At 08:28 AM 5/27/2004 -0500, Mike Hord wrote:

>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.


It's the parallel combination of the two resistors.
AC impedance can be lowered by adding a cap, but that won't help you with fighting input offset or leakage on the ADC pins.

An emitter follower will though.  You aren't really interested in the exact voltage anyway, you can easily compensate for the error introduced by the EF.

Now your input will be in the 10's of ohms, dependent on the gain of the transistor used.

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