Issue link: https://iconnect007.uberflip.com/i/1219242
40 DESIGN007 MAGAZINE I MARCH 2020 3. Differential Pair Termination When a differential signal reaches the open end of a differential pair, it will see a high im- pedance, and it will be reflected. A common method of reducing these reflections is to place a resistive load at the end of the pair, which matches the differential impedance of the transmission line. However, the single resistor differential ter- mination (Figure 5a) only terminates the dif- ferential mode signal—not the common-mode signal. Any transient common-mode signal moving down the differential pair will see a high impedance at the end of the pair and re- flect back to the source. The noise created can be seen at the top and bottom of the eye di- agram. The common-mode signal, having the same voltage between the two signals, will not see the termination. Depending on the imped- ance of the driver, the common-mode signals created will bounce back and forth down the transmission lines. And because the termina- tion resistor never perfectly matches the driver impedance, there is always a fixed offset. Any asymmetry in the differential pair will convert the differential signal into a common-mode signal. Whereas the differential center tapped ter- mination (Figure 5b) terminates both differen- tial and common-mode signals, note the nice clear eye. It is best used when you have a low impedance driver on a long transmission line. This strategy introduces less DC offset. The DC blocking capacitor is for level adjustment of DC balanced circuits, such as clocks, 8B10B coding, etc. When Do We Need a Termination? If the transmission line is short, reflections still occur but will be overwhelmed by the ris- ing or falling edge of the signal and may not pose a problem. But even if the trace is short, termination may still be required if the load is capacitive or highly inductive to prevent Figure 4: DDR3 fly-by topology with termination after the final load (simulated in HyperLynx).