ADC Reference voltage

ADC Reference voltage

ADC Introduction 

ADC Prescaler  

ADC Reference voltage

ADC Channel

ADC Resolution

ADC Interrupt

What is ADC Reference voltage??

ADC reference voltage is nothing but it is used by ADC hardware while reading analogue input.It is the maximum voltage ADC can read.

The Reference Voltage (VREF) is the standard voltage against which the Analog Input Voltage must be measured. VREF can be an input voltage provided through external pin. Some ADCs are capable of generating VREF for the ADC module from the Analog VCC of the MCU device. The range of VREF varies among different devices and the respective device datasheet must be referred to know the exact value. Typically, VREF can be selected by configuring the bit field of the corresponding register.

For example :If we selected reference voltage as 2.5V then our ADC can read MAX 2.5 volt. If we select 10V then it can read MAX 10V.
BUT if we select more value of reference voltage then the resolution will decrease.
Example :
Think your controller having the 10bit ADC
If you select 10V as reference voltage then = 10V/1024 = 0.009V minimum  you can read

If you select 2.5V as reference voltage then = 2.5V1024 = 0.002V minimum value you can read
In AT90CAN128 we have selecte the ADC reference voltage using ADMUX register that having 2 modes
1.Internal reference
2.External reference

if you want to select Internal reference then set both

REFS1 and REFS0
if  you want to select External reference then clear  REFS1 and set  REFS0



Reference
http://www.atmel.com/Images/Atmel-8456-8-and-32-bit-AVR-Microcontrollers-AVR127-Understanding-ADC-Parameters_Application-Note.pdf