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AD7764

Data Sheet

Rev. B | Page 20 of 33

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 Σ-Δ modulator of the device is fully

differential, the effective differential reference range is 8.192 V.

VREF + (Diff) = 2 × 4.096 = 8.192 V

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

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

differential reference can be applied to the differential inputs of

the modulator.

Modulator_InputFULL SCALE = 8.192 V × 0.8 = 6.5536 V

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 36 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 36. AD7764 Scaling: Modulator Input Voltage vs. Digital Output Code

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