[Repost] Impedance matching and 0 ohm resistor in PCB design

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[Repost] Impedance matching and 0 ohm resistor in PCB design [Copy link]

1. Impedance matching Impedance matching refers to a suitable matching method between a signal source or a transmission line and a load. According to the access method, impedance matching can be divided into two types: serial and parallel; according to the frequency of the signal source, impedance matching can be divided into low frequency and high frequency. (1) Serial impedance matching is generally used for high-frequency signals. The resistance of the serial resistor is 20~75Ω, which is proportional to the signal frequency and inversely proportional to the width and length of the PCB trace. In embedded systems, serial matching resistors are generally required when the PCB trace length of signals with a frequency greater than 20M is greater than 5cm, such as clock signals, data and address bus signals in the system. The functions of serial matching resistors are twofold: ◆ Reduce high-frequency noise and edge overshoot. If the edge of a signal is very steep, it contains a large amount of high-frequency components, which will radiate interference. In addition, it is also easy to produce overshoot. The series resistor forms an RC circuit with the distributed capacitance of the signal line and the load input capacitance, which will reduce the steepness of the signal edge. ◆ Reduce high-frequency reflection and self-oscillation. When the frequency of the signal is very high, the wavelength of the signal is very short. When the wavelength is short enough to be comparable to the length of the transmission line, the reflected signal superimposed on the original signal will change the shape of the original signal. If the characteristic impedance of the transmission line is not equal to the load impedance (i.e., mismatched), reflection will occur at the load end, causing self-oscillation. The low-frequency signal of the PCB board can be directly connected, and generally no series matching resistor is required. (2) Parallel impedance matching is also called "terminal impedance matching", which is generally used at the input/output interface end, mainly referring to the impedance matching with the transmission cable. For example, the input end matching resistance of LVDS and RS422/485 using Category 5 twisted pair is 100~120Ω; the matching resistance of video signal using coaxial cable is 75Ω or 50Ω, and that of parallel cable is 300Ω. The resistance value of parallel matching resistance is related to the medium of transmission cable, not to the length. Its main function is to prevent signal reflection and reduce self-excited oscillation. It is worth mentioning that impedance matching can improve the EMI performance of the system. In addition, in addition to using series/parallel resistors to solve impedance matching, transformers can also be used for impedance transformation. Typical examples include Ethernet interface and CAN bus. 2. The role of 0 ohm resistor

(1) The simplest is to make a jumper. If a certain section of the circuit is not used, just do not solder the resistor (it will not affect the appearance).

[color=rgb(25, (25, 25)]（2）When the matching circuit parameters are uncertain, use 0 ohm instead. When actually debugging, determine the parameters and then replace them with components with specific values.

（3）When you want to measure the working current of a certain part of the circuit, you can remove the 0 ohm resistor and connect an ammeter to make it easier to measure the current.

（4）When wiring, if it is really impossible to wire, you can also add a 0 ohm resistor to act as a jumper. (5) In high-frequency signal networks, it acts as an inductor or capacitor (impedance matching, 0 ohm resistor also has impedance!). When used as an inductor, it is mainly used to solve EMC problems. (6) Single-point grounding, such as the single-point connection between analog ground and digital ground. (7) Configuration circuit, which can replace jumpers and dip switches. Sometimes users will mess with the settings, which may cause misunderstandings. In order to reduce maintenance costs, 0 ohm resistors should be soldered on the board instead of jumpers. (8) For system debugging, for example, the system can be divided into several modules, and the power supply and ground between modules can be separated by 0 ohm resistors. When a power or ground short circuit is found during the debugging stage, removing the 0 ohm resistor can narrow the search range.

The above functions can also be replaced by "magnetic beads". Although the 0 ohm resistor and the magnetic bead are somewhat similar in function, there are essential differences. The former has impedance characteristics, and the latter has inductive reactance characteristics. Magnetic beads are generally used in power supply and ground networks for filtering.

Source: Internet, please delete if infringed.

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