Circuit board (PCB) level EMC design

1. introduction

Printed circuit board (PCB) is a support for circuit components and devices in electronic products. It provides electrical connections between circuit components and devices. It is the most basic component of various electronic equipment. Its performance is directly related to electronics. The quality of the equipment. With the development of the information society, various electronic products often work together, and the interference between them is becoming more and more serious. Therefore, the electromagnetic compatibility problem has become the key to whether an electronic system can work normally. Similarly, with the development of electric technology, the density of PCBs is getting higher and higher. The quality of PCB design has a great influence on the interference and anti-interference ability of the circuit. To obtain the best performance of electronic circuits, in addition to the selection of components and circuit design, good PCB layout is also a very important factor in electromagnetic compatibility.

Since PCB is an inherent component of the system, enhancing electromagnetic compatibility in PCB wiring will not bring additional costs to the final product. However, in the design of printed circuit boards, product designers often only focus on increasing the density, reducing the occupied space, making the production simple, or pursuing a beautiful appearance, uniform layout, neglecting the influence of the circuit layout on electromagnetic compatibility, causing a large amount of signal radiation Into the space to form harassment. A poor PCB layout can cause more electromagnetic compatibility problems than eliminate them. In many cases, even adding filters and components will not solve these problems. In the end, the entire board had to be rewired. Therefore, it is the most cost-effective way to develop good PCB wiring habits at the beginning.

One thing to note is that there are no strict rules for PCB wiring, and there are no special rules that cover all PCB wiring. Most PCB wiring is limited by the size of the circuit board and the number of layers of copper clad laminate. Some wiring techniques can be applied to one circuit, but not to another. This mainly depends on the experience of the wiring engineer. However, there are still some general rules that will be discussed below.

In order to design a PCB of good quality and low cost, the following general principles should be followed:

                   2. PCB component layout

First, the PCB size should be considered. When the PCB size is too large, the printed lines are long, the impedance increases, the noise resistance decreases, and the cost also increases; if it is too small, the heat dissipation is not good, and the adjacent lines are easily interfered. After determining the PCB size. Then determine the location of special components. Finally, according to the functional unit of the circuit, all the components of the circuit are laid out.

The layout and wiring of the components of digital circuits, analog circuits, and power circuits in electronic equipment have different characteristics. The interference they generate and the methods for suppressing them are different. In addition, due to different frequencies of high-frequency and low-frequency circuits, their interference and methods of suppressing interference are also different. Therefore, when placing components, digital circuits, analog circuits, and power circuits should be placed separately to separate high-frequency circuits from low-frequency circuits. If possible, they should be isolated or made into a circuit board. In addition, the layout should pay special attention to the distribution of devices with strong and weak signals and the direction of signal transmission.

When the high-speed, medium-speed and low-speed logic circuits are arranged on the printed board, the components should be arranged in the manner as shown in Figure 1-①.

In the layout of components and components, like other logic circuits, the related devices should be placed as close as possible, so as to obtain better anti-noise effect. The position of the components on the printed circuit board should fully consider the problem of anti-electromagnetic interference. One of the principles is to keep the leads between the components as short as possible. In the layout, the three parts of the analog signal part, the high-speed digital circuit part, and the noise source part (such as relays, high current switches, etc.) should be reasonably separated to minimize the signal coupling between each other. As shown in Figure 1-②.

The clock generator, crystal oscillator, and the clock input terminals of the CPU are all susceptible to noise and should be close to each other. Noise-prone devices, small current circuits, and high current circuits should be kept away from logic circuits as much as possible. If possible, it is important to make another circuit board.

2.1 Observe the following principles when determining the location of special components:

(1) Shorten the connection between high-frequency components as much as possible, try to reduce their distribution parameters and mutual electromagnetic interference. Components that are susceptible to interference should not be too close to each other, and input and output components should be as far away as possible.

(2) There may be a higher potential difference between some components or wires, and the distance between them should be increased to avoid accidental short circuit caused by discharge. Components with high voltage should be placed in places that are not easily accessible by hand during commissioning.

(3) Components weighing more than 15g should be fixed with brackets and then soldered. Components that are large, heavy, and generate a lot of heat should not be installed on the printed board, but should be installed on the chassis bottom of the whole machine, and heat dissipation should be considered. The thermal element should be far away from the heating element.

(4) For the layout of adjustable components such as potentiometers, adjustable inductance coils, variable capacitors, and micro switches, the structural requirements of the whole machine should be considered. If it is adjusted inside the machine, it should be placed on a printed board where it is convenient to adjust; if it is adjusted outside the machine, its position should be adjusted to the position of the adjustment knob on the chassis panel.

(5) The position occupied by the positioning holes of the printed board and the fixing bracket should be reserved.

2.2 When laying out all the components of the circuit according to the functional unit of the circuit, the following principles must be met:

(1) Arrange the position of each functional circuit unit according to the flow of the circuit, make the layout convenient for signal circulation, and keep the signal in the same direction as possible.

(2) Centering on the core component of each functional circuit, lay out around it. The components should be evenly, neatly and compactly arranged on the PCB to minimize and shorten the leads and connections between the components.

(3) For circuits operating at high frequencies, the distribution parameters between components should be considered. The general circuit should arrange the components in parallel as much as possible. In this way, it is not only beautiful, but also easy to install and weld, and easy to mass produce.

(4) Components located on the edge of the circuit board are generally not less than 2mm away from the edge of the circuit board. The best shape of the circuit board is rectangular. The aspect ratio is 3:2 or 4:3. When the circuit board size is greater than 200150mm. The mechanical strength of the circuit board should be considered.

2.3 General layout requirements of PCB components:

The layout of circuit components and signal paths must minimize the coupling of unwanted signals:

(1) The low electronic signal channel cannot be close to the high-level signal channel and the unfiltered power line, including circuits that can generate transient processes.

(2) Separate low-level analog circuits and digital circuits to avoid common impedance coupling between analog circuits, digital circuits, and power supply common return lines.

(3) High-, medium-, and low-speed logic circuits use different areas on the PCB.

(4) Arrange the circuit to minimize the length of the signal line.

(5) Ensure that there are no excessively long parallel signal lines between adjacent boards, between adjacent layers on the same board, or between adjacent wiring on the same layer.

(6) The electromagnetic interference (EMI) filter should be as close as possible to the EMI source and placed on the same circuit board.

(7) DC/DC converters, switching elements and rectifiers should be placed as close to the transformer as possible to minimize the length of their wires.

(8) Place the voltage regulating element and filter capacitor as close as possible to the rectifier diode.

(9) The printed boards are divided by frequency and current switching characteristics, and the noise components and non-noise components should be further away.

(10) The wiring sensitive to noise should not be parallel to the large current and high-speed switching lines.

3. PCB wiring

3.1 High-frequency characteristics of printed circuit boards and components:

The composition of a PCB is a multi-layer structure using a series of lamination, wiring and prepreg treatment on the vertical stack. In a multi-layer PCB, in order to facilitate debugging, the designer will lay the signal line on the outermost layer.

The wiring on the PCB has impedance, capacitance and inductance characteristics.

Impedance: The impedance of the wiring is determined by the weight of the copper and the cross-sectional area. For example, 1 ounce of copper has an impedance of 0.49mΩ/unit area.

Capacitance: The capacitance of the wiring is determined by the range (A) of the current reached by the insulator (EoEr) and the track spacing (h).

Expressed by the equation as C=EoErA/h, Eo is the dielectric constant of free space (8.854pF/m), and Er is the relative dielectric constant of the PCB substrate (this value is 4.7 in the FR4 laminate)

Inductance: The inductance of the wiring is evenly distributed in the wiring, about 1nH/mm.

For a 1 ounce copper wire, on a 0.25mm (10mil) thick FR4 laminated board, a 0.5mm (20mil) wide and 20mm (800mil) long wire above the ground layer can produce an impedance of 9.8mΩ, 20nH Inductance and coupling capacitance of 1.66pF to ground.

In the case of high frequencies, the traces, vias, resistors, capacitors, distributed inductance and capacitance of the printed circuit board cannot be ignored. The distributed inductance of the capacitor cannot be ignored, and the distributed capacitance of the inductor cannot be ignored. The resistance produces reflection and absorption of high frequency signals. The distributed capacitance of the trace will also play a role. When the length of the trace is greater than 1/20 of the corresponding wavelength of the noise frequency, an antenna effect is generated, and the noise is emitted outward through the trace.

The printed circuit board vias cause about 0.5pF of capacitance. The packaging material of an integrated circuit itself introduces 2~6pF capacitor. A connector on a circuit board has a distributed inductance of 520nH. A dual-in-line 24-pin integrated circuit socket with a distributed inductance of 4~18nH.

These small distribution parameters are negligible for microcontroller systems operating at lower frequencies; special attention must be paid to high-speed systems.

The following are the general requirements that should be followed to avoid the influence of PCB layout parameters:

(1) Increase the distance between traces to reduce capacitive coupling crosstalk;

(2) Route the power and ground wires in parallel to optimize the PCB capacitance;

(3) Route sensitive high-frequency lines away from high-noise power lines to reduce mutual coupling;

(4) Widen the power and ground wires to reduce the impedance of the power and ground wires.

3.2 Segmentation:

Splitting refers to using physical splitting to reduce the coupling between different types of lines, especially through the coupling of power lines and ground lines.

Figure 2 shows an example of dividing four different types of circuits using a dividing technique. In the ground plane, non-metallic trenches are used to isolate the four ground planes. L and C are used as filters for each part of the board, reducing the coupling between different circuit power planes. High-speed digital circuits are required to be placed near the power supply entrance due to their higher instantaneous power requirements. The interface circuit may require anti-static discharge (ESD) and transient suppression devices or circuits to improve its electromagnetic immunity, and the area should be divided independently. For L and C, it is better to use separate L and C for different divided regions instead of one large L and C, because it can provide different filtering characteristics for different circuits.

3.3 RF current suppression of the reference plane:

Regardless of the reference ground layer of a multilayer PCB or the ground wire of a single-layer PCB, the path of current always returns from the load to the power supply. The lower the impedance of the return path, the better the electromagnetic compatibility of the PCB. Due to the influence of the RF current flowing between the load and the power supply, long return paths will produce RF coupling between each other, so the return path should be as short as possible and the loop area should be as small as possible.

3.4 Separation of wiring:

The role of wiring separation is to minimize crosstalk and noise coupling between adjacent lines on the same layer of the PCB.

All signals (clock, video, audio, reset, etc.) should be spaced away from line to line, edge to edge. In order to further reduce electromagnetic coupling, the reference ground is placed near or between key signals to isolate coupling noise generated on other signal lines or between signal lines.

3.5 Power cord design:

According to the size of the printed circuit board current, try to increase the width of the power line to reduce the loop resistance. At the same time, make the direction of the power line and ground line consistent with the direction of data transmission, which helps to enhance the ability to resist noise.

3.6 Suppression of reflected interference and terminal matching:

In order to suppress the reflection interference appearing at the end of the printed line, in addition to special needs, the length of the printed line should be shortened as much as possible and a slow circuit should be used. Terminal matching can be added if necessary. There are many terminal matching methods. Common terminal matching methods are shown in Figure 3. According to experience, terminal matching measures should be adopted when the printed lines are longer than 10cm for the generally faster TTL circuits. The resistance of the matching resistor should be determined according to the maximum value of the output drive current and the sink current of the integrated circuit. The clock signals are mostly matched in series, as shown in Figure 4.

3.7 Protection and shunt lines:

In clock circuits, local decoupling capacitors play a very important role in reducing noise propagation along power rails. But the clock line also needs to be protected from interference from other sources of electromagnetic interference. Otherwise, the disturbed clock signal will cause problems elsewhere in the circuit.

Setting shunt and protection circuits is a very effective method to isolate and protect critical signals (for example, system clock signals in a noisy environment). The shunt or protection circuit in the PCB is to lay isolated protection lines along both sides of the critical signal line. The protection circuit not only isolates the coupling magnetic flux generated by other signal lines, but also isolates the key signals from the coupling with other signal lines.

The difference between the shunt line and the protection line is that the shunt line does not have to be terminated at both ends (connected to ground), but both ends of the protection line must be connected to ground. In order to further reduce coupling, the protection circuit in the multilayer PCB can be added to the ground path every other period.

3.8 Decoupling between local power supply and IC:

In the DC power supply circuit, changes in the load can cause power supply noise. For example, in digital circuits, when the circuit transitions from one state to another, a large spike current will be generated on the power line, forming a transient noise voltage. Local decoupling can reduce noise propagation along the power rail. The large-capacity bypass capacitor connected between the power input port and the PCB acts as a low-frequency disturbance filter and at the same time serves as an electrical energy storage device to meet sudden power requirements. In addition, there should be decoupling capacitors between the power supply and ground of each IC. These decoupling capacitors should be as close as possible to the IC pins, which will help filter out the switching noise of the IC.

Configuring decoupling capacitors can suppress noise caused by load changes. It is a common practice in the reliability design of printed circuit boards. The configuration principles are as follows:

(1) Connect 10~100μF electrolytic capacitor across the power input. If possible, it is better to connect more than 100μF.

(2) In principle, each integrated circuit chip should be arranged with a 0.01μF ceramic capacitor. If the printed circuit board gap is not enough, a 1~10μF tantalum capacitor can be arranged every 4~8 chips. The high-frequency impedance of this device is particularly small, the impedance is less than 1Ω in the range of 500kHz to 20MHz, and the leakage current is very small (below 0.5μA). It is better not to use electrolytic capacitors. The electrolytic capacitors are rolled up with two layers of membranes. This structure behaves as an inductance at high frequencies.

(3) For devices with weak anti-noise capability and large power changes during shutdown, such as RAM and ROM storage devices, high-frequency decoupling capacitors should be directly connected between the power and ground lines of the chip.

(4) The lead wire of the capacitor should not be too long, especially the high-frequency bypass capacitor must not have a lead wire.

The selection of the decoupling capacitor value is not strict, and can be calculated as C=1/f: that is, 10MHz takes 0.1μF. For the system formed by the microcontroller, it can be between 0.1~0.01μF. A good high-frequency decoupling capacitor can remove high-frequency components up to 1GHz. Ceramic chip capacitors or multilayer ceramic capacitors have better high-frequency characteristics.

In addition, the following two points should be noted:

(1) When there are contactors, relays, buttons and other components in the printed board. When operating them, a large spark discharge will occur, and an RC absorption circuit must be used to absorb the discharge current. Generally R takes 1~2kΩ, C takes 2.2~4.7μF.

(2) The input impedance of CMOS is very high, and it is susceptible to induction. Therefore, when in use, the unused end should be grounded through a resistor or connected to a positive power supply.

3.9 Wiring technology:

3.9.1 Via

Vias are commonly used in multilayer printed wiring boards. When it is a high-speed signal, the via produces an inductance of 1 to 4nH and a capacitance of 0.3 to 0.5pF. Therefore, when laying high-speed signal channels, vias should be kept to an absolute minimum. For high-speed parallel lines (such as address and data lines), if layer changes are inevitable, you should ensure that each signal line has the same number of vias.

3.9.2 45 degree angle path

Similar to vias, right-angle turning paths should be avoided because it generates concentrated electric fields at the inner edges. This field can couple strong noise to adjacent paths, so all the right-angle paths should use 45 degrees when rotating the path. Figure 5 is a general rule for a 45 degree path.

3.9.3 Stub

As shown in Figure 6, the stub will produce reflections, while also potentially increasing the possibility of radiating antennas. Although the length of the stub may not be a quarter of an integer of the known signal wavelength of any system, the incident radiation may oscillate on the stub. Therefore, avoid using stubs in the path of transmitting high frequencies and sensitive signals.

3.9.4 Arrangement of tree-shaped signal lines

Although the tree arrangement is suitable for the ground connection of multiple PCB printed circuit boards, it has a signal path that can generate multiple stubs. Therefore, high-speed and sensitive signal lines should not be arranged in a tree.

3.9.5 Radial signal line arrangement

Radial signal arrangements usually have the shortest path and produce the smallest delay from the source to the receiver, but this can also produce multiple reflections and radiated interference, so high-speed and sensitive signal lines with radiating arrangements should be avoided.

3.9.6 Constant path width

The width of the signal path should be constant from drive to load. When the path width is changed, the path impedance (resistance, inductance, and capacitance) changes, resulting in reflection and unbalanced line impedance. So it is best to keep the path width unchanged.

3.9.7 Dense holes and vias

The denseness of vias passing through the power supply and the ground layer will produce localized impedance differences near the vias. This area not only becomes a "hot spot" for signal activity, but the power supply surface is high-impedance at this point, affecting RF current transfer.

3.9.8 Split pores

Similar to the dense holes and vias, the power layer or ground layer splitting the pores (that is, long holes or wide channels) will produce inconsistent areas in the range of the power layer and the ground layer, just like the insulating layer to reduce their effectiveness, but also local Increase the impedance of the power layer and the ground layer.

3.9.9 Ground metallized filling area

All metallized fill areas should be connected to ground, otherwise these large metal areas can act as radiating antennas.

3.9.10 Minimize the ring area

Keeping the signal path and its ground return line close together will help to minimize the ground loop and, therefore, avoid potential antenna loops. For high-speed single-ended signals, sometimes if the signal path does not go along the low-resistance ground plane, the ground loop may also have to flow along the signal path.

3.10 Other wiring strategies:

The use of parallel wiring can reduce the inductance of the wire, but the mutual inductance and distributed capacitance between the wires will increase. If the layout permits, the power line and the ground wire should preferably use a cross-shaped mesh wiring structure. The specific method is to laterally route the printed board. , The other side is wired longitudinally, and then connected with metallized holes at the cross holes.

In order to suppress the crosstalk between the conductors of the printed circuit board, long-distance parallel wiring should be avoided when designing the wiring, the distance between the wires should be widened as much as possible, and the signal wire should not cross the ground wire and the power wire as much as possible. Setting a grounded printed line between some signal lines that are very sensitive to interference can effectively suppress crosstalk.

3.10.1 In order to avoid the electromagnetic radiation generated when the high-frequency signal passes through the printed wire, pay attention to the following points when wiring the printed circuit board:

(1) Wiring uses the same output current and opposite direction signals as much as possible to use parallel layout to eliminate magnetic field interference.

(2) Minimize the discontinuity of the printed conductors. For example, the width of the conductors should not be abrupt, the corners of the conductors should be greater than 90 degrees, and loop routing is prohibited.

(3) The clock signal lead is most likely to produce electromagnetic radiation interference, and should be close to the ground loop when routing.

(4) The bus driver should be close to the bus it wants to drive. For those leads that leave the printed circuit board, the driver should be next to the connector.

(5) Since the impact of the transient current on the printed lines is mainly caused by the inductance of the printed wires, the inductance of the printed wires should be reduced as much as possible. The inductance of the printed wire is proportional to its length and inversely proportional to its width, so short and precise wires are beneficial to suppress interference. Clock leads, signal lines of row drivers or bus drivers often carry large transient currents, and printed wires should be as short as possible. For discrete component circuits, when the width of the printed conductor is about 1.5mm, it can fully meet the requirements; for integrated circuits, the width of the printed conductor can be selected between 0.2 ~ 1.0mm.

(6) Avoid using large-area copper foil around the heating element or the lead wire passing through a large current. Otherwise, the copper foil will swell and fall off when heated for a long time. When it is necessary to use a large area of copper foil, it is best to use a grid shape, which is helpful to exclude the volatile gas generated by the heat between the adhesive between the copper foil and the substrate.

(7) The center hole of the pad is slightly larger than the diameter of the device lead. If the pad is too large, it may form a virtual solder. The outer diameter D of the pad is generally not less than (d+1.2) mm, where d is the lead hole diameter. For high-density digital circuits, the minimum diameter of the pad can be (d + 1.0) mm.

3.10.2 The wiring of printed circuit board should also pay attention to the following issues:

(1) Dedicated zero-volt line, the width of the power line is ≥ 1mm;

(2) The power line and the ground line should be as close as possible, so that the current of the distribution line can be balanced;

(3) To provide a zero-volt line for analog circuits;

(4) To reduce crosstalk between lines, the distance between printed lines can be increased if necessary;

(5) Intentionally installing some zero-volt lines as line-to-line isolation;

(6) The plug of the printed circuit should also be arranged with some zero-volt lines as line-to-line isolation;

(7) Pay special attention to the size of the wire loop in the current flow;

(8) If possible, add an R-C filter at the entrance of the control line (on the printed board) to decouple in order to eliminate possible interference factors in the transmission.

3.11 General rules for PCB wiring:

When designing printed circuit boards, the following points should be noted:

(1) From the perspective of reducing radiation disturbance, multi-layer boards should be selected as much as possible. The inner layer is used as the power layer and the ground layer to reduce the impedance of the power supply line, suppress the common impedance noise, and form a uniform ground plane for the signal line. , Increase the distributed capacitance between the signal line and the ground plane to suppress its ability to radiate into space.

(2) The power line, ground line, and printed circuit board traces should maintain low impedance to high-frequency signals. In the case of very high frequencies, the power cord, ground wire, or printed board traces will become small antennas for receiving and transmitting harassment. In addition to adding filter capacitors, the method of reducing this disturbance is more important to reduce the high-frequency impedance of the power line, ground line and other printed circuit board traces. Therefore, the traces of various printed boards should be short and thick, and the lines should be uniform.

(3) The arrangement of the power cord, ground wire and printed wire on the printed board should be appropriate, as short and straight as possible to reduce the loop area formed between the signal line and the return line.

(4) The clock generator should be as close as possible to the device using the clock.

(5) The quartz crystal oscillator case should be grounded.

(6) Circle the clock area with a ground wire. Keep the clock wire as short as possible.

(7) Printed boards should use 45° fold lines instead of 90° fold lines to reduce the transmission and coupling of high-frequency signals to the outside.

(8) Single-point and double-sided power supplies and single-point grounding are used for the single and double panels; the power and ground wires should be as thick as possible.

(9) The I/O drive circuit should be as close as possible to the connector on the side of the printed board, and let it leave the printed board as soon as possible.

(10) The key line should be as thick as possible, and add protective ground on both sides. The high-speed line should be short and straight.

(11) The component pins should be as short as possible, and the decoupling capacitor pins should be as short as possible. It is best to use leadless chip capacitors for decoupling capacitors.

(12) For class A/D devices, the digital and analog grounds are preferably unified and do not cross.

(13) Keep clock, bus, and chip select signals away from I/O lines and connectors.

(14) The analog voltage input line and reference voltage terminal should be as far away as possible from the digital circuit signal line, especially the clock.

(15) The clock line is perpendicular to the I/O line and has less interference than the parallel I/O line. The clock component pins should be far away from the I/O cable.

(16) Do not run traces under the quartz crystal and under devices sensitive to noise.

(17) For weak signal circuits, do not form current loops around low-frequency circuits.

(18) Do not form a loop for any signal. If unavoidable, keep the loop area as small as possible.

4. PCB grounding design

In electronic equipment, grounding is an important method of controlling interference. If the grounding and shielding can be used correctly, most interference problems can be solved. The ground structure of electronic equipment is roughly system ground, chassis ground (shield ground), digital ground (logic ground) and analog ground.

In the grounding design of the PCB board, the grounding technology is applied to both multilayer PCBs and single-layer PCBs. The goal of grounding technology is to minimize the grounding impedance and thus reduce the potential of the ground loop from the circuit back to the power supply.

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

In low-frequency circuits, the working frequency of the signal is less than 1MHz, its wiring and the inductance between the devices have little effect, and the circulating current formed by the grounding circuit has a great influence on the interference, so one point of grounding should be used. When the signal operating frequency is greater than 10MHz, the impedance of the ground wire becomes very large. At this time, the impedance of the ground wire should be reduced as much as possible, and the nearest multi-point ground should be used. When the operating frequency is 1~10MHz, if one point is used for grounding, the length of the ground wire should not exceed 1/20 of the wavelength, otherwise the multi-point grounding method should be used. High-frequency circuits should be grounded at multiple points in series, the ground wire should be short and thick, and grid-like large-area grounding copper foil should be arranged around the high-frequency components as much as possible.

(2) Separate the digital circuit from the analog circuit

There are both high-speed 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 end. Try to increase the ground area of the linear circuit as much as possible.

(3) Try to thicken the ground wire

If the grounding wire is very thin, the grounding potential will change with the change of current, which will cause the timing signal level of the electronic equipment to be unstable, and the anti-noise performance will deteriorate. Therefore, the ground wire should be thickened as much as possible, so that it can pass three times the allowable current of the printed circuit board. If possible, the width of the ground wire should be greater than 3mm.

(4) Make the ground wire form a closed loop

When designing a grounding system for a printed circuit board composed only of digital circuits, making the grounding line into a closed loop can significantly improve noise immunity. The reason is that there are many integrated circuit components on the printed circuit board, especially when there are components that consume a lot of power, due to the limitation of the thickness of the ground wire, a large potential difference will be generated on the ground junction, resulting in a decline in the ability to resist noise If the grounding structure is formed into a loop, it will reduce the potential difference and improve the anti-noise ability of electronic equipment.

(5) When using a multi-layer circuit board design, one of the layers can be used as a "full ground plane", which can reduce the ground impedance and at the same time play a shielding role. We often lay a wide circle of ground wire around the printed board, which also plays the same role.

(6) Ground wire of single-layer PCB

In a single-layer (single-sided) PCB, the width of the ground wire should be as wide as possible, and should be at least 1.5mm (60mil). Since star routing cannot be implemented on a single-layer PCB, changes in jumper and ground width should be kept to a minimum, otherwise it will cause changes in line impedance and inductance.

(7) Ground wire of double-layer PCB

In double-layer (double-sided) PCBs, ground grid/dot matrix wiring is preferred for digital circuits. This wiring method can reduce ground impedance, ground loops, and signal loops. As in a single-layer PCB, the width of the ground and power lines should be at least 1.5mm.

Another layout is to put the ground plane on one side and the signal and power lines on the other side. In this arrangement, the ground loop and impedance will be further reduced. At this time, the decoupling capacitor can be placed as close as possible between the IC power supply line and the ground plane.

(8) PCB capacitance

On a multi-layer board, the PCB capacitor is created by a thin insulating layer separating the power plane and the ground. On a single-layer board, the parallel arrangement of power lines and ground lines will also have this capacitive effect. One advantage of PCB capacitors is that it has a very high frequency response and low series inductance evenly distributed over the entire surface or the entire line, which is equivalent to a decoupling capacitor evenly distributed across the board. No single discrete component has this feature.

(9) High-speed circuit and low-speed circuit

Place high-speed circuits and components closer to the ground plane, and low-speed circuits and components closer to the power plane.

(10) Ground copper filling

In some analog circuits, the unused board area is covered by a large ground plane to provide shielding and increase decoupling capabilities. But if this copper area is suspended (for example, it is not connected to ground), then it may appear as an antenna and will cause electromagnetic compatibility problems.

(11) Ground plane and power plane in multilayer PCB

In a multi-layer PCB, it is recommended to place the power plane and the ground plane as close as possible to adjacent layers in order to generate a large PCB capacitor on the entire board. The fastest critical signal should be close to the side of the ground plane, and non-critical signals should be placed close to the power plane.

(12) Power requirements

When the circuit requires more than one power supply, ground is used to separate each power supply. However, it is impossible to ground multiple points in a single-layer PCB. One solution is to separate the power and ground wires from one power supply from other power and ground wires, which also helps to avoid noise coupling between power supplies.

5. Design of analog-digital mixed circuit board

How to reduce the mutual interference between digital and analog signals? There are two basic principles: the first principle is to reduce the area of the current loop as much as possible; the second principle is that the system uses only one reference plane. Conversely, if there are two reference planes in the system, a dipole antenna may be formed (Note: the radiation size of a small dipole antenna is proportional to the length of the line, the amount of current flowing, and the frequency); and if the signal cannot pass through as much as possible A small loop returns, it is possible to form a large loop antenna (Note: the size of the radiation of the small loop antenna is proportional to the loop area, the current flowing through the loop, and the square of the frequency). Avoid these two situations as much as possible in the design.

It was suggested that the digital and analog grounds on the mixed-signal circuit board be separated to achieve isolation between the digital and analog grounds. Although this method is feasible, there are many potential problems, especially in complex large-scale systems. The most critical issue is that the wiring cannot be crossed across the division gap. Once the wiring across the division gap is crossed, the electromagnetic radiation and signal crosstalk will increase dramatically. The most common problem in PCB design is that the signal line crosses the split ground or power supply and generates EMI.

Understanding the path and method of current return to ground is the key to optimizing mixed-signal circuit board design. Many design engineers only consider where the signal current flows, and ignore the specific path of the current. If the ground layer must be divided and must be routed through the gap between the divisions, a single point connection can be made between the divided grounds first to form a connection bridge between the two grounds and then routed through the connection bridge. In this way, a direct current return path can be provided under each signal line, so that the loop area formed is small.

The use of optical isolation devices or transformers can also achieve the signal across the division gap. For the former, the optical signal is crossed across the division gap; in the case of a transformer, the magnetic field is crossed across the division gap. Another feasible method is to use differential signals: the signal flows in from one line and returns from another signal line. In this case, it is not necessary to serve as a return path.

In actual work, it is generally preferred to use a unified way to partition the PCB into analog and digital parts. Analog signals are routed in the analog area on all layers of the circuit board, while digital signals are routed in the digital circuit area. In this case, the digital signal return current does not flow into the analog signal ground. Only when the digital signal is routed on the analog part of the circuit board or the analog signal is routed on the digital part of the circuit board, will the digital signal interfere with the analog signal. This problem does not occur because there is no split ground, the real reason is that the digital signal wiring is not appropriate.

When connecting the analog and digital ground pins of the A/D converter together, most A/D converter manufacturers recommend that the AGND and DGND pins be connected to the same low impedance ground through the shortest lead . If the system has only one A/D converter, the above problem can be easily solved. Split the ground and connect the analog ground and digital ground together under the A/D converter. When this method is adopted, the width of the connecting bridge between the two grounds must be equal to the width of the IC, and no signal line can cross the division gap. If there are many A/D converters in the system, for example, how to connect 10 A/D converters? If the analog ground and the digital ground are connected together under each A/D converter, multiple points are connected, and the isolation between the analog ground and the digital ground is meaningless. If you do not connect in this way, you violate the manufacturer's requirements.

The best way is to use uniformly at the beginning. The unified ground is divided into an analog part and a digital part. This layout and wiring not only meet the requirements of IC device manufacturers for low impedance connection of analog ground and digital ground pins, but also will not form a loop antenna or dipole antenna and cause EMC problems.

Mixed-signal PCB design is a complicated process. The design process should pay attention to the following points:

(1) The PCB partition is divided into independent analog and digital parts.

(2) Appropriate component layout.

(3) A/D converters are placed across partitions.

(4) Do not divide the ground. Lay uniformly under the analog and digital parts of the circuit board.

(5) In all layers of the circuit board, digital signals can only be routed in the digital part of the circuit board; analog signals can only be routed in the analog part of the circuit board.

(6) Realize the separation of analog and digital power supply.

(7) The wiring cannot cross the gap between the divided power supply surfaces.

(8) The signal lines that must cross the gap between the divided power supplies should be located on the wiring layer next to a large area of ground.

(9) Analyze the path and method of the actual return current through the ground.

(10) Use correct wiring rules.

6. Circuit measures in PCB design

When we design electronic circuits, we pay more attention to the actual performance of the product, but not too much to consider the electromagnetic compatibility characteristics of the product and the suppression of electromagnetic disturbance and electromagnetic anti-interference characteristics. In order to achieve the purpose of electromagnetic compatibility when using the circuit schematic diagram to arrange the PCB, the necessary circuit measures must be taken, that is, the necessary additional circuits are added on the basis of the circuit schematic diagram to improve the electromagnetic compatibility performance of the product. The following circuit measures can be used in actual PCB design:

(1) A resistor can be connected in series on the PCB trace to reduce the transition rate of the lower and upper edges of the control signal line.

(2) Try to provide some form of damping (high-frequency capacitors, reverse diodes, etc.) for relays, etc.

(3) Filter the signal entering the printed board, and filter the signal from the high-noise area to the low-noise area. At the same time, reduce the signal reflection by using the method of string termination resistance.

(4) The useless end of the MCU should be connected to the power supply or ground through the corresponding matching resistor. Or defined as an output terminal, the power and ground terminals of the integrated circuit should be connected, not floating.

(5) Don't leave the unused gate circuit input terminal floating, but connect the power supply or ground through the corresponding matching resistor. The positive input of the unused op amp is grounded, and the negative input is connected to the output.

(6) Set a high-frequency decoupling capacitor for each integrated circuit. A small high-frequency bypass capacitor must be added to each electrolytic capacitor.

(7) Use large-capacity tantalum capacitors or polyester capacitors instead of electrolytic capacitors as charging and discharging energy storage capacitors on the circuit board. When using tubular capacitors, the housing should be grounded.

7. Conclusion

Printed circuit boards are the most basic components of electronic products and the carrier of most electronic components. After the design of the printed circuit board of a product is completed, it can be said that the nuisance and anti-interference characteristics of its core circuit have been basically determined. To improve its electromagnetic compatibility characteristics, it can only be achieved through the filtering of the interface circuit and the shell Shielding is "chasing and blocking", which not only greatly increases the subsequent cost of the product, but also increases the complexity of the product and reduces the reliability of the product. It can be said that a good printed circuit board can solve most of the electromagnetic disturbance problems, as long as the appropriate transient suppression devices and filter circuits are added to the interface circuit board at the same time, most of the immunity problems can be solved at the same time.

The EMC design of printed circuit boards is a very skillful job, and it also requires a lot of experience accumulation. A well-designed printed circuit board with electromagnetic compatibility is a perfect "handicraft" that cannot be copied and copied. But this is not to say that our printed circuit boards do not have to consider the electromagnetic compatibility performance of the products, only through the peripheral circuits and shells to remedy. As long as we can abide by the design rules listed in this article, we can also solve most of the electromagnetic compatibility problems, and then through a small number of peripheral transient suppression devices and filter circuits and proper shell shielding and correct grounding, we can complete A product that meets the requirements of electromagnetic compatibility. If we pay attention to the accumulation and summary of usual experience and technology, we can eventually become masters of PCB "crafts" design and design our own PCB "craftsmanship".