Datasheet

200
ATmega32A [DATASHEET]
Atmel-8155D-AVR-ATmega32A-Datasheet_02/2014
AVCC is connected to the ADC through a passive switch. The internal 2.56V reference is generated from the inter-
nal bandgap reference (V
BG
) through an internal amplifier. In either case, the external AREF pin is directly
connected to the ADC, and the reference voltage can be made more immune to noise by connecting a capacitor
between the AREF pin and ground. V
REF
can also be measured at the AREF pin with a high impedant voltmeter.
Note that V
REF
is a high impedant source, and only a capacitive load should be connected in a system.
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 AVCC 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 indicated in Table 27-4
on page 303.
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:
1. 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.
2. Enter ADC Noise Reduction mode (or Idle mode). The ADC will start a conversion once the CPU has
been halted.
3. 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 entering such sleep modes to avoid
excessive power consumption. If the ADC is enabled in such sleep modes and the user wants to perform differen-
tial 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
The Analog Input Circuitry for single ended channels is illustrated in Figure 22-8. An 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 resistance in the input path).
The ADC is optimized for analog signals with an output impedance of approximately 10 k or less. If such a source
is used, the sampling time will be negligible. If a source with higher impedance 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 recom-
mended 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.
Signal components higher than the Nyquist frequency (f
ADC
/2) should not be present for either kind of channels, to
avoid distortion from unpredictable signal convolution. The user is advised to remove high frequency components
with a low-pass filter before applying the signals as inputs to the ADC.