ADC sampling frequency two signal in 1 degree phase
If you have two signals of 1.5 MHz and want to discriminate their phase shift to 1°, then you have two options
The hardware obvious way, sample at 360*1.5 MHz = 540 MHz. Each sample will be 1° of phase. Count the samples between zero crossings. You can't do that with an AD9642, the fastest grade is 250 MHz, and cheaper ones are 170 MHz.
If you are prepared to do some processing, then a bit more than Nyquist on your highest expected frequency, say 5 to 10 MHz, though with a 9642 you can sample rather faster. Do a DFT of the two sampled waveforms, and extract the phases. I've discriminated 0.01° between two 10 kHz signals sampled at 44.1 kHz on a PC soundcard
As an alternative, if those 1.5 MHz +/- are not going to change in frequency between measurements, then sample at pretty much any frequency you like, except an exact multiple or sub-multiple of 1.5 MHz. The input signals will alias, but the phase relationship between the two input signals will be preserved in their aliased versions. If you sampled at 501 kHz, then you'd get 3 kHz aliased signals. If the 1.5 MHz signals were 1° apart in phase, then so would the 3 kHz signals.