I'm a DFAE in Japan. The customer is thinking of adopting ADF4154 or ADF4156, but he wants to use the input frequency at 78MHz. In this case, is it possible to operate at the low frequency if the slew rate is kept at 400V / µs?
Yes this should be okay provided slew rate requirements are met as you have said. I would advise confirming this with an evaluation board if possible