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The default for the Arduino A/D capability is a 5v reference. Given the 10-bit A/D converter, this is 4.88mV per number from 0 to 1023. The Arduino also supports setting the analog reference voltage to 1.1V, giving an accuracy approximately 5 times higher.

I wanted to try increasing that further. The Arduino has an internal 32k ohm resistor on AREF, so I connected a 1M ohm resistor to Vcc. The voltage reads 0.16V, but then A/D converter doesn't work. Presumably there is a lower limit. With 100k, it works, but the reference voltage is more than 1.1V, so there isn't much point.

Is it possible to set the reference voltage to less?

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  • \$\begingroup\$ It's much better to get an instrumentation amplifier and use it to increase the voltage to the 5V range, if you're trying to detect a very small voltage. \$\endgroup\$ – pjc50 Oct 29 '12 at 11:36
  • \$\begingroup\$ Accuracy is a strange word to chose for the effect of decreasing the AREF. \$\endgroup\$ – Cybergibbons Nov 27 '12 at 6:44
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Not sure which Arduino you have but on page 377 the datasheet for the atmega2560 says Vref min for the adc is 1.0V. So unfortunately that probably won't help you.

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analogReference(type) EXTERNAL

Aref can be 0 to 5V but needs to be low impedance source and noise free.

Can you ensure the noise on Aref, signal and analog ground is < 1mV ?

The Uno is already obsolete and the Due has a 12bit ADC. The more resolution you have, the greater attention is required to EMI, conducted and radiated noise on signals, ground and Aref.

Consider boosting the signal.

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    \$\begingroup\$ The Uno is obsolete? \$\endgroup\$ – Cybergibbons Nov 27 '12 at 6:44

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