Talk about impedance matching and 0 ohm resistors in embedded system PCB design
1.Impedance matching
Impedance matching refers to a suitable matching method between a signal source or transmission line and a load. According to the access method, impedance matching can be divided into two types: serial and parallel; according to the signal source frequency, impedance matching can be divided into low frequency and high frequency.
(1) Serial impedance matching is generally used for high-frequency signals. The resistance value of the serial resistor is 20~75Ω, and the resistance value is proportional to the signal frequency and inversely proportional to the width and length of the PCB trace. In embedded systems, serial matching resistors are generally added when the PCB trace length of signals with a frequency greater than 20M is greater than 5cm, such as clock signals, data and address bus signals in the system. The functions of serial matching resistors are twofold:
◆ Reduce high-frequency noise and edge overshoot.
If the edge of a signal is very steep, it contains a large amount of high-frequency components, which will radiate interference. In addition, it is also easy to produce 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 short enough to be 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., mismatched), reflection will occur at the load end, causing self-oscillation. Low-frequency signals routed in the PCB board can be directly connected, and generally no series matching resistor is required.
(2) Parallel impedance matching is also called “terminal impedance matching”.
It is generally used at the input/output interface end, mainly referring to the impedance matching with the transmission cable. For example, the input end matching resistance of LVDS and RS422/485 using Category 5 twisted pair is 100~120Ω; the matching resistance of the video signal using coaxial cable is 75Ω or 50Ω, and the matching resistance of the video signal using flat cable is 300Ω. The resistance value of the parallel matching resistor is related to the medium of the transmission cable, not 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 to using series/parallel resistors to solve impedance matching, transformers can also be used to transform impedance. Typical examples include Ethernet interfaces and CAN buses.
2.The role of 0 ohm resistors
(1) The simplest use is to make jumpers. If a certain section of the line is not used, just do not solder the resistor (it does not affect the appearance).
(2) When the matching circuit parameters are uncertain, use 0 ohms instead. During actual debugging, determine the parameters and then replace them with components with specific values.
(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 difficult to lay, you can also add a 0 ohm resistor to act as a jumper.
(5) In high-frequency signal networks, it acts as an inductor or capacitor (acts as an impedance matching function, and 0 ohm resistors also have impedance!). When used as an inductor, it is mainly used to solve EMC problems.
(6) Single-point grounding, such as the single-point connection between analog ground and digital ground.
(7) Circuit configuration, which can replace jumpers and dip switches. Sometimes users will mess with the settings, which may cause misunderstandings. In order to reduce maintenance costs, 0 ohm resistors should be used instead of jumpers and soldered on the board.
(8) For system debugging, for example, the system is divided into several modules, and the power supply and ground between the modules are separated by 0 ohm resistors. If a power or ground short circuit is found during the debugging phase, removing the 0 ohm resistor can narrow the search range.