Can you change the thermistor to one with a larger value so that %R change
is larger? Or change the lower divider resistor
to get a lower starting voltage in the divider.
This will make your reading less linear, but may be acceptable trade off if
all you are doing is looking for  a threshold to trigger at.

Can you can add a rail to rail op amp to increase the gain (-offset) on
your voltage divider so that you use the full input range of your
A/D? You could also use two op amps to split the input range (with higher
gain) across two A/D channels. One channel handles the lower range, the
other the upper range.

You could also make a boost converter to get 6V (or more) from your 3.3 V
using one of the Maxim RS232 parts (or 555 (or spare PIC pin)
and a cockcroft walton voltage multiplier (capacitor and diode tree). I
once used C-W booster to get 40V from 5V for a project).

On some PICS you can set the A/D reference voltage to a lower value (V
divider or Voltage ref IC) to get more resolution.
e.g. match the max voltage you expect from the thermistor cct.
The A/D resolution is Vref/ADcount.

Some of these options were discussed a couple years ago on PIC list, so
check the archives for 'increase resolution' thread.
circa 2009.

R

On Tue, Feb 7, 2012 at 1:36 PM, alan smith <micro_eng2@yahoo.com> wrote:

> hi group..me again
>
> I want to bounce something off the members.
>
>
> 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
> 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, 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.
>
> So the ONLY way to get better resolution is to
> be able to increase the source voltage from 3.3V to 5V or use a higher
> resolution ADC.  Now for either case, the
> micro thats reading this is 3.3V, so using 5V is out of the question.
>
> Sorta thinking....using a PIC running at 5V to
> read the thermistor, and then either using a PWM output (with RC filter)
> to regenerate
> a voltage of controlled steps over the temperature range they want to
> measure
> (pretty sure its 0 to 100F) for the ADC in their part to measure.
>
> Any  other possible clever ideas?
>
>
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>
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