Impedance matching refers to an appropriate matching method between the signal source or transmission line and the load. According to the access mode, there are two ways of impedance matching: serial and parallel; according to the frequency of the signal source, impedance matching can be divided into low frequency and high frequency.

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 an embedded system, a series matching resistor must be added 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. The series matching resistance has two functions:

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 easy to produce 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.

Reduce high-frequency reflection and self-oscillation. When the frequency of the signal is high, the wavelength of the signal is very short. When 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 it is generally not necessary to add a series matching resistor.

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 are Ethernet interfaces and CAN buses.

zero ohm resistance

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

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

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. When wiring, if it is really not available, you can also add a zero-ohm resistor to act as a jumper. In the high-frequency signal network, it acts as an inductor or capacitor (for impedance matching, zero-ohm resistance also has impedance). When used as an inductor, it mainly solves the EMC problem.

Configuration circuit can replace jumpers and DIP switches. Sometimes users may change the settings, which may cause misunderstanding. In order to reduce maintenance costs, zero-ohm resistors are used instead of jumper wires to be soldered on the board. For system debugging, for example, divide the system into several modules, and separate the power and ground between the modules with a zero-ohm resistor. When the power or ground is short-circuited during the debugging stage, removing the zero-ohm resistor can narrow the search range.