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In a high-speed PCB board system, the impedance mismatch on the transmission line will cause signal reflection, and signal distortion such as overshoot, undershoot and ringing will occur. When the delay TD of the transmission line is greater than 20% of the signal rise time RT, the reflected The impact cannot be ignored, otherwise there will be signal integrity problems. The method of reducing reflection is: according to the characteristic impedance of the transmission line, connect the resistance in series at the driving end to match the source impedance with the impedance of the transmission line, or connect the resistance in parallel at the receiving end to match the impedance of the load with the impedance of the transmission line, so that the source reflection coefficient or load reflection coefficient is zero. Commonly used termination methods are: series termination, simple parallel termination, Thevenin termination, RC network termination and diode termination, etc. These termination methods will be analyzed separately below.
(1) Series termination
Series termination means that a resistor RS is connected in series as close as possible to the source end to match the impedance of the signal source, so that the reflection coefficient of the source end is zero, thereby suppressing the signal reflected from the load to be reflected back from the source end to the load end. PS plus the output impedance ZS of the driving source should be equal to the transmission line impedance Zo, that is, RS=ZO-Zs. The value of the series resistance is usually selected in the range of 15 to 75Ω, and more often it is selected as 33Ω. The advantage of series termination is that only one termination resistor is required for each line, and there is no need to connect the DC power supply, so it does not consume too much power; when driving high capacitive loads, it can provide current limiting, which can Helps reduce ground bounce noise. The disadvantage is: due to the voltage divider effect of the series resistance, in the middle of the trace path, the voltage is only half of the source voltage, so it cannot drive the distributed load; due to the series resistance on the signal path, the RC time constant is increased, thereby slowing down the load. Therefore, it is not suitable for high-frequency signal paths (such as high-speed clocks, etc.). It is important to note that this series resistor must be as close as possible to the output of the source driver, and that vias should not be used on the PCB because vias have capacitance and inductance.
(2) Parallel termination
Parallel termination is also called DC parallel termination. In this way, a termination resistor Pp (Ap=Zo) is connected to the input end of the receiver (ie, the end of the wiring network) to pull down to ground or pull up to the DC power supply to achieve matching. Reflections are eliminated at the load. The advantage of this termination method is that the design is simple and easy to implement, but the disadvantage is that it consumes DC power and cannot be used in portable devices that require low power consumption. In addition, this pull-up to power can increase the drive capability of the driver, but it will raise the low level of the signal; while the pull-down to ground can improve the current sink capability, but it will pull down the high level of the signal.
(3) Thevenin termination
Thevenin termination is a voltage divider termination, which uses a pull-up resistor Rpl and a pull-down resistor Rp; constitutes a termination resistor and absorbs reflections through Rp1 and Rp2. The equivalent impedance of the Thevenin termination must be equal to the characteristic impedance Zo of the transmission line to achieve matching. The selection of the resistance values of the termination resistors Rpl and Rp2 should focus on avoiding setting an inappropriate load voltage reference level, which is used for the high and low logic transition points. The Rp1/Rp2 ratio determines the relative proportions of logic high and low drive currents. When Rp1=Rp2, the drive requirements for high and low logic are the same; when Rpl
(4) RC network termination
RC network termination, also known as AC load termination, uses a series RC network as the termination impedance to eliminate reflections at the end of the network. The termination resistance Rp should be equal to the transmission line impedance Zo, and the selection of the capacitor Cp should ensure that the time constant of the RC network should be greater than twice the propagation delay, that is, "RpCp>2TD", usually 0.1μF multilayer ceramic capacitors are used. For specific designs, the capacitance values are determined by simulation. The benefit of AC termination is that the capacitor blocks the DC path without additional DC power dissipation, while allowing high-frequency energy to pass through and acting as a low-pass filter. The downside is that the time constant of the RC network slows down the signal. Additionally, additional resistors and capacitors take up board space and add cost.
(5) Diode termination
In the diode termination method, one diode is connected in series between the end of the transmission line and the power supply Vcc, and the other diode is connected in series between the end of the transmission line and the ground. Schottky diodes are usually used because of their low turn-on voltage. Unlike other terminations, diode termination is not an attempt to match the characteristic impedance of the transmission line to eliminate reflections. When the receiver voltage overshoots, the diode starts to work to stabilize the voltage. Although it prevents overshoot, it has two disadvantages: reflections still exist in the system; reflections on high-speed signals are slower. In order to obtain the advantages of this technique, it can be used in combination with the previous methods on PCB board.
