AD7490 datasheet(17/28 Pages) AD

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Data Sheet

AD7490

Rev. E | Page 17 of 28

When no amplifier is used to drive the analog input, the source

impedance should be limited to low values. The maximum

source impedance depends on the amount of total harmonic

distortion (THD) that can be tolerated. The THD increases as

the source impedance increases, and performance degrades (see

Figure 9).

ADC TRANSFER FUNCTION

The output coding of the AD7490 is either straight binary or

twos complement depending on the status of the LSB

(CODING bit) in the control register. The designed code

transitions occur midway between successive LSB values (that

is, 1 LSB, 2 LSBs, and so on). The LSB size is equal to

REFIN/4096. The ideal transfer characteristic for the AD7490

when straight binary coding is selected is shown in Figure 18.

111...111

111...110

111...000

000...010

011...111

000...001

000...000

1LSB

+VREF – 1LSB

1LSB = VREF/4096

VREF IS EITHER REFIN OR 2 × REFIN

ANALOG INPUT

Figure 18. Straight Binary Transfer Characteristic

Handling Bipolar Input Signals

Figure 20 shows how useful the combination of the 2 × REFIN

input range and the twos complement output coding scheme is

for handling bipolar input signals. If the bipolar input signal is

biased about REFIN and twos complement output coding is

selected, REFIN becomes the zero code point, −REFIN is negative

full scale, and +REFIN becomes positive full scale, with a

dynamic range of 2 × REFIN.

011...111

011...110

000...001

000...000

111...111

100...010

100...001

100...000

+VREF – 1LSB

–VREF + 1LSB

1LSB = 2 × VREF/4096

VREF – 1LSB

ANALOG INPUT

Figure 19. Twos Complement Transfer Characteristic

with REFIN ± REFIN Input Range

VREF

REFIN

VIN0

DOUT

TWOS

COMPLEMENT

DSP/µP

VDRIVE

VDD

AD7490

VIN15

0.1µF

R1 = R2 = R3 = R4

+REFIN

(= 2 × REFIN)

–REFIN

(= 0V)

REFIN

011...111

000...000

100...000

Figure 20. Handling Bipolar Signals

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