ATmega64A datasheet(241/392 Pages) ATMEL

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241

8160C–AVR–07/09

ATmega64A

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 AVCC 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 AVCC than

indicated in Table 28-7 on page 336.

23.7

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:

Make sure that the ADC is enabled and is not busy converting. Single Conversion

mode must be selected and the ADC conversion complete interrupt must be

enabled.

Enter ADC Noise Reduction mode (or Idle mode). The ADC will start a conversion

once the CPU has been halted.

If no other interrupts occur before the ADC conversion completes, the ADC interrupt

will wake up the CPU and execute the ADC Conversion Complete interrupt routine. If

another interrupt wakes up the CPU before the ADC conversion is complete, that

interrupt will be executed, and an ADC Conversion Complete interrupt request will be

generated when the ADC conversion completes. The CPU will remain in Active mode

until a new sleep command is executed.

Note that the ADC will not be automatically turned off when entering other sleep modes than Idle

mode and ADC Noise Reduction mode. The user is advised to write zero to ADEN before enter-

ing such sleep modes to avoid excessive power consumption. If the ADC is enabled in such

sleep modes and the user wants to perform differential conversions, the user is advised to

switch the ADC off and on after waking up from sleep to prompt an extended conversion to get a

valid result.

23.7.1

Analog Input Circuitry

The analog input circuitry for single ended channels is illustrated in Figure 23-8. An analog

source applied to ADCn is subjected to the pin capacitance and input leakage of that pin, regard-

less of whether that channel is selected as input for the ADC. When the channel is selected, the

source must drive the S/H capacitor through the series resistance (combined resistance in the

input path).

The ADC is optimized for analog signals with an output impedance of approximately 10 k

Ω or

less. If such a source is used, the sampling time will be negligible. If a source with higher imped-

ance is used, the sampling time will depend on how long time the source needs to charge the

S/H capacitor, with can vary widely. The user is recommended to only use low impedant sources

with slowly varying signals, since this minimizes the required charge transfer to the S/H

capacitor.

If differential gain channels are used, the input circuitry looks somewhat different, although

source impedances of a few hundred k

Ω or less is recommended.

Signal components higher than the Nyquist frequency (f

ADC/2) should not be present for either

kind of channels, to avoid distortion from unpredictable signal convolution. The user is advised

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