359
8331B–AVR–03/12
Atmel AVR XMEGA AU
Figure 28-4.
Single-ended measurement in signed mode.
In unsigned mode, the negative input is connected to half of the voltage reference (VREF) volt-
age minus a fixed offset. The nominal value for the offset is:
Since the ADC is differential, the input range is VREF to zero for the positive single-ended input.
The offset enables the ADC to measure zero crossing in unsigned mode, and allows for calibra-
tion of any positive offset when the internal ground in the device is higher than the external
ground. See
for details.
Figure 28-5.
Single-ended measurement in unsigned mode.
28.3.4
Internal Inputs
These internal signals can be measured or used by the ADC.
• Temperature sensor
• Bandgap voltage
• V
CC
scaled
• DAC output
• Pad and Internal Ground
The temperature sensor gives an output voltage that increases linearly with the internal temper-
ature of the device. One or more calibration points are needed to compute the temperature from
a measurement of the temperature sensor. The temperature sensor is calibrated at one point in
production test, and the result is stored to TEMPESENSE0 and TEMPSENSE1 in the production
signature row. For more calibration condition details, refer to the device datasheet.
The bandgap voltage is an accurate internal voltage reference.
V
CC
can be measured directly by scaling it down by a factor of 10 before the ADC input. Thus, a
V
CC
of 1.8V will be measured as 0.18V, and V
CC
of 3.6V will be measured as 0.36V. This
enables easy measurement of the V
CC
voltage.
The internal signals need to be enabled before they can be measured. Refer to their manual
sections for Bandgap and DAC for details of how to enable these. The sample rate for the inter-
-
ADC0
ADC15
•
•
•
-
Δ
V
VREF
0.05
×
=
ADC0
ADC15
V
VREF
Δ
−
2
+
-
•
•
•
•
•
•
