241
2467S–AVR–07/09
ATmega128
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
where V
IN
is the voltage on the selected input pin and V
REF
the selected voltage reference (see
Table 97 on page 242 and Table 98 on page 243). 0x000 represents ground, and 0x3FF repre-
sents the selected reference voltage minus one LSB.
If differential channels are used, the result is
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 posi-
tive. Figure 119 shows the decoding of the differential input range.
Table 96 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 V
REF
.
Figure 119. Differential Measurement Range
ADC
V
IN
1024⋅
V
REF
--------------------------=
ADC
V
POS
V
NEG
–()GAIN 512⋅⋅
V
REF
------------------------------------------------------------------------=
0
Output Code
0x1FF
0x000
V
REF
/GAIN
Differential Input
Voltage (Volts)
0x3FF
0x200
- V
REF
/GAIN
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