High speed systems face many simultaneous impedance requirements that must be achieved in the same stack-up – differential impedances at 100 ohms for Ethernet, USB at 90 ohms, PCIe at 85 ohms, and a myriad of single-ended DDR requirements. We want to follow the specification, selecting components and routing traces at the correct impedance. However, sometimes there are hurdles – the preferred component is not available in 85 ohms, or the upstream package is yet another impedance. 

Learn more here.