AD7765 Analog Devices, AD7765 Datasheet - Page 17

• Manufacturer Part Number: AD7765
• Description: 24-Bit, 156 kSPS, 112 dB Sigma-Delta ADC with On-Chip Buffers and Serial Interface
• Manufacturer: Analog Devices

Specifications of AD7765

• Resolution (bits): 24bit
• # Chan: 1
• Sample Rate: 40MSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: 6.5 V p-p
• Adc Architecture: Sigma-Delta
• Pkg Type: SOP

AD7765 INPUT STRUCTURE
The AD7765 requires a 4.096 V input to the reference pin V
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.
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.
V
Modulator
INPUT TO MODULATOR
REF
+ Diff
(
PIN 5 AND PIN 6
V
)
IN
=
– AND V
_
2
Input
×
. 4
IN
096
+
FULLSCALE
=
V
V
V
V
. 8
V
V
IN
–0.5dBFS INPUT
–0.5dBFS INPUT
IN
IN
IN
IN
IN
192
+ = 3.6855V
– = 0.4105V
+ = 0.4105V
– = 3.6855V
+ = 2.048V
– = 2.048V
=
V
INPUT VOLTAGE (V)
. 8
192
Figure 28. AD7765 Scaling; Modulator Input Voltage vs. Digital Output Code
V
×
+3.2768V = MODULATOR FULL-SCALE = 80% OF 4.096V
80% OF 4.096V = MODULATOR FULL-SCALE = –3.2768V
0
OVERRANGE REGION
8 .
=
. 6
+4.096V
5536
V
REF
OVERRANGE REGION
Rev. A | Page 17 of 32
+,
–4.096V
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.
TWOS COMPLEMENT
DIGITAL OUTPUT
0111 1111 1111 1111 1111 1111
0111 1000 1101 0110 1111 1101
0000 0000 0000 0000 0000 0001
0000 0000 0000 0000 0000 0000
1111 1111 1111 1111 1111 1111
1000 0111 0010 1001 0000 0010
1000 0000 0000 0000 0000 0000
DIGITAL OUTPUT
ON SDO PIN
AD7765