AD7764 Analog Devices, AD7764 Datasheet - Page 19

• Manufacturer Part Number: AD7764
• Description: 24-Bit, 312 kSPS, 109 dB Sigma Delta ADC with On-Chip Buffers and Serial Interface
• Manufacturer: Analog Devices

Specifications of AD7764

• 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

ON-CHIP DIFFERENTIAL AMPLIFIER
The AD7764 contains an on-board differential amplifier that is
recommended to drive the modulator input pins. Pin 1, Pin 2,
Pin 3, and Pin 4 on the AD7764 are the differential input and
output pins of the amplifier. The external components, R
C
the recommended configuration.
To achieve the specified performance, the differential amplifier
should be configured as a first-order antialias filter, as shown in
Figure 35, using the component values listed in Table 7. The
inputs to the differential amplifier are then routed through the
external component network before being applied to the mod-
ulator inputs, V
optimal values in the table as an example yields a 25 dB
attenuation at the first alias point of 19.6 MHz.
Table 7. On-Chip Differential Filter Component Values
Optimal
Tolerance
Range
1
The range of values for each of the components in the differ-
ential amplifier configuration is listed in Table 7. When using
the differential amplifier to gain the input voltages to the
required modulator input range, it is advisable to implement
the gain function by changing R
optimal value.
Values shown are the acceptable tolerances for each component when
altered relative to the optimal values used to achieve the stated
specifications of the device.
FB
, C
S
1
, and R
R
(kΩ)
4.75
2.37 to
5.76
Figure 35. Differential Amplifier Configuration
M
IN
, are placed around Pin 1 through Pin 6 to create
IN
A
B
− and V
V
V
R
R
IN
IN
IN
IN
R
(kΩ)
3.01
2.4 to
4.87
A–
A+
C
FB
S
IN
+ (Pin 5 and Pin 6). Using the
1
3
DIFF
AMP
C
C
R
R
IN
FB
FB
FB
FB
R
(Ω)
43
36 to
47
and leaving R
M
2
4
V
V
R
R
OUT
OUT
C
(pF)
8.2
0 to
10
M
M
S
A+
A–
C
M
5
6
FB
C
(pF)
47
20 to
100
V
V
as the listed
IN
IN
FB
–
+
IN
, R
C
(pF)
33
33 to
56
M
Rev. A | Page 19 of 32
FB
,
The common-mode input at each of the differential amplifier
inputs (Pin V
2.2 V dc. The amplifier has a constant output common-mode
voltage of 2.048 V, that is, V
voltage for the modulator input pins (V
Figure 36 shows the signal conditioning that occurs using the
differential amplifier configuration detailed in Table 7 with a
±2.5 V input signal to the differential amplifier. The amplifier in
this example is biased around ground and is scaled to provide
±3.168 V p-p (−0.5 dBFS) on each modulator input with a
2.048 V common mode.
To obtain maximum performance from the AD7764, it is
advisable to drive the ADC with differential signals. Figure 37
shows how a bipolar, single-ended signal biased around ground
can drive the AD7764 with the use of an external op amp, such
as the AD8021.
+2.5V
+2.5V
V
–2.5V
–2.5V
IN
0V
0V
Figure 36. Differential Amplifier Signal Conditioning
A
B
Figure 37. Single-Ended-to-Differential Conversion
IN
A+ and Pin V
2R
R
AD8021
2R
REF
IN
/2, the requisite common mode
A−) can range from −0.5 V dc to
+3.632V
+2.048V
+0.464V
+3.632V
+2.048V
+0.464V
R
R
IN
IN
C
S
IN
+ and V
R
C
C
R
DIFF
AMP
FB
FB
FB
FB
IN
−).
AD7764
R
R
M
M
V
V
IN
IN
+
–
C
M
V
V
IN
IN
–
+