How to calculate Capacitor value for AC Coupling.

I'm using Cyclone V 5CGXFC5C6F23I7N FPGA.

In Transceiver section,the clock for the REFCLK is provided by External OSCILLATOR and its frequency is 125MHz, LVDS IO Standard,and I'm using AC COUPLING,for AC Coupling we need capacitor,so when I start to find the value of the capacitor I understand that I need to know the INPUT IMPEDANCE at the dedicated REFCLK PINS of the FPGA, but I didn't found the information about INPUT IMPEDANCE at the at the dedicated REFCLK PINS.

so what would be the INPUT IMPEDANCE at the dedicated REFCLK PINS of theCYCLONE V FPGA?