Usage of analogReference() on a Teensy 3.2

[KrisKasprzak]

All,

I was experimenting with using analogReference and have yet to get anything reliable working. I have a Teensy 3.2 powered by a 3v3 buck converter and a 10uf + a 0.1 uf cap across A3 to ground. This setup is used for both analogReference on and off.

My voltage applied to A3 is approx. 0.753 volts as read with a voltmeter. I'm not worrying about measuring exactly as my voltmeter reports, but I expect analogRead() to be close and somewhat reliable.

1. with analogReference (INTERNAL) commented out, vVolts is 0.7476 and around +/- 0.001 (which I consider close enough to input)
2. with analogReference(INTERNAL) included, vVolts is 2.065 and around +/- 0.005

It seems using internal analogReference requires a different REFERENCE_VOLTAGE and is noisier. I read somewhere that internal reference for Teensy 3.2 is 3v3 but something is wrong with my implementation.

Thoughts?

#define BIT_CONVERSION 4096
#define REFERENCE_VOLTAGE 3.3f
#define VM_PIN A3

float vVolts = 0.0f;

void setup() {

  analogReadRes(12);
  analogReadAveraging(4);

  // constant voltage at A3 is about .753 volts
  analogReference(INTERNAL);
}

void loop() {

  // get the voltage
  vVolts = analogRead(VM_PIN);
  vVolts = vVolts / (BIT_CONVERSION / REFERENCE_VOLTAGE);

  // analogReference commented out, vVolts is 0.7476 and around +/- 0.001 (which I consider close enough to input)
  // analogReference(INTERNAL);     vVolts is 2.065 and around +/- 0.005
  // seems using internal requires a different REFERENCE_VOLTAGE and is noisier

  Serial.println(vVolts, 3);
  delay(100);
}

 

[defragster]

Check this thread - for T_3.5?
analogReference(1); // Vref set to 1.2V

Quick Math suggests that might explain ~2.065 ????