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Technical Article

The Role Of Matching Resistors In PCB Design

Time:2021-02-26 14:51:12 Click:

1. PCB design impedance matching

PCB design impedance matching refers to a suitable matching method between the signal source or transmission line and the load. According to the access method, the impedance matching of PCB design has two methods: Serial and parallel; And in the light of the frequency of the signal source, the impedance matching of PCB design can be divided into another two types:Low frequency and high frequency.





High frequency signals generally use serial PCB design impedance matching

The resistance value of the series resistor is 20~75Ω, and the resistance value is proportional to the signal frequency and inversely proportional to the PCB trace width. In embedded systems, serial matching resistors are generally required for signals with frequencies greater than 20M and PCB trace lengths greater than 5cm, such as clock signals, data and address bus signals in the system. There are two functions of series matching resistance:


1> Reduce high frequency noise and edge overshoot. If the edge of a signal is very steep, it contains a lot of high-frequency components, which will radiate interference. In addition, it is also prone to overshoot. The series resistance and the distributed capacitance of the signal line and the load input capacitance form an RC circuit, which will reduce the steepness of the signal edge.


2> Reduce high frequency reflection and self-excited oscillation. When the frequency of the signal is high, the wavelength of the signal is very short. However, if the wavelength is as short as 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 (that is, it does not match), reflections will occur at the load end, causing self-oscillation. The low-frequency signal of the wiring in the PCB board can be directly connected, and generally no serial matching resistor is required.





Parallel PCB design impedance matching is also called "terminal PCB design impedance matching"

Generally used in the input/output interface, which mainly refers to the impedance matching with the PCB design of the transmission cable. For example, LVDS and RS422/485 use Category 5 twisted-pair cables with input end matching resistance of 100~120Ω; Video signals use coaxial cables with matching resistance of 75Ω or 50Ω, and flat cables with a matching resistance of 300Ω. The resistance value of the parallel matching resistor is related to the medium of the transmission cable and has nothing to do with the length. Its main function is to prevent signal reflection and reduce self-excited oscillation.


It is worth mentioning that the impedance matching of PCB design can improve the EMI performance of the system. In addition, in order to solve the impedance matching of PCB design, in addition to using series/parallel resistors, transformers can also be used for impedance transformation. Typical examples are Ethernet interfaces and CAN buses.





2. Zero ohm resistance

1> The easiest way is to use it as a jumper. If a certain section of the circuit is not used, just do not solder the resistor directly (it will not affect the appearance).

2>When the matching circuit parameters are uncertain, replace them with zero ohms. In actual debugging, determine the parameters and replace them with specific numerical components.

3> When you want to measure the working current of a certain part of the circuit, you can remove the zero-ohm resistance and connect an ammeter to facilitate current measurement.

4> In the PCB design and layout, if the layout is really impossible, you can also add a zero ohm resistor to act as a jumper.

5> In the high-frequency signal network, it acts as an inductor or a capacitor (for PCB design impedance matching, zero-ohm resistance also has impedance). When used as an inductor, it mainly solves the EMC problem.

6> Single-point grounding, such as the single-point connection of analog ground and digital ground.

7> The configuration circuit can replace jumpers and DIP switches. Sometimes users will change the settings randomly, which is easy to cause misunderstanding. In order to reduce maintenance costs, zero-ohm resistors are used instead of jumper wires to be soldered on the board.

8> For system debugging, for example, divide the system into several modules, and separate the power supply and ground between the modules with a zero-ohm resistance. When the power supply or ground is short-circuited during the debugging stage, removing the zero-ohm resistance can narrow the search range.






The above functions can also be replaced by "magnetic beads". Although zero-ohm resistance and magnetic beads are somewhat similar in function, there are essential differences. The former has an impedance characteristic, and the latter has an inductive reactance characteristic. Magnetic beads are generally used in power and ground networks for filtering.

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