Atmel XMEGA B, User Manual

Page: 328 / 415

328 XMEGA B [DATASHEET]
8291B–AVR–01/2013
Figure 26-4. Single-ended measurement in signed mode.
In unsigned mode, the negative input is connected to half of the voltage reference (VREF) voltage 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 calibration of any positive offset when the internal
ground in the device is higher than the external ground. See
Figure 26-11 on page 331 for details.
Figure 26-5. Single-ended measurement in unsigned mode.
26.3.4 Internal Inputs
These internal signals can be measured or used by the ADC.

Temperature sensor

Bandgap voltage

V
CC
scaled

Pad and Internal Ground
The temperature sensor gives an output voltage that increases linearly with the internal temperature 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 for
details of how to enable these. The sample rate for the internal 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.
•
+
ADC
-
ADC0
ADC15
••

V VREF 0.05

=
••
+
ADC
-
ADC0
ADC15
−ΔV
VREF
____
2
•