Hi,
* No (unused) floating inputs at the microcontroller? Every pin has valid HIGH or LOW state?
* No varying current through the GND pin of the microcontroller?
* no excessive noise source around? (switching regulator, charge pump, RS232 transmitter...)
* correct ADC_clock setting, and correct ADC timing?
* fast decoupling capacitor very close to uC VCC pin, plus large bulk capacitor at VCC?
* low impedance true GND_plane?
* source impedance (@ sampling frequency) <10kOhms?
* Add a C of about 10nF very close to the A_IN pin.
* low noise linear regulator for VCC voltage?
* no other noisy load at VCC?
...
Klaus
Added:
According to the datasheet (9.3.1 p82) total unadjusted error can be as high as 4 LSB. So it's not your mistake.
This unadjusted error includes offset, gain and linearity (INL) error...
But I assume "fluctuation" means noise. And noise should be as low as +/-1LSB.
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If you don´t find a hardware problem, then you may add a digital low pass filter to reduce noise.
Klaus