Datasheet

202
7728G–AVR–06/10
ATtiny87/ATtiny167
17.8 ADC Conversion Result
After the conversion is complete (ADIF is high), the conversion result can be found in the ADC
Result Registers (ADCL, ADCH). The form of the conversion result depends on the type of the
conversion as there are three types of conversions: single ended conversion, unipolar differ-
ential conversion and bipolar differential conversion.
17.8.1 Single Ended Conversion
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 17-4 on page 205 and Table 17-5 on page 206). 0x000 represents analog ground, and
0x3FF represents the selected voltage reference minus one LSB. The result is presented in
one-sided form, from 0x3FF to 0x000.
17.8.2 Unipolar Differential Conversion
If differential channels and an unipolar input mode are used, the result is:
where V
POS is the voltage on the positive input pin, VNEG the voltage on the negative input pin,
and V
REF the selected voltage reference (see Table 17-4 on page 205 and Table 17-5 on page
206). The voltage on the positive pin must always be larger than the voltage on the negative
pin or otherwise the voltage difference is saturated to zero. The result is presented in
one-sided form, from 0x000 (0
d
) to 0x3FF (+1023
d
). The GAIN is either 8x or 20x.
17.8.3 Bipolar Differential Conversion
As default the ADC converter operates in the unipolar input mode, but the bipolar input mode
can be selected by writing the BIN bit in the ADCSRB register to one. In the bipolar input mode
two-sided voltage differences are allowed and thus the voltage on the negative input pin can
also be larger than the voltage on the positive input pin. If differential channels and a bipolar
input mode are used, the result is:
where V
POS is the voltage on the positive input pin, VNEG the voltage on the negative input pin,
and V
REF the selected voltage reference. The result is presented in two’s complement form,
from 0x200 (-512
d
) through 0x000 (+0
d
) to 0x1FF (+511
d
). The GAIN is either 8x or 20x.
ADC
V
IN
1024⋅
V
REF
-----------------------------=
ADC
V
POS
V
NEG
–()1024⋅
V
REF
-----------------------------------------------------------
GAIN⋅=
ADC
V
POS
V
NEG
–()512⋅
V
REF
-------------------------------------------------------
GAIN⋅=