At 04:12 PM 07/02/2012, you wrote:
> >
> >
> > Now a thermistor, it simply generates a voltage
> > as a divider with a source impedance of course.  As the temperature
> > changes, the voltage that
> > is
> > generated across it changes, and the resolution
> > of what you can measure is going to be dependent on the source voltage =
as
> > well
> > as the ADC resolution.
> >
>
>
>What value is the fixed resistor in the divider circuit?
>
>It's been a few years but I seem to recall a homework question about
>picking the best fixed resistor to optimize the output delta of the
>thermistor. As memory serves, the optimum choice was a less than intuitive
>solution.

The optimum choice is to have the series resistor equal to the
thermistor at the temperature of interest. Resolution declines
on either side of that. If you need high resolution over a wide
range of temperature, and the ADC resolution isn't sufficient,
then you need to do something else.

Consumer thermometers use a RC timing arrangement with a precision
(well, 1%-ish isn't really precision these days) reference resistor.
You could probably implement that using the SR FF that's in some PICs.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the rewar=
d"
speff@interlog.com             Info for manufacturers: http://www.trexon.co=
m
Embedded software/hardware/analog  Info for designers:  http://www.speff.co=
m



--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.