ADC Conversion Result
After the conversion is complete (ADIF is high), the conversion result can be found in
the ADC Result Registers (ADCL, ADCH).
For single ended conversion, the result is
V
⋅ 1024
IN
ADC = --------------------------
V
REF
where VIN is the voltage on the selected input pin and VREF the selected voltage refer-
ence (see Table 125 on page 287 and Table 126 on page 288). 0x000 represents
analog ground, and 0x3FF represents the selected reference voltage minus one LSB.
If differential channels are used, the result is
(V
– V
) ⋅ 512
NEG
POS
ADC = ----------------------------------------------------
V
REF
where VPOS is the voltage on the positive input pin, VNEG the voltage on the negative
input pin, and VREF the selected voltage reference. The result is presented in two’s com-
plement form, from 0x200 (-512d) through 0x1FF (+511d). Note that if the user wants to
perform a quick polarity check of the result, it is sufficient to read the MSB of the result
(ADC9 in ADCH). If the bit is one, the result is negative, and if this bit is zero, the result
is positive. Figure 128 shows the decoding of the differential input range.
Table 124 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
.
Figure 128. Differential Measurement Range
Output Code
0x1FF
0x000
0
Differential Input
Voltage (Volts)
- VREF
VREF
0x3FF
0x200
286
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATMEL [ ATMEL ]
