Figuring more data might be helpful here’s the results of my similar experimentation using the TMP36. As background, I have a 0.1uF decoupling cap across pins 1&3 of the TMP36 and have modified the sketch so I take 100 readings and average them before reporting them as a variable via the cloud. In all cases I’m monitoring the voltage of the 3.3v pin and it runs around 3.28v.
Case 1: Multimeter reading of the TMP36 output voltage without it being hooked to the Core – I get 0.718 volts which works out to be 21.8 degrees C, probably close to ambient in my dining room
Case 2: Multimeter reading of the TMP36 output voltage with it also being hooked to Core pin A0 – I still get 0.718 volts. So far, so good
Case 3: Analog reading from Core pin A0 via my sketch with the multimeter still connected to the TMP36 – The multimeter still says 0.718 volts but the sketch reports the average sensor reading to be 990, which works out to be .793 volts. Strange that the Core analog input sees a significantly higher voltage than the multimeter so something is awry. (I suppose it might be related to the sampling interval @BDub reported.)
Case 4: Analog reading from Core pin A0 via my sketch with the multimeter disconnected – The sketch reports the average sensor reading to be 1128, which works out to be .904 volts. This is much higher than the actual TMP36 output voltage and if converted to temperature would be just over 40 degrees C. Certainly not ambient temperature in my dining room. Alas, I can’t say what the TMP36 is actually reporting given the multimeter is disconnected in this case.
Clearly there are two problems here – the Core isn’t accurately reporting the TMP36 voltage in the case where I can compare it to the multimeter (Case 3), and without the multimeter the value is even more in error (Case 4).
Happy to experiment further if that’d help.