> Ignoring the power dissipation in the resistors, I'm being told that the
> following pairs, are all within spec:
>
> 1M/10K
> 100K/1K
> 10K/100ohm
> 1K/10ohm
> 100ohm/1ohm.
>
> I would have guessed that the top one is going to charge the capacitor a
> lot slower than the bottom one.... but I'm starting to think that
> perhaps they are all equivalent? =A0 Did I miss/forget something in my
> electronics training?

A lot slower is correct BUT "fast enough to meet spec" is the aim.
Examples 2 to 5 are lower than needed - which is fine.

You can have eg 0.1V fed in through 10k in series, OR 50v fed in
through 490k with 10k to ground.
In both cases you see 0.1V at the pin. In the second case the
impedance is SLIGHTLY lower as it is 10k in parallel with 10k.

In the initial case, input current is 0.1V/10k =3D 10 uA.

In the 2nd case input current is (50-0.1)/500k ~=3D 10 uA.
So the higher resistance is matched by a higher voltage to get the
same voltage at the PIC pin . and 'it all balances out'.

In example1, for a given PIC ADC input capacitor, which is only going
to be charged to 0.1V in this example, the large input resistance is
offset by the large input voltage which can be accepted. All the other
examples provide MORE input current than is needed.


 Russell McMahon

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