AD9627_15 Datasheet, PDF (27/76 Pages) Analog Devices – 12-Bit, 80 MSPS/105 MSPS/125 MSPS/150 MSPS

AD9627
The output common-mode voltage of the AD8138 is easily set
with the CML pin of the AD9627 (see Figure 46), and the driver
can be configured in a Sallen-Key filter topology to provide
band limiting of the input signal.
499Ω
1V p-p
49.9Ω
R
499Ω
VIN+ _AVDD
AD8138 ^C
AD9627
0.1µF
523Ω
R
VIN–
CML
499Ω
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 secondary winding of the transformer.
2V p-p
49.9Ω
R
VIN+
C
AD9627
R
VIN– ^CML
0.1µF
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
recommended input configuration (see Figure 49).
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)
0 to 70
33
70 to 200
33
200 to 300
15
>300
15
C Differential (pF)
15
5
5
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
49.9Ω
10µF
10µF
AVDD
1kΩ
R
0.1µF _1kΩ
AVDD
C
1kΩ _R
0.1µF ^1kΩ
VIN+
AD9627
VIN–
Figure 48. Single-Ended Input Configuration
2V p-p
0.1µF
P_A
SS
0.1µF
25Ω
P
25Ω
0.1µF
R
C
0.1µF
R
VIN+
AD9627
VIN–
CML
Figure 49. Differential Double Balun Input Configuration
V_CC
ANALOG INPUT
0.1µF _0Ω
16
1
2
C_D
ANALOG INPUT
R_D
R_{G 3}
4
5
0.1µF ^0Ω
8, 13
11
AD8352
10
14
0.1µF
0.1µF
0.1µF
0.1µF
R
200Ω
C
200Ω
R
0.1µF
VIN+
AD9627
VIN– ^CML
Figure 50. Differential Input Configuration Using the AD8352
Rev. B | Page 27 of 76