I think that the first test you must doing with current limiting about 200 A - R2 obout 22 k
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Int Vref ATmega8 is low acurate:
Quote:
VINT Internal Voltage Reference 2.3-2.7 V and typ. 2.56 V
Table 103. "ADC Characteristics"
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I recomend to use external one, from LEM HASS module - Vref 2.5 ± 0.025 V
That is how.
For lowering voltage for ref 2.5 V ch. ADC0 and ADC1 must be shunted to GND pin 22 mega8 by 100 k resistors reffer my sch modes in page 20.