In all simplicity, a termination resistor placed at the end of the transmission line prevents the signal from bouncing back.For standard termination, you would match the termination resistor value with the differential-mode characteristic impedance of the cabling on both ends of the network. This ensures the proper termination of signals traveling in both directions on the bus.Today qihe smt pick and place machine sharing PCB Trace termination techniques: Introduction to Types .
Let’s first talk about why the circuit needs to be terminated? As we all know, if the impedance in the circuit is discontinuous, it will cause signal reflection, causing overshoot, undershoot, ringing and other signal distortions, seriously affecting the signal quality.
Therefore, impedance matching is a very important consideration when designing circuits. Impedance control of our PCB traces is no longer an advanced technology, it is basically a basic ability that every hardware engineer must have.
So in a specific circuit, it is not enough to only consider the impedance of the traces. The actual circuit is composed of a sending end, a connection, and a receiving end. What we want to achieve is that the impedance of the entire link is consistent.
However, it is difficult to achieve this in actual circuits. Generally, the output impedance of the transmitting end is relatively small, and the input impedance of the receiving end is high. To handle this contradiction, termination becomes a natural method. Therefore, the essence of termination is still impedance matching, which is the top priority in PCB design.
Common termination methods include the following: series termination, parallel termination, Thevenin termination and RC network termination. The following is a brief introduction to the differences, advantages and disadvantages of several termination methods.
his is the easiest and most commonly used termination method for us. The output impedance of the sending end is relatively small, so we directly connect a resistor in series to the circuit so that the total impedance of the output impedance plus the resistor value is equal to the transmission line impedance. This can ensure the continuity of the impedance and reduce signal reflection. Series termination is relatively simple to implement, but its shortcomings are also obvious. Since the resistor is connected in series in the line, it will affect the rise time of the signal, which may cause problems in high-speed circuits. In addition, due to the voltage division of the resistor, the output of the sending end is reduced. The series-terminated resistor should be placed as close as possible to the sending end to play a better role.
When the input impedance of the receiving end is relatively large, we can consider connecting a resistor in parallel to the ground or power supply at the receiving end. The resistance of the resistor is equal to the characteristic impedance of the trace. Impedance matching is achieved in this way. This method is as simple and easy as series terminals, but the disadvantage is that it consumes DC power. When it is pulled up, it can improve the driving ability, and when it is pulled down, it can improve its ability to absorb current.
Thevenin termination uses pull-up resistors and pull-down resistors to form a termination circuit, so that the Thevenin equivalent impedance is equal to the characteristic impedance of the transmission line to achieve impedance matching. The advantage of Thevenin termination is that both pull-up and pull-down resistors can be used to absorb reflections. When there is no signal on the circuit, it can also provide a DC level for the circuit, which is suitable for bus applications. But the disadvantage is also obvious, that is, due to the existence of resistance, there is a DC path between the power supply box and the ground, and the DC power consumption is large.
RC network termination
RC network termination is an upgraded version of parallel termination. Just add a capacitor below the resistor connected in parallel to ground. This can not only reduce reflections like parallel termination, but also isolate DC due to the presence of capacitors, reducing DC power consumption. Of course, the shortcomings are also obvious. The time constant of the RC circuit will affect the rise time of the signal, and must be carefully calculated when using high-speed circuits.Read more: PCB Trace termination techniques: Introduction to Types
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