AFE4490 datasheet(19/85 Pages) TI2

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1.2 V

1 V

0 V

-1 V

-1.2 V

ADC Max

(Differential)

TIA Max

(Differential)

TIA Min

(Differential)

ADC Min

(Differential)

AFE4404

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SBAS689D – JUNE 2015 – REVISED DECEMBER 2016

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8.3.4 Analog-to-Digital Converter (ADC)

The AFE has an ADC that provides a 22-bit representation of the current from the photodiode. The ADC codes

corresponding to the various sampling phases can be read out from 24-bit registers in twos complement format.

The ADC full-scale input range is ±1.2 V and spans bits 21 to 0. The mapping of the ADC input voltage to the

ADC code is shown in Table 3.

Table 3. Mapping the ADC Input Voltage to the ADC Code

DIFFERENTIAL INPUT VOLTAGE AT ADC INPUT

24-BIT ADC OUTPUT CODE

–1.2 V

111000000000000000000000

(–1.2 / 221) V

111111111111111111111111

000000000000000000000000

(1.2 / 221) V

000000000000000000000001

1.2 V

000111111111111111111111

The two MSBs of the 24-bit word serve as sign-extension bits to the 22-bit ADC code and are equal to the MSB

of the 22-bit ADC code when the input to the ADC is within its full-scale range, as shown in Table 4.

Table 4. Using Sign-Extension Bits to Determine the Input Operating Voltage

BITS 23-21

INPUT STATUS

000

Positive and lower than positive full-scale (within full-scale range)

111

Negative and higher than negative full-scale (within full-scale range)

001

Positive and higher than positive full-scale (outside full-scale range)

110

Negative and lower than negative full-scale (outside full-scale range)

Noted that the TIA has an operating range of ±1 V even though the ADC input full-scale range is ±1.2 V, as

shown in Figure 27. When setting the TIA gain, ensure that the signal at the TIA output does not exceed ±1 V.

Figure 27. TIA and ADC Dynamic Ranges

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