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Impedance matching and 0 ohm resistors in PCB design

writer: G February 26, 2019

(1) High frequency signals are typically matched using serial impedance. The resistance of the series resistor is 20~75Ω. The resistance value is proportional to the signal frequency and inversely proportional to the PCB trace width. In embedded systems, when the signal is greater than 20M and the PCB trace length is greater than 5cm, the system adds serial matching resistors (such as clock signals, data and address bus signals) to the system. The serial matching resistor has two functions:

◆ Reduce high frequency noise and edge overshoot. If the edge of the signal is very steep, it will contain a lot of high frequency components, which will cause radiation interference and is also prone to overshoot. The distributed capacitance of the series resistor and signal line and the load input capacitor form an RC circuit to 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 shorter than the length of the transmission line, the superposition of the reflected signal 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 (ie, it does not match), then reflection will occur at the load end, causing self-oscillation. The low frequency signals of the traces on the PCB can be directly connected, and it is usually not necessary to add a series matching resistor.

(2) Parallel impedance matching, also known as "terminal impedance matching", is usually used at the input/output interface end, mainly referring to the impedance of the transmission cable. For example, the input 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 the flat cable is 300Ω. The resistance of the parallel matching resistor is related to the medium of the transmission cable, regardless of 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, impedance matching can be used to perform impedance transformation in addition to series/parallel resistance. Typical examples are Ethernet interfaces, CAN buses, and the like.

2. 0 ohm resistance

(1) The simplest is to use jumpers. If you do not use a wire, you can solder the resistor directly (without affecting the appearance).

(2) When the matching circuit parameters are uncertain, replace them with 0 ohms. When performing actual debugging, determine the parameters and replace them with specific digital components.

(3) When you want to measure the working current of a certain part of the circuit, you can take off the 0 ohm resistor and connect the ammeter to measure the current.

(4) When wiring, if you do not pass the actual fabric, you can also add a 0 ohm resistor to bridge the effect.

(5) In the high-frequency signal network, act as an inductor or capacitor (impedance matching, 0 ohm resistor also has impedance!). When used as an inductor, it is mainly to solve the EMC problem.

(6) Single point grounding, such as single point analog ground and digital ground.

(7) The configuration circuit can replace the jumper and DIP switch. Sometimes users tamper with settings, which can lead to misunderstandings. To reduce maintenance costs, use a 0 ohm resistor instead of a jumper to solder the board.

(8) For example, for system debugging, the system is divided into several modules. The power supply between the modules is separated from ground by a 0 ohm resistor. When the power or ground is shorted during the debug phase, the 0 ohm resistor can be removed to narrow the search.

The above functions can also be replaced by "magnetic beads". Although the 0 ohm resistor and the magnetic bead have similar functions, there are essential differences, the former being the impedance characteristic and the latter being the inductive. Magnetic beads are commonly used in power and terrestrial networks for filtering.

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