The internal reference of the MCU is 1.45 V. I am relying on this to measure ambient temperature ratio-metrically using NTC and a resistor. The MCU datasheet says that the internal voltage reference can vary from 1.38 to 1.5. I am assuming that for the given instant, If I sample the 3 V supply with the internal reference and immediately sample the NTC, the short term variation of the internal reference will not add errors in my measurements.
Question: Is my assumption right? Or, any other suggestions?
[datasheet of MCU:][1]
[1]: http://am.renesas.com/req/product_document_lineup_child.do?REGION_KEY=3&LAYER_KEY=85282&PDF_URL=http://documentation.renesas.com/doc/products/mpumcu/doc/rl78/r01uh0382ej0200_rl78l13.pdf&TKUPDATE=true&APNOTE=true&AUTHORFLG= Temperature Measurement range: -10 to 40C
Best Answer
Since you're measuring ratiometrically slow variations in the supply or the reference (for example with ambient temperature) will not have any effect on the reading. That's the beauty of ratiometric reading- the reference can vary 5% and the reading will depend only on the stability of the reference resistor.. ideally.
However, fast variations (for example, if the supply varies when something is switched or noise in the readings) will show up in your temperature reading. Since the resistance change with temperature of a typical NTC thermistor is very large it won't have much effect on the reading around the point where the thermistor resistance is close to the reference resistor resistance.
If you're looking for very stable readings over a wide range of temperature you may wish to regulate the 3V down to 1.8 or whatever with another dedicated regulator (there are other things that would work, but a regulator takes almost no area and is inexpensive) to keep the variations to a minimum and perhaps digitally low-pass filter at least the reference measurement to reduce the noise in the reading. Of course if you could power the resistor/thermistor from the internal reference it would be ideal, but often the chip designers don't make it externally available.