> > > What is the maximum recommended impedance for the PIC32 A/D? Choose
> > > your resistors to be four times that, or less.

> > How do you get to "four times that, or less"? Isn't that "two times
> > that, or less", for a simple divider like this:
>
> Good catch. That would be my brain skipping a beat :)
>
> Two times would be the correct ratio.
>
> If it were a potentiometer instead of two resistors, the impedence is
> never more than one-fourth the pot value. Must have been my pre-coffee
> fingers typing.

If you are paralleling resistors (as far as signal is concerned) then
you MAY be able to have resistors of higher than specified maximum
load value, as the effective end impedance is what counts.

BUT it is always [tm] a good idea to end up with usefully less than
the maximum value if there are no other constraints. If you use too
low a value you will load the driver (op am) and power dissipation may
be a concern in extreme cases, but staying away from the maximum
allowed value (eg 10k or whatever) does no harm. Max value is set such
that the conversion error caused is no worse than some limit -maybe
1/2 LSB, maybe less. Less than less error is good if it's essentially
free.



             Russell
--=20
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