AD7765 datasheet(16/32 Pages) AD

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AD7765

Rev. 0 | Page 16 of 32

AD7765 INPUT STRUCTURE

5536

192

FULLSCALE

Input

Modulator

The AD7765 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 is

fully differential, the effective differential reference range is

8.192 V.

This means that a maximum of ±3.2768 V p-p can be applied to

each of the AD7765 modulator inputs (Pin 5 and Pin 6), with

the AD7765 being specified with an input −0.5 dB down from

full scale (−0.5 dBFS).

Diff

REF

192

096

)

(

The AD7765 modulator inputs must have a common-mode

input of 2.048 V. Figure 27 shows the relative scaling between

the differential voltages applied to the modulator pins and the

respective 24-bit twos complement digital outputs.

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

full reference may be used. In the case of the AD7765, 80% of

the full differential reference can be applied to the modulator’s

differential inputs.

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 27. AD7765 Scaling; Modulator Input Voltage vs. Digital Output Code

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