I have a NodeMCU 1.0 ESP-12E development board. Trying to hook up a LM35 to measure temperature. For that I need to use the analog pin A0. However it seems my sensor is out by about 4C. I traced this to an offset error - when the board is connected via USB to the PC, and I connect analog pin A0 to analog ground with a jumper on the board, I read 13 from analogRead(A0). I should be getting 0. I confirmed with a voltmeter that A0 is at ground potential. I have nothing else connected to the board. When I feed in 3.3V to A0, I get 1024. When I hook up the LM35 which outputs 210mV, I get a reading of 81 which is 13 off (based on the standard analogValue / 1024 * 3300 calculation).
What am I doing wrong?