Internal Voltage Ref

hello,
i ran internal voltage ref sketch using following on mega adk.  put 10uf cap between aref , gnd.

code: [select]

analogreference( internal1v1 )

when measure voltage of aref, using fluke 179, read 1.074 volts.  here, i'm not entirely how apply this.  5v supply measured @ 5.064 v.

code: [select]

analogreference( internal2v56 )

this provides aref voltage of 2.506v , 5v supply still remains same @ 5.064v.  no analogref code, aref same supply.  got mux

code: [select]

  admux = (0<<refs1) | (0<<refs0) | (0<<adlar) | (1<<mux3) | (1<<mux2) | (1<<mux1) | (0<<mux0);
}

void loop( void )
{
  int value;

  // start conversion 
  adcsra |= _bv( adsc );
 
  // wait complete
  while( ( (adcsra & (1<<adsc)) != 0 ) );

  // scale value
  value = (((internalreferencevoltage * 1023l) / adc) + 5l) / 10l;

  serial.println( value );
  delay( 1000 );
}

my atd readings of 5v supply still pretty off @ point.  notice there's .5v addition make believe lsb jitter.  believe still pretty .5v off.  can maybe explain step missing or potential solution readings more accurate without adding more in "value"?

what trying do?

note "1.1v" reference of uno 10% accurate (1.0v 1.2v expected range).  wouldn't expect mega references more accurate.

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