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AD7764 View Datasheet(PDF) - Analog Devices

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AD7764 Datasheet PDF : 32 Pages
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AD7764
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.
VREF +(Diff ) =2 × 4.096 = 8.192 V
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.
Modulator _ InputFULLSCALE = 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 33 shows the relative scaling between
the differential voltages applied to the modulator pins, and the
respective 24-bit twos complement digital outputs.
INPUT VOLTAGE (V)
+4.096V
OVERRANGE REGION
TWOS COMPLEMENT
DIGITAL OUTPUT
VIN+ = 3.6855V
VIN– = 0.4105V
+3.2768V = MODULATOR FULL-SCALE = 80% OF 4.096V
0111 1111 1111 1111 1111 1111
0111 1000 1101 0110 1111 1101
–0.5dBFS INPUT
INPUT TO MODULATOR
PIN 5 AND PIN 6
VIN– AND VIN+
VIN+ = 2.048V
VIN– = 2.048V
0000 0000 0000 0000 0000 0001
0000 0000 0000 0000 0000 0000
1111 1111 1111 1111 1111 1111
DIGITAL OUTPUT
ON SDO PIN
–0.5dBFS INPUT
VIN+ = 0.4105V
VIN– = 3.6855V
80% OF 4.096V = MODULATOR FULL-SCALE = –3.2768V
1000 0111 0010 1001 0000 0010
1000 0000 0000 0000 0000 0000
OVERRANGE REGION
–4.096V
Figure 33. AD7764 Scaling: Modulator Input Voltage vs. Digital Output Code
Rev. 0 | Page 17 of 32
 

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