252
8272E–AVR–04/2013
ATmega164A/PA/324A/PA/644A/PA/1284/P
If the user has a fixed voltage source connected to the AREF pin, the user may not use the other
reference voltage options in the application, as they will be shorted to the external voltage. If no
external voltage is applied to the AREF pin, the user may switch between AV
CC and 2.56V as
reference selection. The first ADC conversion result after switching reference voltage source
may be inaccurate, and the user is advised to discard this result.
If differential channels are used, the selected reference should not be closer to AVCC than
23.7
ADC Noise Canceler
The ADC features a noise canceler that enables conversion during sleep mode to reduce noise
induced from the CPU core and other I/O peripherals. The noise canceler can be used with ADC
Noise Reduction and Idle mode. To make use of this feature, the following procedure should be
used:
a.
Make sure that the ADC is enabled and is not busy converting. Single Conversion
Mode must be selected and the ADC conversion complete interrupt must be enabled.
b.
Enter ADC Noise Reduction mode (or Idle mode). The ADC will start a conversion
once the CPU has been halted.
c.
If no other interrupts occur before the ADC conversion completes, the ADC interrupt
will wake up the CPU and execute the ADC Conversion Complete interrupt routine. If
another interrupt wakes up the CPU before the ADC conversion is complete, that
interrupt will be executed, and an ADC Conversion Complete interrupt request will be
generated when the ADC conversion completes. The CPU will remain in active mode
until a new sleep command is executed.
Note that the ADC will not be automatically turned off when entering other sleep modes than Idle
mode and ADC Noise Reduction mode. The user is advised to write zero to ADEN before enter-
ing such sleep modes to avoid excessive power consumption. If the ADC is enabled in such
sleep modes and the user wants to perform differential conversions, the user is advised to
switch the ADC off and on after waking up from sleep to prompt an extended conversion to get a
valid result.
23.7.1
Analog Input Circuitry
analog source applied to ADCn is subjected to the pin capacitance and input leakage of that pin,
regardless of whether that channel is selected as input for the ADC. When the channel is
selected, the source must drive the S/H capacitor through the series resistance (combined resis-
tance in the input path).
The ADC is optimized for analog signals with an output impedance of approximately 10k
or
less. If such a source is used, the sampling time will be negligible. If a source with higher imped-
ance is used, the sampling time will depend on how long time the source needs to charge the
S/H capacitor, with can vary widely. The user is recommended to only use low impedant sources
with slowly varying signals, since this minimizes the required charge transfer to the S/H
capacitor.
If differential gain channels are used, the input circuitry looks somewhat different, although
source impedances of a few hundred k
or less is recommended.