Sugiyama-san,
the ADS42JB49 has two overrange functions. The normal OVR (default) indicates when the max code is exceeded - so the input signal exceeding the max fullscale value.
The fast overrange is the second option (can be set via SPI). There is a programmable threshold for the fast OVR which triggers the FOVR indicator. In the excel file the FOVR threshold is set to 10 which is equivalent to about -22dBFS. Based on the info in the excel sheet looks like fullscale is about +12dBm on your signal generator which means the FOVR threshold is at -10dBm approximately.
At -9dBm input power on your signal generator you start seeing few instancesof OVR getting triggered. I'm suspecting your input signal is much slower than your sampling clock (e.g. fin = 50MHz, Fs=250Msps) and you sample the peaks of the input signal that trigger the ovr but you also sample a lot of sample point (eg. around 0 crossing) that don't trigger the FOVR. As you increase the swing more and more sample point exceed the threshold which is what the scope pictures show as well.
The FOVR signal is not flag that stays high once triggered but self clears after each sample.
Please let us know if this answers the question.
Best regards,
Tommy