Hi Michele,
When the part is handling an RF signal then the ADG9xx can handle a 7dBm (0.7Vp) signal and with a 0.5V DC bias it can pass a 16dBm (2Vp) signal.
This is determined by the lower limit in datasheet of -0.5V DC; so to simplify things you can relate the AC signals to the Vrms level. 7dBm = 0.5Vrms and we recommend this is max signal you should use with 0V bias. With 16dBm signal we recommend a 0.5V DC bias to offset your input and this ensures you are not clipping your signal.
The power handling of the ADG9xx decreases with frequency because parasitic diodes will begin to turn on if input signal goes below ~-0.6V. The attached app note (Power Handling section) explains in more detail why this is so.
http://www.analog.com/static/imported-files/application_notes/AN_952.pdf
Rgds,
Theresa.