Rainbow Electronics ATmega128L - Manual (Page 226)

226
ATmega128(L)
2467B–09/01
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 CK
ADC2
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 the falling edge of CK
ADC2
. A conversion initi-
ated by the user (i.e. all single conversions, and the first free running conversion) when
CK
ADC2
is low will thus 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 CK
ADC2
is high will take 14 ADC clock cycles due to the synchronization
mechanism. In free running mode, a new conversion is initiated immediately after the
previous conversion completes, and since CK
ADC2
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. E.g. the ADC clock period may be 6
µs, allowing a channel to be sampled at
12 kSPS, regardless of the bandwidth of this channel.
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.
Special care should be taken when changing differential channels. Once a differential
channel has been selected, the gain stage may take as much as 125
µs to stabilize to
the new value. Thus conversions should not be started within the first 125 µs after
selecting a new differential channel. Alternatively, conversion results obtained within this
period should be discarded.
The same settling time should be observed for the first differential conversion after
changing ADC reference (by changing the REFS1:0 bits in ADMUX).
The settling time and gain stage bandwidth is independent of the ADHSM bit setting.
ADC Input Channels When changing channel selections, the user should observe the following guidelines to
ensure that the correct channel is selected:
Table 94. ADC Conversion Time
Condition
Sample & Hold (Cycles from
Start of Conversion) Conversion Time
(Cycles)
First conversion 14.5 25
Normal conversions, single ended 1.5 13
Normal conversions, differential 1.5/2.5 13/14