AD7765 Analog Devices, AD7765 Datasheet - Page 18

• 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
ON-CHIP DIFFERENTIAL AMPLIFIER
The AD7765 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 AD7765 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 29, using the
component values listed in Table 7. The inputs to the differential
amplifier are then routed through this external component
network before being applied to the modulator inputs V
V
an example yields a 25 dB attenuation at the first alias point of
19.84 MHz.
Table 7. On-Chip Differential Filter Component Values
Optimal
Tolerance
Range
1
The range of values that can be used for each of the listed
components in the differential amplifier configuration is also
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
leaving the R
altered relative to the optimal values used to achieve the stated
specifications of the device.
Values shown were the acceptable tolerances for each component when
FB
IN
, C
+(Pin 5 and Pin 6). Using the optimal values in the table as
S
1
, and R
R
(kΩ)
4.75
2.37
to
5.76
Figure 29. Differential Amplifier Configuration
FB
M
IN
, are placed around Pin 1 through Pin 6 to create
as the listed optimal value.
A
B
R
(kΩ)
3.01
2.4 to
4.87
V
R
R
V
IN
FB
IN
IN
IN
A–
A+
C
S
1
3
R
(Ω)
43
36 to
47
R
C
DIFF
AMP
M
C
R
FB
FB
FB
FB
2
4
C
(pF)
8.2
0 to
10
V
V
S
R
R
OUT
OUT
M
M
A+
A–
C
M
5
6
C
(pF)
47
20 to
100
V
V
FB
IN
IN
–
+
IN
IN
IN
− and
,
, R
C
(pF)
33
39 to
56
M
Rev. A | Page 18 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 30 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 give
±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 AD7765, it is advisable
to drive the ADC with differential signals. Figure 31 shows how a
bipolar, single-ended signal biased around ground can drive the
AD7765 with the use of an external op amp, such as the AD8021.
V
IN
+2.5V
–2.5V
+2.5V
–2.5V
0V
0V
Figure 30. Differential Amplifier Signal Conditioning
Figure 31. Single-Ended-to-Differential Conversion
A
B
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
C
IN
S
IN
+ and V
R
C
C
R
DIFF
AMP
FB
FB
FB
FB
IN
−).
R
R
V
V
M
M
IN
IN
+
–
C
M
V
V
IN
IN
–
+