Datasheet

AD7765
Rev. A | Page 17 of 32
AD7765 INPUT STRUCTURE
The AD7765 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 differ-
ential, 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. In the case of the AD7765, 80% of
the full differential reference can be applied to the modulator’s
differential inputs.
V5536.68.0V192.8_
=×=
FULLSCALE
InputModulator
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). The AD7765 modulator inputs must
have a common-mode input of 2.048 V.
Figure 28 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
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)
06519-120
Figure 28. AD7765 Scaling; Modulator Input Voltage vs. Digital Output Code