368
8331B–AVR–03/12
Atmel AVR XMEGA AU
In order to achieve
n
bits of accuracy, the source output resistance, R
source
, must be less than
the ADC input resistance on a pin:
where the ADC sample time, T
S
is one-half the ADC clock cycle given by:
For details on R
channel
, R
switch
, and C
sample
, refer to the ADC and ADC gain stage electrical char-
acteristic in the device datasheet.
28.10.1
Gain Stage Impedance mode
To support applications with very high source output resistance, the gain stage has a high
impedance mode. In this mode the charge on the S/H capacitor is kept after each sample, and
the S/H capacitor can be fully charged by doing multiple samples on the same input channel.
When low impedance mode is used, the S/H capacitor charge is flushed after each sample.
28.11 DMA Transfer
The DMA controller can be used to transfer ADC conversion results to memory or other periph-
erals. A new conversion result for any of the ADC channels can trigger a DMA transaction for
one or several ADC channels. Refer to
”DMAC - Direct Memory Access Controller” on page 54
for more details on DMA transfers.
28.12 Interrupts and Events
The ADC can generate interrupt requests and events. Each ADC channel has individual inter-
rupt settings and interrupt vectors. Interrupt requests and events can be generated when an
ADC conversion is complete or when an ADC measurement is above or below the ADC com-
pare register value.
28.13 Calibration
The ADC has built-in linearity calibration. The value from the production test calibration must be
loaded from the signature row and into the ADC calibration register from software to achieve
specified accuracy. User calibration of the linearity is not needed, hence not possible. Offset and
gain calibration must be done in software.
28.14 Channel Priority
Since the peripheral clock is faster than the ADC clock, it is possible to set the start conversion
bit for several ADC channels within the same ADC clock period. Events may also trigger conver-
sions on several ADC channels and give the same scenario. In this case, the ADC channel with
the lowest number will be prioritized. This is shown the timing diagrams in
.
R
source
T
s
C
sample
2
n
1
+
(
)
ln
⋅
-----------------------------------------------
R
channel
–
R
switch
–
≤
T
s
1
2
f
⋅
ADC
-------------------
≤