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Do I need to assemble an additional 100 Ohm resistor myself ? I am sampling at 20 MHz, is it really necessary to attach it ?
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Yes, you need the 100-Ohm resistor. LVDS drivers output a current, and that current develops a voltage over the 100-Ohm resistor. Without the resistor the LVDS interface will not function.
I believe the DE115 board has locations on the PCB for the 100-Ohm termination resistors, but you need to solder them on. You also need to make sure that the I/O bank voltage is changed to 2.5V to be compatible with LVDS - telling Quarus that the bank VCCIO = 2.5V is not sufficient to change the voltage, you have to both change the constraint and physically change the voltage.
Cheers,
Dave
Honored Contributor
