What is the max crest factor for RMS operation?

There is no real limit on the crest factor when using ADL5920. What you will find is that there will be a increasing measurement error for signals which have a large crest factor. The rms detector used in ADL5920 is very similar to ADL5906. In the ADL5906 datasheet, you can find some plots which show the measurement error (compared to cw) for a number of different modulation schemes (Figures 4,5,7 and 8).

There is no real limit on the crest factor when using ADL5920. What you will find is that there will be a increasing measurement error for signals which have a large crest factor. The rms detector used in ADL5920 is very similar to ADL5906. In the ADL5906 datasheet, you can find some plots which show the measurement error (compared to cw) for a number of different modulation schemes (Figures 4,5,7 and 8).