ATtiny261A datasheet(151/292 Pages) ATMEL

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8197A–AVR–10/09

ATtiny261A/461A/861A

• Quantization Error: Due to the quantization of the input voltage into a finite number of codes,

a range of input voltages (1 LSB wide) will code to the same value. Always ± 0.5 LSB.

• Absolute Accuracy: The maximum deviation of an actual (unadjusted) transition compared to

an ideal transition for any code. This is the compound effect of offset, gain error, differential

error, non-linearity, and quantization error. Ideal value: ± 0.5 LSB.

15.11 ADC Conversion Result

After the conversion is complete (ADIF is high), the conversion result can be found in the ADC

Result Registers (ADCL, ADCH). The form of the conversion result depends on the type of the

conversio as there are three types of conversions: single ended conversion, unipolar differential

conversion and bipolar differential conversion.

15.11.1

Single Ended Conversion

For single ended conversion, the result is

where VIN is the voltage on the selected input pin and VREF the selected voltage reference (see

Table 15-4 on page 155 and Table 15-5 on page 156). 0x000 represents analog ground, and

0x3FF represents the selected voltage reference minus one LSB. The result is presented in one-

sided form, from 0x3FF to 0x000.

15.11.2

Unipolar Differential Conversion

If differential channels and an unipolar input mode are used, the result is

where VPOS is the voltage on the positive input pin, VNEG the voltage on the negative input pin,

and VREF the selected voltage reference (see Table 15-4 on page 155 and Table 15-5 on page

156). The voltage on the positive pin must always be larger than the voltage on the negative pin

or otherwise the voltage difference is saturated to zero. The result is presented in one-sided

form, from 0x000 (0d) to 0x3FF (+1023d). The GAIN is either 1x, 8x, 20x or 32x.

15.11.3

Bipolar Differential Conversion

As default the ADC converter operates in the unipolar input mode, but the bipolar input mode

can be selected by writting the BIN bit in the ADCSRB to one. In the bipolar input mode two-

sided voltage differences are allowed and thus the voltage on the negative input pin can also be

larger than the voltage on the positive input pin. If differential channels and a bipolar input mode

are used, the result is

ADC

VIN 1024

⋅

VREF

--------------------------

ADC

VPOS VNEG

–

() 1024

⋅

VREF

-------------------------------------------------------- GAIN

⋅

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