Hi Fernando,
The Johnson current noise of the total 20K ohm input resistance (18K external plus 2K internal) at room temp & 20KHz BW, works out to 0.13nA RMS. This is 118dB below the 100uA RMS full-scale input current. Overall, the -1701's ADCs are noisier, at -100dB SNR. This suggests that we can vary the ADC input resistance together with its input voltage over a moderate range with little change in the ADC's SNR.
With the standard 18K ohm external resistor, full-scale input is 2VRMS. If your driving stage is capable of more swing than that, it would make sense to increase both its gain and the ADC input resistor, to make that stage and the ADC clip at about the same level. This should provide the best SNR between them. Since the ADAU1701's ADCs are current input, the usual advice to keep impedance low doesn't apply here.
Best regards,
Bob