f2"25 Hz at 300 bauds) using AD7725 ?
The AD7725 cannot be used to directly demodulate. It samples the input and
provides a programmable digital filter into which you can load any
co-efficients. This allows the user to implement a band pass, high pass, low
pass, band stop filter, in fact you are only limited by the number of taps
You can use the AD7725 to sample a spectrum and the process the signal further
in a DSP for example to extract the FSK information.
Alternatively you could consider a complete RF strip such as the AD607, AD608.
The AD607 has a built in PLL which is ideal for demodulating FSK.