By selecting the Average Mode equal Accumulate, you allow the number to grow up to a 16-bit value.
If you want to make sure the value do not grown beyond the maximum value for the given resolution (12-bits), choose the Fixed Resolution.
If i select fixed resolution then the adc reading's are coming half of the actual value .
That makes sense. Refer to the table in page 8 in the ADC datasheet:
2*Vref -> 0x0FFF
Vref -> 0x0800
Vss -> 0x0000
Your Vref is set to VDDA, that means if your input is VDDA, you will get a value around 0x800 (2048).