Datasheet
AD7764
Rev. A | Page 19 of 32
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
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 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
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 19.6 MHz.
R
FB
C
FB
R
IN
R
IN
R
M
R
M
C
S
C
M
V
IN
–
V
IN
+
06518-024
DIFF
AMP
R
FB
C
FB
A
B
1
3
2
4
5
6
V
IN
A–
V
IN
A+
V
OUT
A+
V
OUT
A–
Figure 35. 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
Tole ran ce
Range
1
2.37 to
5.76
2.4 to
4.87
36 to
47
0 to
10
20 to
100
33 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 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
IN
and leaving 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 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.
06518-122
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
–
Figure 36. Differential Amplifier Signal Conditioning
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.
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
06518-026
C
M
Figure 37. Single-Ended-to-Differential Conversion