I need the high impedance so that we don't take current from the circuit being monitored.
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.