Rainbow Electronics ATmega64C1 - Manual (Page 247)

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7647A–AVR–02/08
ATmega32/64/M1/C1
analog signal is applied to an analog pin and the digital input from this pin is not needed, this bit
should be written logic one to reduce power consumption in the digital input buffer.
18.10 Amplifier
The ATmega32/64/M1/C1 features three differential amplified channels with programmable 5,
10, 20, and 40 gain stage.
Because the amplifiers are switching capacitor amplifiers, they need to be clocked by a synchro-
nization signal called in this document the amplifier synchronization clock. To ensure an
accurate result, the amplifier input needs to have a quite stable input value during at least 4
Amplifier synchronization clock periods.
To ensure an accurate result, the amplifier input needs to have a quite stable input value at the
sampling point during at least 4 amplifier synchronization clock periods.
Amplified conversions can be synchronized to PSC events (See “Synchronization Source
Description in One Ramp Mode” on page 151 and “Synchronization Source Description in Cen-
tered Mode” on page 151 ) or to the internal clock CK
ADC
equal to eighth the ADC clock
frequency. In case the synchronization is done the ADC clock divided by 8, this synchronization
is done automatically by the ADC interface in such a way that the sample-and-hold occurs at a
specific phase of CK
ADC2
. A conversion initiated by the user (i.e., all single conversions, and the
first free running conversion) when CK
ADC2
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 initi-
ated by the user when CK
ADC2
is high will take 14 ADC clock cycles due to the synchronization
mechanism.
The normal way to use the amplifier is to select a synchronization clock via the AMPxTS1:0 bits
in the AMPxCSR register. Then the amplifier can be switched on, and the amplification is done
on each synchronization event.
In order to start an amplified Analog to Digital Conversion on the amplified channel, the ADMUX
must be configured as specified on
Table 18-5 on page 243 .
The ADC starting requirement is done by setting the ADSC bit of the ADCSRA Register.
Until the conversion is not achieved, it is not possible to start a conversion on another channel.
In order to have a better understanding of the functioning of the amplifier synchronization, two
timing diagram examples are shown
Figure 18-16 and Figure 18-17 .
As soon as a conversion is requested thanks to the ADSC bit, the Analog to Digital Conversion
is started. In case the amplifier output is modified during the sample phase of the ADC, the on-
going conversion is aborted and restarted as soon as the output of the amplifier is stable. This
ensure a fast response time. The only precaution to take is to be sure that the trig signal (PSC)
frequency is lower than ADCclk/4.