Em 7/2/2012 18:36, alan smith escreveu:
> 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. =20
> =20
> Now a thermistor, it simply generates a voltage
> as a divider with a source impedance of course.  As the temperature chang=
es, 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.
> =20
> 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.
> =20
> 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 mea=
sure
> (pretty sure its 0 to 100F) for the ADC in their part to measure.
> =20
> Any  other possible clever ideas?


Can't you simply set the AD's Vref- and Vref+ to the lower and higher
limits of your input signal? This way, you won't lose any codes. That
is, when the input signal is at its minimum value (=3DVref-) the AD will
read 0 and when the input signal is at its maximum (=3DVref+) the AD will
read 1023.

Beware, if Vref- and VRef+ voltages are too close to each other the
circuit will be more susceptible to noise and will be less precise.

Another solution would be using op-amps to scale and shift your signal
so it fills the range 0V to 3.3V.

Isaac

--=20
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