What determines the AD6676’s full-scale input setting?

To a first order, the full-scale setting (PIN_0dBFS) is given by the following equation with IDACI_FS being the full-scale current setting of the 1^st stage feedback DAC and  R_IN being the IF input resistance.

PIN_0dBFS = 10 × log10(1/2 × R_IN × IDAC1_FS²)

IDACI_FS can be varied over a 4 to 1 mA range thus allowing PIN_0dBFS to be varied over a 12 dB range. At the max setting of 4 mA and an R_IN of 60 ohms, the theoretical PIN_0dBFS is -3.2 dBm.  In practice, the IF, F_ADC, and L_ext settings have a secondary affect that may result in the PIN_0dBFS being up to 1 dB higher than the theoretical value.