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AD9629
Rev. 0 | Page 18 of 32
Differential Input Configurations
Optimum performance is achieved while driving the AD9629 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 AD9629 (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
ADC
VIN–
VIN+
VCM
08
54
0-
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
0-
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
~10 MHz where SNR is a key parameter, differential double balun
coupling is the recommended input configuration (see
th
An alternative to using a transformer-coupled input at frequencies
e true SNR performance of the AD9629. For applications above
ue of Shunt Capacitor C is dependent
Frequency Range (MHz)
Series
C Differential (pF)
in the second Nyquist zone is to use the
AD8352 differential driver.
for more information.
In any configuration, the val
on the input frequency and source impedance and may need to
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
R
(Ω Each)
0 to 70
33
22
70 to 200
n
125
Ope
Single-Ended Input Configuration
ate performance in
n-
,
A single-ended input can provide adequ
cost-sensitive applications. In this configuration, SFDR and
distortion performance degrade due to the large input commo
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
AVDD
ADC
VIN+
VIN–
08
54
0-
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
0-
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
0-
0
11
Figure 41. Differential Input Configuration Using the AD8352