The role of circuit termination

Termination is what we often use in circuits. Termination is especially important in high-speed circuits. If the circuit is not properly terminated at the time of design, the circuit may not work at all.

Common termination methods include the following types: series termination, parallel termination, and the termination of the Daviding and RC network termination. The following briefly describes the differences and advantages and disadvantages of several termination methods.

(1) Series termination. This is one of the easiest and most commonly used termination methods. The output impedance of the transmitting end is relatively small, then we directly connect a resistor in series on the circuit so that the total impedance of the output impedance plus the resistance is equal to the impedance of the transmission line, so that the continuity of the impedance can be ensured and the signal reflection can be reduced. Serial termination is relatively simple and the disadvantages are obvious. Since the series resistance in the line will affect the rise time of the signal, it may cause problems in high-speed circuits. In addition, because of the partial pressure of the resistance, the output of the transmitting end is reduced. The series termination resistors should be placed as close as possible to the transmit end to perform better.

(2) Parallel termination. When the input impedance of the receiving end is relatively large, we can consider connecting a resistor to the ground or to the power supply in parallel 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 a series terminal. The disadvantage is that it consumes DC power. When you pull up, you can increase the driving ability. When you pull down, it can increase the absorption of current.

(3) Daviding termination. The Thevenin termination is the use of pull-up resistors and pull-down resistors together to form the termination circuit, so that the Thevenin equivalent impedance is equal to the characteristic impedance of the transmission line in order to achieve impedance matching. The advantage of the Thevenin termination is that both the 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. However, the disadvantages are also obvious. That is due to the presence of resistance, there is a DC path between the power box grounds and the DC power consumption is large.

(4) RC network termination. RC network termination is an upgraded version of parallel termination. It is to add a capacitor below the resistance connected in parallel to ground. This can reduce the reflection as well as the parallel termination, and at the same time, because of the presence of the capacitor, the DC is isolated and the DC power consumption is reduced. Of course, the disadvantages are obvious. The time constant of the RC circuit will affect the rise time of the signal and must be carefully calculated during the use of the high-speed circuit.