>So I have a board that I designed with a 10K
>thermistor, but its feeding a 3.3V micro (and that is fixed...cant change=
=20
>this
>part). They have come back and asked for
>a better resolution.

>Now a thermistor, it simply generates a voltage
>as a divider with a source impedance of course. As the temperature changes=
,=20
>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=
=20
>well
>as the ADC resolution.

>Sorta thinking....using a PIC running at 5V to
>read the thermistor, and then either using a PWM output (with RC filter) t=
o=20
>regenerate
>a voltage of controlled steps over the temperature range they want to=20
>measure
>(pretty sure its 0 to 100F) for the ADC in their part to measure.

>Any other possible clever ideas?

Perhaps...

Go ahead and use +5V at the top of your voltage divider. If you pick the=20
source resistor so that the voltage out of the divider never exceeds the=20
3.3V of the PIC over the range of possible values of the thermistor you'll=
=20
be Ok. If you are concerned that the voltage will jump up if the thermistor=
=20
is disconnected or reaches a higher resistance than you thought, then you=20
can include a schotky diode from the middle of the voltage divider to the=20
+3.3V rail.

-- Bob Ammerman
RAm Systems

--=20
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