Incorrect AnalogRead values

Hello guys,

I have 3 Arduino Due boards and I am receiving incorrect AnalogRead values compared with my multimeter measurement.
Very same symptoms as here

  • Same issue on all analog input ports.

  • Simple voltage divider scheme (attached my scheme). I am using potentiometer. It is just testing board where on real board instead of potentiometer will be 10k NTC thermistor.

  • Serial output only shows 2 values. For 0-0.3V it show 512, for 0.3-3 it is 768 (10bit resolution)

  • Board is supplied from my PCs USB port

  • In my case it is same on all 3 boards.

  • All my boards have desoldered DC jack and ICSP pins. I recieved this boards from colleague in this state. But I don’t think it should have some impact.

  • What is most strange, I have one clone of Due board from ebay and there same scheme and code works correct. Attached photos of original and clone board.

I tried without success:

  • Put 10k resistor between analog pin and GND
  • Put 100nF capacitor between analog pin and GND
  • Set INPUT_PULLUP

Here is my code

#define ReadPin A5

void setup() {
 Serial.begin(9600);
 pinMode(ReadPin,INPUT);
}

void loop() {

 int sensorValue = analogRead(ReadPin);
 Serial.println(sensorValue); //768 or 512
 delay(250);      
}

Thank you for your time!

This might help you:

http://www.djerickson.com/arduino/due_adc.html

ard_newbie:
This might help you:

Evaluating Arduino and Due ADCs

Thank you for this.

Based on your topic I found, that on original boards resistor jumper was set to AREF pin instead of supply 3.3V. So colleague probably forgot to mention me, that I should set reference voltage.

When I shoreted 3.3 pin with AREF, it started to measure values.

Problem solved!