Hi,
The digital sample outputs are use LVDS signalling, so search for LVDS and read up on that. LVDS is a differential signalling, so every 'signal' is really a pair signal pins that are complement of each other. While one pin is 'high' the other pin is 'low'.
if you use a multimeter to look at the voltage level of the pins, then for pins that are not switching you would see one pin of the differential output high and the matching pin low. But for output pins that are switching the multimeter will simply show you the average of the high and low voltage levels over time.
Also, the analog input signal needs to be differential and the voltage levels need to correspond to the data sheet requirements. So if you wish to enter a constant input voltage, then that voltage needs to be converted to differential and each side of the differential signal needs to have its voltage swing centered around the datasheet requirement for the common mode voltage (called VCM or CM). The EVM you bought has a transformer coupled input circuit and the trnasformer does the conversion from a single ended input signal to differential and the transformer also provides the AC coupling so that the level of the differential signal could be biased to the desired voltage levels. But transformers do this for AC signals - not for static DC signals. The transformer coupling on the EVM will only work for input frequencies above the low frequency cutoff specified for the transformer used - usually about 1MHz or more. I don't know where you are injecting your voltage from your 500 ohm potentiometer - but this voltage needs to conform to datasheet requiremnts: differential and biased to be centered about 2.5V common mode.
Regards,
Richard P.