What reference source are you using? The onboard ref voltage has been rather dissapointing in my experience, sitting as high as 2.7V. A bit off, for a 2.5V "precision" reference. I've used various external band gap ref chips that output 2.49V or 4.096V depending on the application.
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Re: [AVR-Chat] Why ADC result is not accurate in the ATtiny15L?
2004-02-08 by Dave VanHorn