ATmega644PA datasheet(252/468 Pages) ATMEL

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252

8152G–AVR–11/09

ATmega164PA/324PA/644PA/1284P

22.7.2

Analog Noise Canceling Techniques

Digital circuitry inside and outside the device generates EMI which might affect the accuracy of

analog measurements. If conversion accuracy is critical, the noise level can be reduced by

applying the following techniques:

Keep analog signal paths as short as possible. Make sure analog tracks run over the

analog ground plane, and keep them well away from high-speed switching digital

tracks.

The AVCC pin on the device should be connected to the digital V

CC supply voltage

via an LC network as shown in Figure 22-9.

Use the ADC noise canceler function to reduce induced noise from the CPU.

If any ADC port pins are used as digital outputs, it is essential that these do not

switch while a conversion is in progress.

Figure 22-9. ADC Power Connections

22.7.3

Offset Compensation Schemes

The gain stage has a built-in offset cancellation circuitry that nulls the offset of differential mea-

surements as much as possible. The remaining offset in the analog path can be measured

directly by selecting the same channel for both differential inputs. This offset residue can be then

subtracted in software from the measurement results. Using this kind of software based offset

correction, offset on any channel can be reduced below one LSB.

PA4 (ADC4)

PA5 (ADC5)

PA6 (ADC6)

PA7 (ADC7)

AREF

AVCC

GND

PC7

ATmega644PA Datasheet(PDF) 252 Page - ATMEL Corporation