18.104.22.168. Programmable Common Mode Voltage (VCM)
The Quartus® Prime software automatically chooses the optimal setting for RX VCM.
On-chip biasing circuitry is available only if you select OCT. If you select external termination, you must implement off-chip biasing circuitry to establish the VCM at the receiver input buffer.
Manual VCM adjustment is not supported and is only adjusted by calibrations.