Simulation or lab measurement shows that DCI does not behave in a linear fashion. In other words, the impedance seems to vary depending on the input voltage. What is the reason for this? Shouldn't the impedance continuously match the reference resistors?
The Virtex-II DCI technology is implemented with transistors, not resistors, so it can be expected to behave in a non-linear fashion. Any variation in the impedance is within the normal goals of signal integrity engineering, and should not affect the fidelity of the signal.