Unity Gain Amplifier / Voltage Follower - Selecting resistor

[tocpcs]

For the purpose of this discussion impedance means the same thing as resistance.

What's needed is a comparatively low impedance connection to the analog in, that way practically all of the voltage is across the high impedance.
If you have a 1000? resistor in series with a 100M? resistor, practically all of the applied voltage is where? It is across the 100M?, 1000? : 100M?.

You missed the part where I said to run the op-amp from "Vin", but you can't do that if you're stuck on running from 5V.

I caught that - VIN will be empty - the plan is to not use the voltage regulator and bring in +5 from the already regulated +5 at the other controller - (saves getting an additional supply and then wasted energy from that).

I was going to go and chuck a 9V supply in, but if I'm not going to have an issue wiring the input to the arduino (i.e. I don't want the arduino loading down the sensor), then I won't worry about the Op-Amp.
The entire point of the op-amp is so I don't load it down - if the analog inputs are indeed 100Mohm then I would hope that's fine? The sensor reading won't get loaded down, it'll be accurate and I'll get on with reading the voltage...