Ah, didn't know this dc-dc-converters.
Yes all thermistors have common ground.
I have 4 so i need 2 ADUM6401.
The central heating connections are:
AF1 = 0 volt ground
AF2 = outside air thermistor
KF2 = furnace thermistor
VF2 = underfloor heating water thermistor
BF2 = hot water storage tank thermistor
The ADC chip means i read the voltage on the Arduinos digital inputs?
Do you have an example circuit diagram to give me a kick-start?
For the evaluation board i think you mean this one: http://www.analog.com/en/evaluation/eval-adumqs/eb.html
But it's a little bit to difficult to get here in Germany.