<warning> Arduino newbie </warning>
Both Adafruit & this forum being terrific resources, I'm posting to see if more experience users confirm my hesitation about relying on the internal voltage reference.
Starting with the great tutorial http://www.ladyada.net/learn/sensors/tmp36.html
I found that the TMP36 sensor output voltage measured by the ATMega's ADC was different from what I measured directly using a Sperry DMM - difference of about 10-20mV. Then tried the self-calibration mentioned in the tutorial, but still a discrepancy. And the imputed V+ even with the self-calibration came to 5.14V but the DMM measured 5.03V.
Now the tutorial states the internal bandgap as 1.05; the 328 datasheet says 1.1V. And then (RTFM!) reading the datasheet again & nudged in that direction by another forum post, apparently the tolerance on that voltage is +/- 0.1V.
So...here's the question: is it true then, that the bandgap is a pretty crappy reference? I'm tempted instead to rely on the stability of the onboard 5V regulator, measure it (in my case 5.03V), then code that value into the ADC calibration. Sure it applies only to my particular Arduino, but for one-off use seems a better way to go.
Thanks for any advice / correction.