Resistance Termination in High Speed Circuits
First of all, why does the circuit need to be terminated?
It is well known that if the impedance is discontinuous in the circuit, it will cause signal reflection, upstream and downward rush, ringing and other signal distortion, seriously affecting the quality of the signal.
So impedance matching is an important consideration in circuit design. Impedance control of our PCB routing is no longer a sophisticated technology, it is basically the basic ability of every hardware engineer.
So in the specific circuit, it is not enough to consider only the impedance of the line.
The actual circuit is composed of sender, connection and receiver. What we want to achieve is that the impedance of the whole link is the same. However, it is very difficult to do this in practical circuits. Generally, the output impedance of the transmitter is relatively small, while the input impedance of the receiver is very high. To deal with this contradiction, termination becomes a very natural means. Therefore, the essence of termination is still impedance matching, which is the most important part of PCB design.
Common terminal modes are as follows: series terminal, parallel terminal, Daviding terminal and RC network terminal. Here is a brief introduction to the differences, advantages and disadvantages of several terminal modes.
(1) Series termination.
This is one of the easiest and most commonly used termination methods we can think of.
Series termination is relatively simple and has obvious shortcomings. Because of the series resistance in the line, the rise time of the signal will be affected, which may cause problems in high-speed circuits. In addition, the output of the transmitter is reduced due to the voltage dividing of the resistance.
Series terminal resistance should be placed as close as possible to the sender, which can play a better role.
(2) Parallel connection.
When the input impedance of the receiver is relatively large, we can consider connecting a resistor to the ground or to the power supply at the parallel terminal of the receiver.
The resistance value of the resistance is equal to the characteristic impedance of the line. Impedance matching is realized in this way. This method is as simple as the series terminal, but it consumes DC power. When pulling up, it can improve the driving capacity, and when pulling down, it can improve the current absorption capacity.
(3) Thevenin terminal.
The Davidinian terminal circuit is composed of pull-up resistance and pull-down resistance, which makes the equivalent impedance of Davidinian equal to the characteristic impedance of transmission line to achieve impedance matching.
The advantage of Davidinian termination is that both pull-up resistance and pull-down resistance can be used to absorb reflection. When there is no signal in the circuit, it can also provide a DC level for the circuit, which is suitable for bus application. But the drawback is also obvious, that is, because of the existence of resistance, there is a DC path between the power box and the ground, and the DC power consumption is large.
(4) RC network terminal.
RC network terminal is an upgraded version of parallel terminal.
It is to add another capacitor under the shunt resistance to the ground. In this way, the reflection can be reduced as parallel connection, and the DC power consumption can be reduced due to the isolation of capacitors. Of course, the shortcomings are also obvious. The time constant of RC circuit will affect the rise time of the signal. It should be carefully calculated in the use of high-speed circuit.
