would you mind to provide more information about analog input specifications as below:
1. Typical DC Leakage Current
2. Analog input impedance
3.DC Leakage Current test condition(input voltages etc.)/data/curve
For AD7927, the typical Analog input DC Leakage current is around -13nA. It was taken on the same condition stated on the datasheet. While Analog input impedance is very low since SAR ADC consists of DAC capacitors. This is the reason why it needs a buffer to drive those capacitors.
Regarding to your concern, can I ask what kind of configuration you are try to implement on AD7927. This will help us to give you recommendation for your configuration.