AD7764 datasheet(17/32 Pages) AD

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AD7764

Rev. 0 | Page 17 of 32

AD7764 INPUT STRUCTURE

The AD7764 requires a 4.096 V input to the reference pin,

VREF+, supplied by a high precision reference, such as the

ADR444. Because the input to the device’s Σ-Δ modulator are

fully differential, the effective differential reference range is

8.192 V.

192

096

)

(

+ Diff

REF

As is inherent in Σ-Δ modulators, only a certain portion of this

full reference may be used. With the AD7764, 80% of the full

differential reference may be applied to the modulator’s

differential inputs.

5536

192

FULLSCALE

Input

Modulator

This means that a maximum of ±3.2768 V p-p full-scale can be

applied to each of the AD7764 modulator inputs (Pin 5 and

Pin 6), with the AD7764 being specified with an input −0.5 dB

down from full scale(−0.5 dBFS).

The AD7764 modulator inputs must have a common-mode

input of 2.048 V. Figure 33 shows the relative scaling between

the differential voltages applied to the modulator pins, and the

respective 24-bit twos complement digital outputs.

INPUT TO MODULATOR

PIN 5 AND PIN 6

VIN– AND VIN+

VIN+ = 3.6855V

0111 1111 1111 1111 1111 1111

0000 0000 0000 0000 0000 0000

+3.2768V = MODULATOR FULL-SCALE = 80% OF 4.096V

80% OF 4.096V = MODULATOR FULL-SCALE = –3.2768V

+4.096V

–4.096V

VIN– = 0.4105V

VIN+ = 2.048V

VIN– = 2.048V

VIN– = 3.6855V

VIN+ = 0.4105V

1000 0000 0000 0000 0000 0000

0111 1000 1101 0110 1111 1101

1000 0111 0010 1001 0000 0010

–0.5dBFS INPUT

OVERRANGE REGION

DIGITAL OUTPUT

ON SDO PIN

0000 0000 0000 0000 0000 0001

1111 1111 1111 1111 1111 1111

TWOS COMPLEMENT

DIGITAL OUTPUT

INPUT VOLTAGE (V)

Figure 33. AD7764 Scaling: Modulator Input Voltage vs. Digital Output Code

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