I'd like to use a ADuM1250 to isolate a AD8557 digital instrument amp from the FPGA it's connected to. Is this possible or is the ADuM1250 interface only compatible with I2C devices?
The ADuM1250 can probably be made to work, and it is not exclusively for use with I2C, but I think it is not a good fit for this application. The AD8557 digital input is generated on high pulses, which means that the idle state is low, so the ADuM1250 will have to be actively driving against its output pull up to maintain an idle state at the DIGIN pin. Side 1 will not be reliably read as a low in all conditions since it drives its output as high as 0.9V, and the Vil for this pin must be 0.2 x Vdd which can be below 0.9V. The side 2 drivers would have to be used. There are a lot of odd situations to verify with this combination. As I said, it can likely work, it is just a lot of work.
It would be simpler to use a default low standard digital isolator like the ADuM1286.