componentsspecificationsstandardtemperature
Why not -50 to 90?
How was the temperature range of -40 to 85c chosen to become the industrial standard? (Some products may have a different operating temperature range, but for the majority, the chosen spec is 85 to -40)
Is there any scientific paper that shows this is a good range to qualify component?
Have you actually measured the voltage the LM35 is outputting with a multimeter? I would trust the LM35 far more than the ATmega's ADC to go completely down to 0 V.
From the graph you included, it looks like something is hitting a rail somewhere.
If it is indeed the LM35 outputting the errant voltages, try a pulldown (~50 kilohm?) on the output to ground (or better yet, -5 V).
You're measuring a very small voltage with the built-in ADC - 20 °C is only 200 mV. Furthermore, without a negative power rail, the sensor is only really good to 2 °C, and any bias current on the output (from the ADC) will probably affect the readings you get.
The simplest analog circuit I can come up with is this:
V1 represents the temperature sensor output value.
The values of R1 and R3 may need to be adjusted specially if you use other transistors (you can use variable resistors to find out the correct values then replace them with fixed value resistors).
You may also need a voltage divider on the base terminal of Q1.
This is the output signal analysis.
This assumes you are using a common anode RGB LED.
Best Answer
Why not -35 to 75 ? Probably because that range covers the temperatures most often encountered by most equipment produced for use in society - excluding, of course, equipment produced for exceptional locations...
Care has to be taken that items should be used within their working range of temperature : something so simple as a rubber O ring can fail if used outside or below its defined working temperature range.