AVR® ADC Voltage Reference

The Analog Reference (AREF) is the reference voltage to the on-chip Analog to Digital Converter (ADC) on AVR® devices. The reference voltage for the ADC, VREF, indicates the voltage range of the ADC conversion. Single-ended channels that exceed VREF results in a maximum conversion value result. VREF can be measured at the AREF pin with a high impedance voltmeter.

VREF Options

VREF can be selected as either AVCC, internal 1.1 V reference, or external AREF pin.

  • AVCC is the voltage connected to the AVCC pin which is internally connected to the ADC through a passive switch.
  • The internal 1.1 V reference is generated from the internal bandgap reference (VBG) through an internal amplifier.
  • The external AREF pin is directly connected to the ADC, and the reference voltage can be made more immune to noise by connecting a capacitor between the AREF pin and ground.

If a fixed voltage source is connected to the external AREF pin, the application may not use the other reference voltage options in the application, as they will be shorted to the external voltage.

If no external voltage is applied to the AREF pin, you may switch between AVCC and 1.1 V as the reference selection. The first ADC conversion result after switching reference voltage source may be inaccurate, and you are advised to discard this result.

If differential channels are used, the selected reference should not be closer to AVCC than indicated in the "ADC Characteristics of Electrical Characteristics" in the device datasheet.

Additional Information

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