PCB News - Impedance matching and ohm resistance in PCB Layout

1.In PCB design. 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 two types: low frequency and high frequency.

(1) High-frequency signals generally use serial 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 added 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:

◆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.

◆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 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, 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.

(2) Parallel impedance matching is also called "terminal impedance matching", which is generally used at the input/output interface and mainly refers to the impedance matching with the transmission cable. For example, LVDS and RS422/485 use Category 5 twisted pair input end matching resistance is 100~120Ω; video signal using coaxial cable matching resistance is 75Ω or 50Ω, using flat cable is 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 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.

2. The role of 0 ohm resistance

(1) 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 will not affect the appearance).

(2) When the matching circuit parameters are uncertain, replace it with 0 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 0 ohm resistor and connect an ammeter to facilitate current measurement.

(4) When wiring, if it is really impossible to deploy, you can also add a 0 ohm resistor to make a jumper.

(5) In the high-frequency signal network, it acts as an inductor or a capacitor (for impedance matching, 0 ohm resistors also have impedance!). When used as an inductor, it mainly solves the EMC problem.

(6) Single-point grounding, such as the single-point docking 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, 0 ohm resistors are used instead of jumpers to be soldered on the board.

(8) For system debugging, for example, divide the system into several modules, and separate the power and ground between the modules with a 0 ohm resistor. When the power or ground is short-circuited 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 0 ohm resistance and magnetic beads are 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 and ground networks for filtering.

The above is the introduction of impedance matching and 0 ohm resistance in PCB Layout. Ipcb is also provided to PCB manufacturers and PCB manufacturing technology