It seems to be that this is not possible to find in one chip.
I found ADAS3022 which has 8 inputs high impedance, 0-5V and 0-10V input 16bit. But this consumes quite much current.
I also found AD7171 which is low power but needs external reference 5V for 0-5V, external MUX and may be external buffer for high impedance.
I know that there were multimeter circuits like ICL7109 which had very high input impedance, stable values, but unfortunately no 0-5V input and no multiplexer in front.
I would need a solution for a data logger - battery driven - and therefore need something with very small current consumption that does not need to be very fast. 100 Sps or so would be ok.
May be somebody has an idea.
Thank you very much !