Comes down to accuracy, both on the sensor and reference.  Higher accuracy will cost you more.  Better than a voltage divider is a precision reference source, simple 3 terminal devices.  On the sensor, depends on how critical the reading is...turning on a fan or controlling a life support system?


--- On Sun, 2/8/09, Matthew Mucker <matthew@mucker.net> wrote:

> From: Matthew Mucker <matthew@mucker.net>
> Subject: [EE] Voltage Divider
> To: piclist@mit.edu
> Date: Sunday, February 8, 2009, 7:31 PM
> While I'm a moderately competent digital electronics
> hobbyist, I avoid
> delving into the analog realm whenever possible. I have
> what is probably a
> simple problem here, but I don't know the best approach
> to solving it.
> 
> I'd like to use an LM20 to measure temperatures from
> -55C to +50C. The LM20
> has a mostly linear voltage output in response to
> temperature. At -55C, the
> output is 2485mV. At 80C, the output will be 919mV.  (The
> datasheet doesn't
> give a specific number for 50C and the chart is too small
> to interpolate
> with any accuracy so let's play with the 80C value.)
> The LM20 is relatively
> inexpensive ($1 in onesies) and is usable for the
> temperature range in which
> I'm interested. 
> 
> The LM20 and a PIC18F2xK20 are going to be powered from a
> 3.3V power supply.
> The PIC chip has VREF- and VREF+ inputs to the ADC. I
> believe that if I can
> set VREF- at 0.9V and VREF+ at 2.5V, I can greatly increase
> the resolution
> of the ADC over the input range that the LM20 will deliver.
> 
> 
> How do I approach building a voltage divider to provide me
> with these
> voltage references? How do I select the best
> order-of-magnitude for the
> resistors? What other considerations do I need to pay
> attention to?
> 
> (Now... I am aware that this may be a bit extreme. The PIC
> has a 10bit ADC,
> and even if I leave VREF- and GND and VREF+ at 3.3V, I can
> probably get +/-
> 1 degree C resolution, but this still makes for a nice
> topic of discussion,
> IMHO, and I'm bound to learn something new from the
> discussion.)
> 
> Another potential topic: what are the pro's and
> con's of the LM20 versus a
> NTC thermistor for this application? The thermistor seems
> to require more
> claibration and interpolation to correct for
> nonlinearities.
> 
> Thanks,
> 
> -Matthew
> -- 
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