Datasheet

299
7766F–AVR–11/10
ATmega16/32U4
The settling time and gain stage bandwidth is independent of the ADHSM bit setting.
24.5.1 ADC Input Channels
When changing channel selections, the user should observe the following guidelines to ensure
that the correct channel is selected:
• In Single Conversion mode, always select the channel before starting the conversion. The
channel selection may be changed one ADC clock cycle after writing one to ADSC. However,
the simplest method is to wait for the conversion to complete before changing the channel
selection.
• In Free Running mode, always select the channel before starting the first conversion. The
channel selection may be changed one ADC clock cycle after writing one to ADSC. However,
the simplest method is to wait for the first conversion to complete, and then change the
channel selection. Since the next conversion has already started automatically, the next
result will reflect the previous channel selection. Subsequent conversions will reflect the new
channel selection.
When switching to a differential gain channel, the first conversion result may have a poor accu-
racy due to the required settling time for the automatic offset cancellation circuitry. The user
should preferably disregard the first conversion result.
24.5.2 ADC Voltage Reference
The reference voltage for the ADC (V
REF
) indicates the conversion range for the ADC. Single
ended channels that exceed V
REF
will result in codes close to 0x3FF. V
REF
can be selected as
either AV
CC
, internal 2.56V reference, or external AREF pin.
AV
CC
is connected to the ADC through a passive switch. The internal 2.56V reference is gener-
ated from the internal 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 impedance voltmeter. Note that V
REF
is a high
impudent 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 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 AV
CC
than indi-
cated in Table 29-5 on page 384.
24.6 Temperature Sensor
The ATmega16U4/ATmega32U4 includes an on-chip temperature sensor, whose the value can
be read through the A/D Converter.
The temperature measurement is based on an on-chip temperature sensor that is coupled to a
single ended ADC input. MUX[5..0] bits in ADMUX register enables the temperature sensor. The
nternal 2.56V voltage reference must also be selected for the ADC voltage reference source in
he temperature sensor measurement. When the temperature sensor is enabled, the ADC con-
verter can be used in single conversion mode to measure the voltage over the temperature
sensor.