360
8331B–AVR–03/12
Atmel AVR XMEGA AU
nal signals is lower than that of the ADC. Refer to the ADC characteristics in the device
datasheets for details.
For differential measurement Pad Ground (Gnd) and Internal Gnd can be selected as negative
input. Pad Gnd is the gnd level on the pin and identical or very close to the external gnd. Internal
Gnd is the internal device gnd level.
Internal Gnd is used as the negative input when other internal signals are measured in single-
ended signed mode.
Figure 28-6.
Internal measurements in single-ended signed mode.
To measure the internal signals in unsigned mode, the negative input is connected to a fixed
value given by the formula below, which is half of the voltage reference (VREF) minus a fixed
offset, as it is for single-ended unsigned input. Refer to
for details.
VINN = VREF/2 -
Δ
V
Figure 28-7.
Internal measurements in unsigned mode.
28.4
ADC Channels
To facilitate the maximum utilization of the ADC, it has four separate pairs of MUX control regis-
ters with corresponding result registers. Each pair forms an ADC channel. See
. The ADC can then keep and use four parallel configurations of input sources and trig-
gers. Each channel has dedicated result register, events and interrupts, and DMA triggers.
As an example of the ADC channel usage, one channel can be setup for single-ended measure-
ments triggered by an event channel, the second channel can measure a differential input using
a different event, and the two last channels can measure two other input sources started by the
application software.
All the ADC channels use the same ADC pipeline for the conversions, and the pipeline enables
a new conversion to be started for each ADC clock cycle. This means that multiple ADC mea-
surements from different channels can be converted simultaneously and independently. The
channels' result registers are individually updated and are unaffected by conversions on other
channels. This can help reduce software complexity by allowing different software modules to
start conversions and read conversion results fully independently of each other.
+
ADC
-
TEMP REF
VCC SCALED
DAC
BANDGAP REF
TEMP REF
VCC SCALED
BANDGAP REF
+
-
V
VREF
Δ
−
2
DAC