the frequency that needs to be grounded. Pay attention to the resonance problem of the capacitor, and the capacitive reactance of the capacitor is the smallest at the resonance point.

Figure b shows a system disturbed by ground loop currents. If the safety ground of the device is disconnected, the ground loop is cut off, and the ground loop current interference can be solved. But for safety reasons, the chassis must be connected to a safe ground. Therefore, for the high frequency ground loop current, the ground wire is disconnected, and for the 50Hz AC power, the chassis is reliably grounded.

floating

Floating ground means not connecting to the ground. The grounding system of the equipment is electrically insulated from the grounding system of the housing components. As shown in Figure 8-9, it is a floating method. Its purpose is to isolate the circuit or equipment from the common ground or common conductors that may cause circulating currents, thereby suppressing interference from the ground wire. The advantage is that the circuit is not affected by the electrical properties of the earth; the disadvantage is that the circuit is easily affected by parasitic capacitance, which makes the ground potential of the circuit fluctuate and increases the inductive interference to the analog circuit, so the effect of floating ground not only depends on It depends on the size of the insulation resistance of the floating ground, and also depends on the size of the floating parasitic capacitance and the frequency of the signal; since the equipment is not directly connected to the ground, it is prone to electrostatic accumulation. There will be electrostatic breakdown phenomenon with strong discharge current between it and the ground, which is a very destructive interference source. Therefore, when using the floating method, a bleeder resistor with a large resistance value should be connected between the equipment and the ground to eliminate the influence of static electricity accumulation.

floating way

Using floating equipment, the unit is susceptible to space coupling interference, pay attention to the use of electromagnetic shielding technology.

Analog circuit ground

Many analog circuits operate at low frequencies, and for these sensitive circuits, single-point grounding is the best grounding method. The primary purpose of grounding is to prevent large ground currents from other noisy components (eg, digital logic, motors, power supplies, relays) from competing for the sensitive analog ground. The noise-free level required for the analog ground depends on the sensitivity of the analog input. E.g,

For low-level analog amplifiers, those requiring a 10 μV input signal are more susceptible to interference than those requiring a 10 V input signal.

Digital circuit ground

Multi-point grounding is preferred in high-speed digital circuits. Its main purpose is to establish a unified potential common mode reference system. Many digital loops do not require a filtered ground reference. Digital circuits have a noise margin of a few hundred millivolts and can withstand ground noise gradients of tens to hundreds of millivolts.

For the ground wire system of the printed board consisting of only digital circuits, the ground wire is made into a closed loop to reduce the potential difference, which can significantly improve the anti-interference ability. For lower frequency analog signals, the more consideration is to avoid mutual interference between loop currents, so it cannot be connected to a closed loop.

The layout of the power line should be as thick as possible according to the size of the current. At the end of the wiring work, use the ground wire to cover (large area) the parts of the board that are not traced. It is also necessary to avoid the common impedance path when grounding. As shown in Figure 8-10, the "sampling point of the adjustment terminal" or "common point" of the voltage regulator circuit must not be connected to the output line and the common ground where the load current flows. As shown in Figure (a) on the line, it should be led out from the 'root' of the pin with a separate lead as shown in Figure (b).

Avoid common impedance paths

Grounding of digital-analog hybrid circuits

Another important part of grounding technology is the common ground processing of digital circuits and analog circuits. Generally speaking, the frequency of digital circuits is high, and the sensitivity of analog circuits to noise is strong. Because of this, high-frequency digital signal lines should be kept away from sensitive analog circuit devices as much as possible. Similarly, the signal loops of each other should also be mutually Isolation, which involves the division of analog and digital ground. The general practice is to separate the analog ground and the digital ground, and connect them only at a certain point, which is usually at the general ground interface of the PCB, or under the digital-to-analog converter. If necessary, magnetic components (such as magnetic beads) connected as shown.

Digital-analog mixed circuit grounding

It should be noted that in the design of the digital-analog hybrid circuit, the analog ground and the digital ground cannot be overlapped, and any signal line cannot cross the ground gap or divide the gap between the power supplies (as shown in Figure 8-12).

Signal across formation gap

In addition, there is also a unified processing method, that is, no ground division is performed, but the respective ranges are specified to ensure that the digital and analog traces and return flow do not pass through each other's areas. This strategy is generally applicable to the situation where the ratio of digital-to-analog devices is equal and there are multiple digital-to-analog conversion devices, which is beneficial to reduce the impedance of the ground plane. The reference ground wire design is shown in Figure 8-13:

unified design

Another important point in the grounding design is to ensure that all ground planes are equipotential. Multiple vias are required to be closely connected between the same grounds, and the connection lines between different grounds (such as analog and digital grounds) should be as short as possible.

The smaller the grounding resistance, the better. Generally, the grounding resistance is required to be less than 4Ω; for mobile devices, the grounding resistance can be less than 10Ω.

Ground resistance consists of ground wire resistance, contact resistance and ground resistance. There are three ways to reduce the ground resistance for this purpose:

①Reduce the resistance of the grounding wire. For this purpose, use a multi-strand thin wire with a large total cross-section and a short length.

②Reduce the contact resistance. For this reason, the grounding wire should be closely and firmly connected with the grounding bolt and the grounding electrode, and the contact area and tightness between the grounding electrode and the soil should be increased.

③Reduce ground resistance by increasing the surface area of the ground electrode and increasing the conductivity of the soil (such as injecting salt water into the soil).

The vertical grounding electrode grounding resistance R is:

R=0.366(ρ/L)lg(4L/d)Ω

In the formula: ρ——soil resistivity, Ω·m; L——depth of grounding electrode in the ground, m; d——diameter of grounding electrode, m.

For example, if loess ρ is 200 Ω·m, L is 2cm, and d is 0.05m, the grounding resistance R of the vertical ground electrode is 80.67Ω. For example, when salt water is injected into the soil to reduce ρ to 20Ω·m, the grounding resistance R of the ground electrode is 8.067Ω.

The most common interference on the ground wire is the ground loop interference caused by the ground loop current. The noise on the ground wire mainly affects the ground level of the digital circuit, and when the digital circuit outputs a low level, it is more sensitive to the noise of the ground wire. The interference on the ground wire may not only cause the malfunction of the circuit, but also cause conduction and radiation emission. Therefore, the key to reducing these interferences is to reduce the impedance of the ground wire as much as possible (for digital circuits, it is particularly important to reduce the ground wire inductance). The inductance of a trace is proportional to its length and the logarithm of its length, and inversely proportional to the logarithm of its width. Therefore, shortening the length of the wire can effectively reduce the inductance, while increasing the width of the trace has a very limited effect on reducing the inductance.

The design of the ground wire should pay attention to the following points:

① Correct selection of single-point grounding and multi-point grounding

②The ground wire should be as thick as possible, and the width of the ground wire should be 1 to 3 times that of the signal and control wires. If the printed board conditions allow, the grounding wire should be more than 2 to 3 mm, and the diameter of the grounding wire on the component pins should be about 1.5 mm.

③The grounding wire forms a closed loop. When there is only a PCB composed of digital circuits, the grounding circuit wiring forms a circle, which can mostly improve the anti-noise ability. The reason is: There are many integrated circuit components on the PCB, especially when encountering components with high energy consumption, due to the limitation of the thickness of the ground wire, a large potential difference will be generated, resulting in a decrease in the anti-noise ability. If the grounding structure is formed into a ring It will reduce the potential difference and improve the anti-noise ability of electronic equipment.

④The input impedance of the CMOS operational amplifier circuit is very high, and it is susceptible to induction. When using it, the unused terminal should be grounded or connected to a positive power supply.

⑤ In the multi-layer board, a ground plane is specially set.

⑥According to different power supply voltages, the digital circuit and the analog circuit are respectively set with ground wire. If there are logic circuits and linear circuits on the circuit board, they should be separated as much as possible, and the ground wires of the two should not be mixed, and they should be connected to the ground wires of the power supply. To maximize the ground area of the linear circuit. The ground of the low-frequency circuit should be grounded in parallel at a single point as far as possible. When the actual wiring is difficult, it can be partially connected in series and then grounded in parallel. The high-frequency circuit should be grounded at multiple points in series, the ground wire should be short and thick, and the high-frequency components should be grounded in a large area around the high-frequency components.

⑦ For digital circuits, set up a ground wire grid in the double-sided panel, that is, arrange as many parallel ground wires as possible on both sides of the double-sided panel, and the parallel lines of the upper and lower layers are perpendicular to each other. Then connect with plated through holes where they cross. The ground wire grid can effectively reduce the loop area of the signal current, which is beneficial to reduce radiation.

⑧The analog circuit is more sensitive, in order to prevent crosstalk, it should be grounded at a single point.

⑨ For circuit boards with both analog and digital functions, the power copper foils of analog ground and digital ground are usually separated; the two copper foils are only connected at the power supply. But there is a problem with this separation method: the signal line must cross the copper boundary line. These boundaries force the signal to reach the power supply first before returning to the device. The solution is to place jumpers at the ground copper foil where the signal traverses. These jumpers provide bridges for signal loops in separate copper foils, helping to reduce current loops.