AD9609 View Datasheet(PDF) - Analog Devices
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AD9609 Datasheet PDF : 32 Pages
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AD9609
Differential Input Configurations
Optimum performance is achieved while driving the AD9609 in a
differential input configuration. For baseband applications, the
AD8138, ADA4937-2, and ADA4938-2 differential drivers provide
excellent performance and a flexible interface to the ADC.
The output common-mode voltage of the ADA4938-2 is easily
set with the VCM pin of the AD9609 (see Figure 37), and the
driver can be configured in a Sallen-Key filter topology to
provide band limiting of the input signal.
VIN
76.8Ω
0.1µF
200Ω
33Ω
90Ω
10pF
ADA4938-2
120Ω
33Ω
200Ω
VIN– AVDD
ADC
VIN+ VCM
Figure 37. Differential Input Configuration Using the ADA4938-2
For baseband applications below ~10 MHz where SNR is a key
parameter, differential transformer-coupling is the recommended
input configuration. An example is shown in Figure 38. To bias
the analog input, the VCM voltage can be connected to the
center tap of the secondary winding of the transformer.
2V p-p
49.9Ω
VIN+
R
C
ADC
R
VIN–
VCM
0.1µF
Figure 38. 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 AD9609. For applications above
~10 MHz where SNR is a key parameter, differential double balun
coupling is the recommended input configuration (see Figure 40).
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 41. 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 9 displays the suggested 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 9. Example RC Network
Frequency Range (MHz)
R Series
(Ω Each)
0 to 70
33
70 to 200
125
C Differential (pF)
22
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 39
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+
ADC
VIN–
Figure 39. Single-Ended Input Configuration
2V p-p
0.1µF
PA
SS
0.1µF
25Ω
P
25Ω
0.1µF
R
C
0.1µF
R
VIN+
ADC
VIN–
VCM
Figure 40. Differential Double Balun Input Configuration
0.1µF
ANALOG INPUT
0Ω 16
1
2
CD
ANALOG INPUT
RD
RG 3
4
5
0.1µF 0Ω
VCC
8, 13
11
AD8352
10
14
0.1µF
0.1µF
0.1µF
0.1µF
R
200Ω
200Ω
C
R
0.1µF
VIN+
ADC
VIN–
VCM
Figure 41. Differential Input Configuration Using the AD8352
Rev. 0 | Page 18 of 32
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