Knowledge

Impedance matching

Impedance matching refers to a suitable matching method between the signal source or transmission line and the load. There are two ways of impedance matching: serial and parallel according to the access method; impedance matching can be divided into two types: low frequency and high frequency according to the signal source frequency.

High frequency signals generally use serial impedance matching

The resistance of the series resistor is 20~75Ω. The resistance 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 a frequency greater than 20M and a PCB trace length greater than 5cm, such as clock signals, data and address bus signals in the system, etc. The series matching resistor has two functions:

§Reduce high-frequency noise and edge overshoot. If the edge of a signal is very steep, it contains a large number of high-frequency components, which will radiate interference. In addition, it is also prone to 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-excited oscillation. When the frequency of the signal is very high, the wavelength of the signal is very short. When the wavelength is so short that it is 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. does not match), reflection will occur at the load end, causing self-oscillation. The low-frequency signals routed within the PCB board can be connected directly, and there is generally no need to add a series matching resistor.

Parallel impedance matching is also called “terminal impedance matching”

Generally used at the input/output interface, mainly refers to the impedance matching with the transmission cable. For example, when LVDS and RS422/485 use Category 5 twisted pair cables, the input end matching resistance is 100~120Ω; when video signals use coaxial cables, the matching resistance is 75Ω or 50Ω, and when using flat cables, the matching resistance is 300Ω. The resistance 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-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 interfaces, CAN buses, etc.

Zero ohm resistor

l It is simply used as a jumper. If a certain section of the line is not used, just do not solder the resistor (it does not affect the appearance).

l When the parameters of the matching circuit are uncertain, use zero ohms instead. During actual debugging, determine the parameters and then replace them with components with specific values.

lWhen you want to measure the operating current of a certain part of the circuit, you can remove the zero-ohm resistor and connect it to an ammeter, which makes it easier to measure the current.

l When wiring, if you really can’t lay it out, you can also add a zero-ohm resistor to act as a jumper.

l In high-frequency signal networks, it acts as an inductor or capacitor (plays an impedance matching role, and zero-ohm resistors also have impedance). When used as an inductor, it is mainly used to solve EMC problems.

lSingle-point grounding, such as single-point connection of analog ground and digital ground to a common ground.

lConfiguration circuit can replace jumpers and DIP switches. Sometimes users will tamper with the settings, which can easily lead to misunderstandings. In order to reduce maintenance costs, zero-ohm resistors should be soldered to the board instead of jumpers.

l For system debugging, for example, divide the system into several modules, and separate the power supply and ground between modules with zero-ohm resistors. When a short circuit is found in the power supply or ground during the debugging stage, removing the zero-ohm resistor can narrow the search range.

The above functions can also be replaced by “magnetic beads”. Although zero-ohm resistors and magnetic beads are somewhat similar in function, they are essentially different. The former has impedance characteristics and the latter has inductive reactance characteristics. Magnetic beads are generally used in power and ground networks to have a filtering effect.

If a worker wants to do his job well, he must first sharpen his tools. A better understanding of impedance matching and zero-ohm resistance will make PCB design and manufacturing easier.
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