Datasheet

AD9640
Rev. B | Page 26 of 52
The output common-mode voltage of the AD8138 is easily set
with the CML pin of the AD9640 (see Figure 46), and the driver
can be configured in a Sallen-Key filter topology to provide
band limiting of the input signal.
AVDD
1V p-p
49.9Ω
523Ω
0.1µF
R
R
C
499Ω
499Ω
499Ω
AD8138
06547-025
AD9640
VIN+
VIN–
CML
Figure 46. Differential Input Configuration Using the AD8138
For baseband applications where SNR is a key parameter,
differential transformer coupling is the recommended input
configuration. An example is shown in Figure 47. To bias the
analog input, the CML voltage can be connected to the center
tap of the transformer’s secondary winding.
2V p-p
49.9Ω
0.1µF
R
R
C
06547-026
AD9640
VIN+
VIN–
CML
Figure 47. Differential Transformer-Coupled Configuration
The signal characteristics must be considered when selecting
a transformer. Most RF transformers saturate at frequencies
below a few MHz, and excessive signal power can also cause
core saturation, which leads to distortion.
At input frequencies in the second Nyquist zone and above, the
noise performance of most amplifiers is not adequate to achieve
the true SNR performance of the AD9640. For applications where
SNR is a key parameter, differential double balun coupling is the
recommended input configuration (see Figure 49 for an example).
An alternative to using a transformer coupled input at frequencies
in the second Nyquist zone is to use the AD8352 differential
driver. An example is shown in Figure 50. See the AD8352 data
sheet for more information.
In any configuration, the value of Shunt Capacitor C is dependent
on the input frequency and source impedance and may need to
be reduced or removed. Table 13 displays recommended values to
set the RC network. However, these values are dependent on the
input signal and should be used only as a starting guide.
Table 13. Example RC Network
Frequency Range (MHz)
R Series
(Ω Each) C Differential (pF)
0 to 70 33 15
70 to 200 33 5
200 to 300 15 5
>300 15 Open
Single-Ended Input Configuration
A single-ended input can provide adequate performance in cost
sensitive applications. In this configuration, SFDR and distortion
performance degrade due to the large input common-mode swing.
If the source impedances on each input are matched, there should
be little effect on SNR performance. Figure 48 details a typical
single-ended input configuration.
1V p-p
R
R
C
49.9Ω
0.1µF
10µF
10µF
0.1µF
AVDD
1kΩ
1kΩ
1kΩ
1kΩ
AD9640
A
V
DD
06547-071
VIN+
VIN–
Figure 48. Single-Ended Input Configuration
AD9640
R
0.1µF
0.1µF
2V p-p
VIN+
VIN–
CML
C
R
0.1µF
S
0.1µF
06547-028
25Ω
25Ω
SP
A
P
Figure 49. Differential Double Balun Input Configuration
AD9640
AD8352
0Ω
R
0Ω
C
D
R
D
R
G
0.1µF
0.1µF
0.1µF
VIN+
VIN–
CML
C
0.1µF
16
1
2
3
4
5
11
R
0.1µF
0.1µF
10
14
0.1µF
8, 13
V
CC
200Ω
200Ω
06547-070
ANALOG INPUT
ANALOG INPUT
Figure 50. Differential Input Configuration Using the AD8352