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AD7764 View Datasheet(PDF) - Analog Devices

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AD7764 Datasheet PDF : 32 Pages
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AD7764
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, RIN, RFB,
CFB, CS, and RM, are placed around Pin 1 through Pin 6 to create
the recommended configuration.
To achieve the specified performance, the differential amplifier
should be configured as a first-order antialias filter, as shown in
Figure 34, 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
modulator inputs, VIN− and VIN+, (Pin 5 and Pin 6). Using the
optimal values in the table as an example yields a 25 dB
attenuation at the first alias point of 19.6 MHz.
CFB
RFB
RIN
A
CS
B
RIN
DIFF
AMP
RFB
RM
VIN
CM
RM
VIN+
CFB
Figure 34. Differential Amplifier Configuration
Table 7. On-Chip Differential Filter Component Values
RIN
(kΩ)
RFB
(kΩ)
RM
CS
CFB
CM
(Ω)
(pF) (pF)
(pF)
Optimal 4.75
3.01
43
8.2
47
33
Tolerance 2.37 to
Range1 5.76
2.4 to
4.87
36 to 0 to
47
10
20 to
100
39 to 56
1 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.
The range of values for each of the components in the
differential 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 RIN and leaving the RFB as the listed
optimal value.
The common-mode input at each of the differential amplifier
inputs (Pin VINA+ and Pin VINA−) can range from −0.5 V dc to
2.2 V dc. The amplifier has a constant output common-mode
voltage of 2.048 V, that is, VREF/2, the requisite common mode
voltage for the modulator input pins (VIN+ and VIN−).
Figure 35 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.
+2.5V
+3.632V
0V
A
–2.5V
+2.048V
+0.464V
VIN+
+2.5V
B
0V
+3.632V
+2.048V
VIN
–2.5V
+0.464V
Figure 35. Differential Amplifier Signal Conditioning
To obtain maximum performance from the AD7764, it is advisable
to drive the ADC with differential signals. Figure 36 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.
CFB
2R
RFB
2R
VIN
RIN
RM
AD8021
VIN
R
CS
DIFF
AMP
RM CM
VIN+
RIN
RFB
CFB
Figure 36. Single-Ended-to-Differential Conversion
Rev. 0 | Page 18 of 32
 

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