ATmega32(L)
Table 81. ADC Conversion Time
Sample & Hold (Cycles
from Start of
Condition
Conversion)
Conversion Time (Cycles)
First conversion
13.5
1.5
25
13
Normal conversions, single ended
Auto Triggered conversions
Normal conversions, differential
2
13.5
13/14
1.5/2.5
Differential Gain Channels
When using differential gain channels, certain aspects of the conversion need to be
taken into consideration.
Differential conversions are synchronized to the internal clock CKADC2 equal to half the
ADC clock. This synchronization is done automatically by the ADC interface in such a
way that the sample-and-hold occurs at a specific phase of CKADC2. A conversion initi-
ated by the user (i.e., all single conversions, and the first free running conversion) when
CKADC2 is low will take the same amount of time as a single ended conversion (13 ADC
clock cycles from the next prescaled clock cycle). A conversion initiated by the user
when CKADC2 is high will take 14 ADC clock cycles due to the synchronization mecha-
nism. In Free Running mode, a new conversion is initiated immediately after the
previous conversion completes, and since CKADC2 is high at this time, all automatically
started (i.e., all but the first) free running conversions will take 14 ADC clock cycles.
The gain stage is optimized for a bandwidth of 4 kHz at all gain settings. Higher frequen-
cies may be subjected to non-linear amplification. An external low-pass filter should be
used if the input signal contains higher frequency components than the gain stage band-
width. Note that the ADC clock frequency is independent of the gain stage bandwidth
limitation. For example, the ADC clock period may be 6 µs, allowing a channel to be
sampled at 12 kSPS, regardless of the bandwidth of this channel.
If differential gain channels are used and conversions are started by Auto Triggering, the
ADC must be switched off between conversions. When Auto Triggering is used, the
ADC prescaler is reset before the conversion is started. Since the gain stage is depen-
dent of a stable ADC clock prior to the conversion, this conversion will not be valid. By
disabling and then re-enabling the ADC between each conversion (writing ADEN in
ADCSRA to “0” then to “1”), only extended conversions are performed. The result from
the extended conversions will be valid. See “Prescaling and Conversion Timing” on
page 204 for timing details.
Changing Channel or
Reference Selection
The MUXn and REFS1:0 bits in the ADMUX Register are single buffered through a tem-
porary register to which the CPU has random access. This ensures that the channels
and reference selection only takes place at a safe point during the conversion. The
channel and reference selection is continuously updated until a conversion is started.
Once the conversion starts, the channel and reference selection is locked to ensure a
sufficient sampling time for the ADC. Continuous updating resumes in the last ADC
clock cycle before the conversion completes (ADIF in ADCSRA is set). Note that the
conversion starts on the following rising ADC clock edge after ADSC is written. The user
is thus advised not to write new channel or reference selection values to ADMUX until
one ADC clock cycle after ADSC is written.
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2503J–AVR–10/06
ATMEL [ ATMEL ]
