The ADCLK925 can accept a signal up to 3.4Vpp differential. A sine wave at 800MHz is ok, however, +22dBm is too high. If you can attenuate the signal by 8-10 dB to get it within the specified range, then it will be work.

The calculation you have done yields RMS voltage, but it is the peak to peak voltage that is relevant to the input receiver voltage specifications. You need to multiple your result by 2*sqrt(2) to get the peak to peak value.