AT90USB646 datasheet(323/434 Pages) ATMEL

323 / 434 page

323

7593A–AVR–02/06

AT90USB64/128

The settling time and gain stage bandwidth is independent of the ADHSM bit setting.

25.5.1

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.

25.5.2

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. VREF can be selected as

either AV

CC, internal 2.56V reference, or external AREF pin.

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 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 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 30-5 on page 407.

25.6 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:

AT90USB646 Datasheet(PDF) 323 Page - ATMEL Corporation

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