Datasheet

AD7765
Rev. A | Page 18 of 32
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
IN
, R
FB
,
C
FB
, C
S
, and R
M
, 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 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
IN
− and
V
IN
+(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
19.84 MHz.
DIFF
AMP
R
IN
R
FB
C
FB
R
IN
R
M
R
M
C
S
C
M
R
FB
C
FB
V
IN
–
A
B
V
IN
+
06519-024
1
3
2
4
5
6
V
IN
A–
V
IN
A+
V
OUT
A+
V
OUT
A–
Figure 29. Differential Amplifier Configuration
Table 7. On-Chip Differential Filter Component Values
R
IN
(kΩ)
R
FB
(kΩ)
R
M
(Ω)
C
S
(pF)
C
FB
(pF)
C
M
(pF)
Optimal 4.75 3.01 43 8.2 47 33
Tolerance
Range
1
2.37
to
5.76
2.4 to
4.87
36 to
47
0 to
10
20 to
100
39 to
56
1
Values shown were 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 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
IN
,
leaving the R
FB
as the listed optimal value.
The common-mode input at each of the differential amplifier
inputs (Pin V
IN
A+ and Pin V
IN
A−) 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, V
REF
/2, the requisite common-mode
voltage for the modulator input pins (V
IN
+ and V
IN
−).
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.
0V
+2.5V
–2.5V
0V
+2.5V
–2.5V
A
B
+3.632V
+2.048V
+0.464V
+3.632V
+2.048V
+0.464V
V
IN
+
V
IN
–
06519-122
Figure 30. Differential Amplifier Signal Conditioning
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.
DIFF
AMP
R
IN
R
FB
C
FB
R
IN
R
M
R
M
C
S
R
FB
C
FB
V
IN
–
V
IN
V
IN
+
AD8021
2R
2R
R
C
M
06519-026
Figure 31. Single-Ended-to-Differential Conversion