Datasheet

AD7764
Rev. A | Page 18 of 32
AD7764 INPUT STRUCTURE
The AD7764 requires a 4.096 V input to the reference pin,
V
REF
+, 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.
V192.8096.42
)(
=×=
+ DiffREF
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 can be applied to the modulator’s differ-
ential inputs.
V5536.68.0V192.8_ =×=
FULLSCALE
InputModulator
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 34 shows the relative scaling between the differential
voltages applied to the modulator pins and the respective 24-bit
twos complement digital outputs.
06518-120
INPUT TO MODULATOR
PIN 5 AND PIN 6
V
IN
– AND V
IN
+
V
IN
+ = 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
V
IN
– = 0.4105V
V
IN
+ = 2.048V
V
IN
– = 2.048V
V
IN
– = 3.6855V
V
IN
+ = 0.4105V
1000 0000 0000 0000 0000 0000
0111 1000 1101 0110 1111 1101
1000 0111 0010 1001 0000 0010
–0.5dBFS INPUT
–0.5dBFS INPUT
OVERRANGE REGION
OVERRANGE REGION
DIGITAL OUTPUT
ON SDO PIN
0000 0000 0000 0000 0000 0001
1111 1111 1111 1111 1111 1111
TWOS COMPLEMENT
DIGITAL OUTPUTINPUT VOLTAGE (V)
Figure 34. AD7764 Scaling: Modulator Input Voltage vs. Digital Output Code