Hi, guys. Thank you for your support.
According to Figure 1 and Figure 2 of the AD654 data sheet, Fout = 0 Hz when VIN = 0 V.
Please let me know that BJT of Fout is ON or OFF at this time?
The state of the BJT is at OFF state. In this case, the amplifier is a voltage to current converter. Since the input in the amplifier is VIN = 0V, there is no current driving the BJT so it's in the cut-off region.
The AD654 is an old product so it's not recommended for new designs. I suggest using new ADC's to have better performance and features.