On Sunday 18 April 2004 01:45 pm, Ake Hedman scribbled:
> Doing mostly software work for the last decade... Why does the ADC inpu=
t
> see the two resistances in the divider in parallel as its input
> impedance? Why isn't it just the LDR that set this? Sorry if this is a
> very dumb question...

Same here (software for the last 12+ years) but from the remnants of know=
ledge=20
from an old EE degree, an ideal voltage source has 0 resistance.  So from=
 an=20
impedance standpoint, the positive voltage rail is 0 ohms away from the=20
negative voltage rail (ground).  In reality it's quite low, and can be=20
considered to be zero for these purposes.

This is good for you, since the parallel combination brings down the impe=
dance=20
seen by the PIC.


> I plan to use the 12F675 and you are right there is a 10K recommended
> impedance of the analog voltage source. I', glad I asked the question.
> Nothing is ever as simple as it first look is it!

No kidding!

Cheers,
-Neil.

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