CAN network impedance
The problem started with the CAN network. The figure below is a basic model of a CAN network with 120 ohm resistors at both ends.
The characteristic impedance of the wire used in the CAN network is also 120 ohms. Here are a few questions to explain separately.
1. Why use a terminal impedance of 120 ohms?
First, the transmission line is used in the CAN network, and the characteristic impedance of the wire is 120 ohms. Let’s discuss this with the problem below the line. Another thing to explain is the device in the CAN network, that is, the CAN transceiver. The output impedance of this device is very low, and the input impedance is relatively high. You can see the block diagram of TJA1050, also That is to say, the signal transmitted with the characteristic impedance of 120 ohms on the transmission line suddenly reaches a place with high impedance, which can be understood as an open circuit, which will cause high signal reflection, which affects the level sampling of the CAN transceiver, and causes information errors. read. If you add a 120 ohm resistor between CANH and CANL, that is, the terminal resistance, because this resistance is the same as the characteristic impedance of the cable, and at the same time, this resistor, which is much smaller than the output impedance of the CAN transceiver, is connected in parallel with the CAN transceiver, and the current is naturally higher. Most of them flow from places with low impedance, so that from the 120 ohm resistance of the flow channel on the cable with the characteristic impedance of 120 ohms, the impedance between them is close, and their signal reflection is much smaller, which can effectively ensure the signal integrity. At the same time, this resistance will not affect the signal itself as shown in the figure below. For example, in a fault-tolerant CAN network, CANH=3.5v, CANL=0.5v, it is dominant, CANH=CANL=2.5v, it is invisible, in the dominant position When the terminal resistance is 3.5v and 1.5v respectively, a CAN transceiver is the output end and the other CAN transceiver is the receiving end, and the output end is the output voltage. Keep the voltages of CANH and CANL at 3.5v and 1.5v. The voltage difference between them will cause the current to be consumed by the terminal resistance. The CANH and CANL at the receiving end can accurately sample the voltage values ​​of 3.5v and 1.5v. Similarly, the terminal resistance does not affect the CAN network signal when in the invisible position. But the effect of impedance matching is achieved.
2. How to define the characteristic impedance of the 120 ohm characteristic impedance wire used in the CAN network?
Characteristic impedance is a specific characteristic of a material that we are talking about here as a wire, which is determined by its own thickness, size and other factors. The characteristic impedance of a wire or coaxial cable does not change with the length and the frequency of the transmission signal (there will be a certain difference with the frequency, but it is very small, theoretically considered not to change with the frequency), and the characteristic impedance also has its own characteristics. Reflecting the conditions, in the DC state, if you use a multimeter to measure the impedance of a section of wire, the result should be Z=0 ohm. But the marked characteristic impedance of this wire is 50 ohms. The so-called 50 ohm characteristic impedance, its 50 ohm impedance is reflected in the high-frequency signal transmission process. Reflected by the following formula, see the figure below:
among them
R=The resistivity of the conductor material (in the case of direct current) per unit length, ohm
G=bypass conductivity per unit length (conductivity of insulating layer), ohm
j= is just a symbol, indicating that this item has a phase angle of +90' (imaginary number)
Ï€=3.1416
L=inductance per unit length of cable
c=Capacity of cable per unit length
Under the condition of direct current, the R and G in the formula can be ignored. The formula can be simplified to find the square root of L/C. If the wire is very short, L should be very close to 0, so the final value is also 0, if this line is very long, L cannot be ignored, and a certain resistance value can also be seen by measuring with a multimeter.
In the process of high-frequency signal transmission, R and G cannot be ignored, and the impedance of the wire is reflected under the action of the inductance and capacitance of the wire during the rise and fall of the AC signal.
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