Datasheet

AD9600
Rev. B | Page 24 of 72
ADC. The output common-mode voltage of the AD8138 is
easily set with the CML pin of the AD9600 (see Figure 46), and
th
e driver can be configured in a Sallen-Key filter topology to
band limit the input signal.
AVDD
1V p-p
49.9Ω
523Ω
0.1µF
R
R
C
499Ω
499Ω
499Ω
AD8138
AD9600
VIN+
VIN–
CML
06909-014
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. The CML
v
o
ltage can be connected to the center tap of the transformer’s
secondary winding to bias the analog input.
The signal characteristics must be considered when selecting
a transformer. Most RF transformers saturate at frequencies
below a few megahertz. Excessive signal power can cause core
saturation, which leads to distortion.
2
V p-
p
49.9Ω
0.1µF
R
R
C
AD9600
VIN+
VIN–
CML
06909-015
Figure 47. Differential Transformer-Coupled Configuration
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 AD9600. For applications where
SNR is a key parameter, differential double-balun coupling is
the recommended input configuration. An example is shown
in Figure 49.
An a
l
ternative to using a transformer-coupled input at
frequencies in the second Nyquist zone is to use the AD8352
dif
f
erential driver. An example is shown in Figure 50. See the
AD8352
data sheet for more information.
I
n any configuration, the value of the shunt capacitor, C, is
dependent on the input frequency and source impedance and may
need to be reduced or removed. Table 10 lists the recommended
values t
o set the RC network. However, the actual values are
dependent on the input signal; therefore, Table 10 should only
b
e us
ed as a starting guide.
Table 10. 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
d
e
tails a typical single-ended input configuration.
2V p-p
R
R
C
49.9Ω
0.1µF
10µF
10µF
0.1µF
AVDD
1kΩ
1kΩ
1kΩ
1kΩ
ADC
AD9600
A
VDD
VIN+
VIN–
06909-018
Figure 48. Single-Ended Input Configuration
AD9600
R
0.1µF
0.1µF
2V p-p
VIN+
VIN–
CML
C
R
0.1µF
S
0.1µF
25Ω
25Ω
SP
A
P
06909-228
Figure 49. Differential Double-Balun Input Configuration
AD9600
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
ANALOG INPUT
ANALOG INPUT
06909-270
Figure 50. Differential Input Configuration Using the AD8352