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249
8210C–AVR–09/11
Atmel AVR XMEGA D
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 21-6.
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 21-7.
Internal measurements in unsigned mode.
21.4
Sampling Time Control
To support applications with high source output resistance, the sampling time can be increased
by steps of a half ADC clock cycle.
21.5
Voltage Reference Selection
The following voltages can be used as the reference voltage (VREF) for the ADC:
• Accurate internal 1.00V voltage generated from the bandgap
• Internal V
CC
/1.6V voltage
• Internal V
CC
/2V voltage
• External voltage applied to AREF pin on PORTA
•
External voltage applied to AREF pin on PORTB
+
ADC
-
TEMP REF
VCC SCALED
BANDGAP REF
TEMP REF
VCC SCALED
BANDGAP REF
+
-
V
VREF
Δ
−
2