ADC Conversion Result

After the conversion is complete (ADCSRA.ADIF is set), the conversion result can be found in the ADC Result Registers (ADCL, ADCH).

For single ended conversion, the result is

ADC=VIN⋅1024VREF

where V_IN is the voltage on the selected input pin, and V_REF the selected voltage reference (see also descriptions of ADMUX.REFSn and ADMUX.MUX). 0x000 represents analog ground, and 0x3FF represents the selected reference voltage minus one LSB.

If differential channels are used, the result is

ADC=(VPOS−VNEG)⋅GAIN⋅512VREF

where V_POS is the voltage on the positive input pin, V_NEG the voltage on the negative input pin, GAIN the selected gain factor, and V_REF the selected voltage reference. The result is presented in two’s complement form, from 0x200 (-512d) through 0x1FF (+511d). Note that if the user wants to perform a quick polarity check of the results, it is sufficient to read the MSB of the result (ADC9 in ADCH). If this bit is one, the result is negative, and if this bit is zero, the result is positive. The figure below shows the decoding of the differential input range.

Figure 1. Differential Measurement Range

The table below shows the resulting output codes if the differential input channel pair (ADCn - ADCm) is selected with a gain of GAIN and a reference voltage of VREF.

Table 1. Correlation between Input Voltage and Output Codes
V_ADCn	Read code	Corresponding Decimal Value
V_ADCm + V_REF/GAIN	0x1FF	511
V_ADCm + 0.999 V_REF/GAIN	0x1FF	511
V_ADCm + 0.998 V_REF/GAIN	0x1FE	510
...	...	...
V_ADCm + 0.001 V_REF/GAIN	0x001	1
V_ADCm	0x000	0
V_ADCm - 0.001 V_REF/GAIN	0x3FF	-1
...	...	...
V_ADCm - 0.999 V_REF/GAIN	0x201	-511
V_ADCm - V_REF/GAIN	0x200	-512

Example:

ADMUX = 0xED (ADC3 - ADC2, 10× gain, 2.56V reference, left adjusted result)

Voltage on ADC3 is 300 mV, voltage on ADC2 is 500 mV.

ADCR = 512 × 10 × (300 - 500) / 2560 = -400 = 0x270

ADCL will thus read 0x00, and ADCH will read 0x9C.

Writing zero to ADLAR right adjusts the result: ADCL = 0x70, ADCH = 0x02.