ADC sampling capacitor impedance

Hi there,

I'm trying to read the voltage from a resistor divider (NTC thermistor and a fixed R) and get always too low values. Now I suspect that the ADC input loads down the rather high impedance source. So what are actually the ADC loading characteristics: Is there a sampling capacitor that loads the input pin - how large? Can you give some recommendation for the maximum source impedance of the signal connected to the ADC input?

/ Eero Heikkinen