Datasheet

329
XMEGA B [DATASHEET]
8291B–AVR–01/2013
Figure 26-6. 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
Figure 26-11 on page 331 for details.
VINN = VREF/2 - V
Figure 26-7. Internal measurements in unsigned mode.
26.4 Sampling Time Control
To support applications with high source output resistance, the sampling time can be increased by steps of one half ADC
clock cycle up to 64 ADC clock cycles.
26.5 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
Figure 26-8. ADC voltage reference selection
+
ADC
-
TEMP REF
VCC SCALED
BANDGAP REF
+
ADC
-
TEMP REF
VCC SCALED
BANDGAP REF
−ΔV
VREF
____
2
Internal 1.00V
AREFB
AREFA
Internal VCC/1.6V
VREF
Internal VCC/2.0V