I know this is rather old thread, but I plan to use ADG604 in one project and I needed to know its distortion too, so I measured it with Audio Precision APX525. It's basically a network analyzer, except it's designed specifically for audio applications. I used these conditions during measurement:
-Symmetrical supply voltage +-5 V, sourced from Agilent E3630A
-ADG604 soldered on SMD breakout board which was then then inserted into standard solderless breadboard
-2x 100 nF decoupling capacitors
-Unbalanced output and input (BNC connectors) used on APX525
-Output connected to S1 pin, input connected to D pin of ADG604
-Output impedance 20 ohm
-Input impedance 100 kohm
-Input bandwidth 90 kHz (APX525 maximum, it's an audio analyzer)
First I measured THD vs. frequency from 20 Hz to 80 kHz, source level was 1 Vrms (my project will use this level). THD was under 0.02% in the entire band. I didn't find any way how to directly attach images in this forum, so here is link to Imgur:
Then I measured THD vs. input level, first for 1 kHz and then for 40 kHz. The input level was sweeped from 1 mVrms to 6.3 Vrms. I'm attaching the results below, distortion is under 0,02% up to 3.5 Vrms at 1 kHz. It gets worse at 40 kHz, but keep in mind that 40 kHz is at the usable limit of the instrument, it can't process any harmonics beyond 90 kHz. Overally, i consider that a great result (at least for my project).
THD Ratio vs level at 1 kHz:
THD Ratio vs level at 40 kHz:
Hopefully, someone will these measurements useful...