AD9627
Rev. B | Page 27 of 76
The output common-mode voltage of the
AD8138 is easily set
can be configured in a Sallen-Key filter topology to provide
band limiting of the input signal.
AVDD
1V p-p
49.9
523
0.1F
R
C
499
499
499
AD8138
AD9627
VIN+
VIN–
CML
065
71-
04
6
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 i
n Figure 47. To bias the
analog input, the CML voltage can be connected to the center
tap of the secondary winding of the transformer.
2V p-p
49.9
0.1F
R
C
AD9627
VIN+
VIN–
CML
06
57
1-
0
47
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 megahertz (MHz). 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 AD9627. For applications where
SNR is a key parameter, differential double balun coupling is the
An alternative to using a transformer-coupled input at frequencies
in the second Nyquist zone is to use the AD8352 differential driver.
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
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 shows a typical
single-ended input configuration.
1V p-p
R
C
49.9
0.1F
10F
0.1F
AVDD
1k
1k
1k
1k
AD9627
AVDD
VIN+
VIN–
0
657
1-
0
48
Figure 48. Single-Ended Input Configuration
AD9627
R
0.1F
2V p-p
VIN+
VIN–
CML
C
R
0.1F
S
0.1F
25
25
S
PA
P
06
57
1-
04
9
Figure 49. Differential Double Balun Input Configuration
AD9627
AD8352
0
R
0
CD
RD
RG
0.1F
VIN+
VIN–
CML
C
0.1F
16
1
2
3
4
5
11
R
0.1F
10
14
0.1F
8, 13
VCC
200
200
ANALOG INPUT
06
57
1-
05
0
Figure 50. Differential Input Configuration Using the AD8352