ATmega128 datasheet(241/386 Pages) ATMEL

241 / 386 page

241

2467S–AVR–07/09

ATmega128

ADC Conversion

Result

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

Result Registers (ADCL, ADCH).

For single ended conversion, the result is

where V

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

Table 97 on page 242 and Table 98 on page 243). 0x000 represents ground, and 0x3FF repre-

sents the selected reference voltage minus one LSB.

If differential channels are used, the result is

where V

POS is the voltage on the positive input pin, VNEG the voltage on the negative input pin,

GAIN the selected gain factor, and V

REF the selected voltage reference. The result is presented

in two’s complement form, from 0x200 (-512d) through 0x1FF (+511d). Note that if the user

wants to perform a quick polarity check of the results, it is sufficient to read the MSB of the result

(ADC9 in ADCH). If this bit is one, the result is negative, and if this bit is zero, the result is posi-

tive. Figure 119 shows the decoding of the differential input range.

Table 96 shows the resulting output codes if the differential input channel pair (ADCn - ADCm) is

selected with a gain of GAIN and a reference voltage of V

REF.

Figure 119. Differential Measurement Range

ADC

VIN 1024

⋅

VREF

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

ADC

VPOS VNEG

–

() GAIN 512

⋅⋅

VREF

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

Output Code

0x1FF

0x000

REF/GAIN

Differential Input

Voltage (Volts)

0x3FF

0x200

- V

REF/GAIN

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