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I am having a problem with the analog readings on my Due. On pin A4 and A5 I am measuring voltages:

double vcc = 3.288  // taken on pin 75 (ADVREF) ;
analogReadResolution(12);
result = analogRead(A4) * (vcc / 4095);   // result = 0.47+- 0.01      actual reading = 0.429
result = analogRead(A5) * (vcc / 4095);   // result = 0.49+- 0.01      actual reading = 0.555

As you may see I could not simply offset the value because both have very different offset: one is positive and the other is negative. The reading is off by 0.05v, which is inaccurate for a 12-bit ADC that offers 0.000805v resolution, almost a 1000x inaccuracy.

Is there any way to calibrate analogRead further without using additional external components on the pin?

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