In my LM35 temp sensor project I have the LM35 connected to the input of a PIC16F1829. It seems to work ok-ish.
However, I notice that some schematics with the LM35 have a 1k resistor between the LM35 and the PIC input or a 1k resistor grounding the LM35 output with the PIC input between the resistor and the LM35.
Both these variations seem to make the measurement, slightly, more accurate but I cannot figure out why!
+ PerranOak
The LM35 is an accurate but very low power device. And, as a result, it is a bit fussy about what it is connected to. To keep the LM35 happy you need to connect it to low noise power lines and connect a
1uf 100nF (correction) or larger low ESR capacitor directly across the LM35 supply and 0V pins pins. Also, the LM35 cannot tolerate much capacitance on it's output. So best to connect a resistor, with short leads directly to the LM35 output pin. You can then connect the free end of the resistor to your MCU. This is most likely the resistor that you have seen on other circuits.
You should also ensure that the LM35 is not exposed to any magnetic fields. In addition, the LM35 should be shielded from heat, cold or draughts: apart from the temperature you are measuring, of course.

There is a pretty good write-up in the TI application report:
http://www.ti.com/lit/ds/symlink/lm35.pdfThe thing to remember is that you are dealing with small millivolt signals.
