Rainbow-electronics ATtiny861_V Manual do Utilizador descarregar pdf (Página 150)

150

2588B–AVR–11/06

ATtiny261/461/861

19.7.2 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:

a. 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.

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

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

while a conversion is in progress.

19.7.3 ADC Accuracy Definitions

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

REF

in 2

steps

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

-1.

Several parameters describe the deviation from the ideal behavior:

• Offset: The deviation of the first transition (0x000 to 0x001) compared to the ideal transition (at

0.5 LSB). Ideal value: 0 LSB.

Figure 19-9. Offset Error

• Gain Error: After adjusting for offset, the Gain Error is found as the deviation of the last

transition (0x3FE to 0x3FF) compared to the ideal transition (at 1.5 LSB below maximum).

Ideal value: 0 LSB

Output Code

REF

Input Voltage

Ideal ADC

Actual ADC

Offset

Error

1 2 ... 145 146 147 148 149 150 151 152 153 154 155 ... 235 236

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