How does one calculate the source impedence for input to
an ADC?

More particularly, given a resistor divider, how do you find
the Rs at the midpoint?  I'm looking at using a 1M-2M
divider in a low power circuit to provide a quick and dirty
battery voltage estimate for a 9V battery driving a 3.3V
system.  Obviously, this VASTLY exceeds the 2.5k max
the datasheet on the PIC18F2320 I'm using recommends,
but since I'm only using one channel of the ADC, I can
leave that connected all the time, allowing for
tremendous acquisition times.  I don't need a lot of
accuracy, just a ballpark figure (say, tenths of a volt,
+/- 50 mV) to let the user know when to change the
battery.

I REALLY don't want to add another opamp to this
circuit, and most of the discrete component voltage
followers that immediately spring to mind would consume
rather more current than I want to spend.

Thanks!
Mike H.

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