PCB Tech - Ground Wire Design to Prevent Common Impedance Interference on PCB board

In electronic printed circuit boards, common impedance interference has a great impact on the normal operation of the circuit. In the PCB circuit design, especially in the PCB design of the high frequency circuit, the influence of the common impedance of the ground wire must be prevented. Through the analysis of the form of common impedance interference, this paper introduces in detail the effect of one-point grounding on common impedance interference in electronic circuits, especially in high-frequency circuits, and the problems that should be paid attention to when using one-point grounding to prevent common impedance. At the same time, it briefly expounds the main forms and requirements of the wiring layout in the PCB board.

In an electronic circuit, most components need to form a loop through the ground wire. Whether the wire design is reasonable or not directly affects the work of the circuit. Minimize the interference to the signal transmission due to the unreasonable design of the ground wire. The voltage, current and signal level of each point of the circuit are represented by the ground wire as the reference voltage. When reading the circuit diagram and understanding the working state of the circuit, the ground wire and each ground point are often regarded as the zero potential point with no potential difference. In the actual circuit work, due to the existence of the impedance (resistance, inductance) of the ground wire, a certain potential difference will be generated. The existence of these potential differences will inevitably affect the operation of the circuit. In PCB design, the effect of ground impedance must be paid attention to and eliminated.

1. The form in which the ground wire interferes with the circuit
1.1 Full current common impedance interference
Circuit 1 and circuit 2 form a loop with the power supply through the common ground AB. The line segment AB can be equivalent to a series loop of resistance and inductance, thus forming a common impedance effect. During operation, the current fluctuation of circuits 1 and 2 will cause the potential change of point A, which will make circuits 1 and 2 interfere with each other. If circuit 2 has output to circuit 3, the interference will also enter into circuit 3, thus forming full current common impedance interference. For example, there is a printed wire with a length of 10cm and a width of 1.5cm, the thickness of the copper foil is 50 microns, and the wire resistance is: if ρ=0.02, then R is about 0.026Ω. When circuit 1 works at a low frequency, the alternating current of the circuit is 1A, then an alternating voltage drop of about 0.026V is generated on this printed wire and acts on circuit 2. At high frequencies, the common impedance interference of the ground wire is mainly based on the inductance of the wire. When the length of a piece of wire is much greater than its width, the self-inductance of the wire can be calculated as 0.8 microhenry/meter. The same 10cm long wire, when the operating frequency it passes through is 30MHz, the inductive reactance presented by this wire is RL= 2π L≈16Ω. It can be seen that when the frequency increases, the inductive reactance of the wire will be several orders of magnitude larger than the resistance of the wire itself. Even if a small high-frequency current flows through the wire, such as 10mA, a high-frequency voltage of 0.16V will be generated on the wire. Therefore, for high-frequency circuits, when making PCBs, the printed wires should be as short as possible to reduce the loss and interference caused by wire inductance to the circuit.

1.2 Local current common impedance interference
When the printed board adopts a ring ground wire, each grounding element is grounded according to the nearest ground. In this way, a part of the AC signal of the final stage forms a loop through the ground wire AD, and an AC voltage drop is generated on the wire AD. Since the transistor emitter and base of the previous stage share the wire BC with the final stage, there is a common impedance interference on the wire BC. This interference is coupled on the common ground wire in the form of local currents, forming local current common impedance interference. The full current common impedance interference mainly exists between the stages. The local current co-impedance interference refers to the interference to other circuits caused by the poor grounding of some and individual components and wires.

2. Methods to prevent common impedance interference
All levels are internally grounded. Internal grounding at all levels is the main method to prevent local current common impedance interference. That is to effectively prevent the AC signal of this stage from escaping to circuits other than this stage through each grounding element, or the AC signal of other circuits from being picked up through each grounding element of this stage. Whether for low-frequency, intermediate-frequency, or high-frequency circuits at all levels, to prevent the common impedance interference of local currents, the effective method is to use one point grounding. In this way, the divergence and reception of the AC signal through the grounding element can be effectively prevented, so that the grounding is pure. In an actual circuit, there are many grounding elements at each level, and it is impossible to pass these elements into a threading hole at the same time. Instead, the grounding elements of this level are arranged as close as possible to a section or an area of the common ground wire.

3. One-point grounding should pay attention to the problem
3.1 Scope of this level grounding element
The scope of the grounding element of this stage refers to the element that is directly connected to the transistor of this stage, or is capacitively coupled. Secondary and components coupled by inductance do not belong to this stage. As shown in Figure 5a and Figure 5b.

3.2 Use grounding branch for one-point grounding
When there are not many components and the volume is not large, the one-point grounding layout is easier to handle; when there are many components and the volume is large, a longer grounding branch can be used. It can also be arranged around the printing plate in the layout, but the components of other levels should not be connected to this ground branch, and the far end of the ground branch should not be connected to other ground wires.

3.3 One-point grounding also includes the off-board components of this level
In addition to the on-board components of this stage, one-point grounding also includes off-board components that are directly or capacitively coupled to this stage. This is often neglected in PCB design, and causes common impedance interference of local currents.

3.4 One point grounding of high frequency circuit
The ground wire of high-frequency circuits generally uses a large area to cover the ground, but this does not mean that the grounding of the internal components at all levels can be scattered.

4. On-board layout
The inner wire of the printed board is used to connect the ground between all levels or parts of the circuit. The layout of the in-board wiring should mainly prevent the interference of the full current common impedance between all levels and parts.

4.1 Requirements for in-plane wiring layout
When the number of circuits in the board is large, the layout of the ground wire must do the following: the ground wires of each part must be separated. In order to eliminate or minimize the common ground segment of each part, the lead-out point of the total ground wire must be reasonable. In order to prevent the common impedance interference of each part through the common lead wire of the general ground wire, the ground wires of some parts of the circuit can be led out separately when necessary.

4.2 The form of in-plane wiring layout
The general ground inside the board is in the lower right corner of the printed board. For the general ground lead-out point in the board, it should be considered uniformly according to the layout of the in-board wiring. The general ground should be selected as close as possible to the ground wires of each part, and the general ground should be led to the ground between the power supplies. Lines are shorter. In digital circuits, since a large number of flip-flops and gate circuits are very sensitive to interference signals, when each circuit is in the switching state, certain pulse interference will be generated, which will cause the flip-flops and gate circuits to trigger falsely. This will directly affect the stability of the circuit operation, which can be designed according to the stage, according to the working state or according to an integrated block. Bus bar type: The bus bar is a strip-shaped symmetrical transmission line. Due to the increase of its thickness and width, the DC resistance decreases, and the main reason is that this symmetrical transmission method has better low impedance characteristics than single-line transmission, and at the same time overcomes the influence of the inductive component of single-line transmission on the circuit. Large-area coverage grounding: In digital circuits with high operating frequency and high-speed switching, the ground wire cannot be distributed in strips, and a large-area coverage grounding method should be used.
The large-area coverage of the ground wire is that when there are many wires in the board, in order to prevent the grounding from being cut off by the wire and affecting the grounding effect, a double-sided printed plate is used, one of which is used for grounding. Large-area grounding is used to prevent the common impedance interference caused by the local current coupling of each grounding element. Therefore, the layout of components at all levels should be centered on the transistors and integrated blocks of this level as much as possible, the components should be concentrated by level, and a grounding area should be set up at the center of the components at this level. In-line ground wire. When the number of stages in the board is small, the in-line ground wire can be used. The circuits at all levels can be arranged in sequence, and the grounding elements of each circuit should be close to the ground. The lead-out point of the board and ground should be placed near the final stage.

5. Conclusion
In a word, in PCB circuit design, especially in high-frequency circuits, we must pay attention to the influence of common impedance interference. Only through a good ground wire design and a reasonable layout structure can the electronic circuit work stably. The above has made a systematic summary of some methods and countermeasures to prevent common impedance interference in PCB circuit design, for the reference of printed circuit boards designers.