> 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.

        Using standard circuit theory you short all voltage sources and open
circuit all current sources, and then solve for R.

        Another solution I see instead of the huge acquisition times is to put a
cap on the line, it will limit how fast the value can change, but if it's a
battery voltage it shouldn't be changing quickly anyways. TTYL

----------------------------------
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http://repatch.dyndns.org:8383/pic_stuff/

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