The short answer is 50 ohms.
That's typical with most detectors.
The datasheet (http://www.analog.com/static/imported-files/data_sheets/ADL5902.pdf) states in figure 54 and supported by table 8 that a 60.4OHM resistor provides the best wideband 50 ohm impedance.
If the input impedance if off a bit, its not the end of the world, but you may give up some (linear in dB or Linear in Volt" performance.
If you need additional help, I have a great friend here ADI (near Chicago) who is the detector king. ;-)