I am designing a board which integrates a Xilinx Ultrascale FPGA and the Jetson TX2 module, and I am wondering about the PCIe differential impedance tracks. The FPGA and Jetson TX2 have an input differential impedance of 100 ohms (from OEM design guidelines). However, the recommended trace impedance is 85 Ohms.
PCI Express® Base Specification Revision 3.0 provides a wide range specification for the input impedance from 80 to 120 Ohms, so the 85 is in the range. However, as I understand, with a design of the traces differential impedance of 85 Ohms, signal reflections will be higher than with a design with 100 Ohms, matching with device termination impedance. Then, why do you still recommend 85 Ohms?
Thank you very much,