AT90USB646 datasheet(325/434 Pages) ATMEL

325 / 434 page

325

7593A–AVR–02/06

AT90USB64/128

25.6.2

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 digi-

tal tracks.

The AV

CC pin on the device should be connected to the digital VCC supply voltage

via an LC network as shown in Figure 25-9.

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 25-9. ADC Power Connections

25.6.3

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.

25.6.4

ADC Accuracy Definitions

An n-bit single-ended ADC converts a voltage linearly between GND and V

REF in 2

n steps

(LSBs). The lowest code is read as 0, and the highest code is read as 2

n-1.

Several parameters describe the deviation from the ideal behavior:

VCC

GND

100nF

Analog Ground Plane

(ADC0) PF0

(ADC7) PF7

(ADC1) PF1

(ADC2) PF2

(ADC3) PF3

(ADC4) PF4

(ADC5) PF5

(ADC6) PF6

AREF

GND

AVCC

(AD0) PA0

μH

AT90USB646 Datasheet(PDF) 325 Page - ATMEL Corporation

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