maximum input impedance of ADC

Hi all, I need to use an external voltage divider (from 24V to 3V) to serve as ADC's input, so I guess I need to know the maximum input impedance of ADC for me to work out the resistance and capacitance. However, I did not find the specific maximum impedance for ADC on SAM3X8E datasheet. Does anyone know the exact value and where to find it? Thanks in advance.