Differential Output Terminations LVPECL, HCSL, LVDS ... - SiTime

Output Terminations for SiT9102/9002/9103LVPECL, LVDS, CML, and HCSL differential driversOUT+Zo = 50ΩZo = 50Ω100 Ω0.1μF0.1μFOUT+OUT-OUT-LVDS, Normal swingFigure 13: LVDS single AC termination at the loadThe double terminations shown in Figure 14 and Figure 15 differ only in the position of the ACcouplingcapacitor. The capacitor in Figure 14 is charged by the common mode current flowingthrough half the differential resistance, which is the equivalent of 50Ω. On the other hand, thecapacitor in Figure 15 is charged by the current through the resistance of the receiver’s inputs,which can be in the range of kilo-ohms. During clock start-up, the capacitor shown in Figure 14will be charged much faster than that shown in Figure 15. Therefore, a valid clock signal will beavailable sooner to the receiver. If fast clock start-up is important, the configuration shown inFigure 14 is preferable.OUT+100 Ω0.1μF0.1μFZo = 50ΩZo = 50Ω100 ΩOUT+OUT-OUT-LVDS, High swingFigure 14: LVDS double AC termination with capacitor close to the sourceIn data transmission applications, the configuration shown in Figure 15 may be moreadvantageous. Because of its higher RC time constant, it can sustain data sequences withlonger 1s and 0s without experiencing significant voltage droop.OUT+100 ΩZo = 50ΩZo = 50Ω100 Ω0.1μF0.1μFOUT+OUT-OUT-LVDS, Normal swingFigure 15: LVDS double AC termination with capacitor close to the load--------------------------------------------------------------------------------------------------------------------------------------------The Smart Timing Choice 10 SiT-AN10009 Rev 1.3