There is a problem in the Quartus® II software version 10.0 and 10.0sp1 with the generation of the IBIS files, the [Diff Pin] section is missing. This will be fixed in a future version of the Quartus II software.
You will need to manually add this section in the IBIS file created by the Quartus II software version 10.0 and 10.0sp1.
The following is an example of the text that needs to be added in the .ibs file for differential pins. The "vdiff" parameter should be set to the differential input specification for the I/O standard you are using for input pins. In this example, pin AB18 is the positive leg of a differential SSTL 18 input pair, pin AB19 is the negative leg. This I/O standard has a Vswing(AC) specification of 500mV. M19 is the positive leg of an LVDS differential pair, M20 is the negative leg. The Vid minimum specification is 100mV.
[Diff Pin] inv_pin vdiff tdelay_typ tdelay_min tdelay_max
AB18 AB19 0.5V NA NA NA
M19 M20 0.1V NA NA NA