Lowest voltage applicable on AREF pin on Uno and Mega

Question

I need to measure an Analog signal in the range of 5-30 mV.

Naturally, using the internal default 5V reference leads to very poor measurement resolution. I would like to use the capability to use an external reference voltage of 50mV on the AREF pin (using an Agilent precision power supply)

However, when I tried this in the lab, I noticed something strange. The ADC readout on the channel remained stuck at 1023. If I used anything above 0.5V on the AREF pin, the readout on the ADC is correct, and the counts reliably go up and down when I ramp up and down the Analog input accordingly.

Any idea how to resolve my issue?

Answer 1

According to the datasheet (page 375) the minimum voltage you are allowed to have as a reference voltage for the ADC is 1V.

Answer 2

In the Arduino Mega it's possible to use differential mode between two analog pins with 10x gain (or 200x). But, the resolution is halved (-512 ... 511) as it's possible to measure negative difference. But with some precision voltage reference switching and 200x gain you might be able to archieve some precision.

The downside is those ADC channels are not supported by analogRead, you have to do it directly by setting ADMUX register (or ADMUX and ADCSRB) to desired differential channel with gain.

byte backup = ADMUX; // store the arduino settings
ADMUX = _BV(REFS1) | _BV(MUX3) | _BV(MUX0); // internal 1.1V reference, differential: ADC1+ ADC0-, gain: 10x

// there should be some delay to stabilize the voltage reference (if another one was selected)
// but it's possible to preset it by  analogReference(INTERNAL1V1); in the setup() so no delay will be needed

ADCSRA |= _BV(ADSC);  // start conversion

while ((ADCSRA & _BV(ADSC)) != 0);  // wait for conversion end (ADSC is cleared after the end by hardware)

int val = ADC; // read the result
ADMUX = backup; // restore original settings (from arduino)

In the Arduino everything else in ADC is configured and ADC is started, so just necessary changes are here. It should be working but I didn't tested it (I don't have the Arduino Mega here)