AD9609
Rev. 0 | Page 18 of 32
Differential Input Configurations
Optimum performance is achieved while driving the AD9609 in a
differential input configuration. For baseband applications, the
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.
AVDD
VIN
76.8
120
0.1F
33
33
10pF
200
200
90
ADA4938-2
ADC
VIN–
VIN+
VCM
08
54
1-
0
07
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
49.9
0.1F
R
C
ADC
VCM
VIN+
VIN–
08
54
1-
00
8
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.
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)
C Differential (pF)
0 to 70
33
22
70 to 200
125
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 39shows a typical single-ended input configuration.
1V p-p
R
C
49.9
0.1F
10F
0.1F
AVDD
1k
1k
1k
1k
ADC
AVDD
VIN+
VIN–
08
54
1-
00
9
Figure 39. Single-Ended Input Configuration
ADC
R
0.1F
2V p-p
VCM
C
R
0.1F
S
0.1F
25
25
S
PA
P
VIN+
VIN–
08
54
1-
01
0
Figure 40. Differential Double Balun Input Configuration
AD8352
0
0
CD
RD
RG
0.1F
16
1
2
3
4
5
11
0.1F
10
14
0.1F
8, 13
VCC
200
200
ANALOG INPUT
R
C
ADC
VCM
VIN+
VIN–
08
54
1-
0
11
Figure 41. Differential Input Configuration Using the AD8352