Datasheet

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
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 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
76.8Ω
120Ω
0.1µF
33Ω
33Ω
10pF
200Ω
200Ω
90Ω
ADA4938
ADC
VIN–
VIN+
VCM
08540-007
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
0.1µF
R
R
C
ADC
VCM
VIN+
VIN–
08540-008
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
Figure 40).
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.
An example is shown in Figure 41. See the AD8352 data sheet
for more information.
In any configuration, the val
on the input frequency and source impedance and may need to
be reduced or removed. Tabl e 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
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 39
shows a typical single-ended input configuration.
1V p-p
R
R
C
49.9Ω
0.1µF
10µF
10µF
0.1µF
AVDD
1kΩ
1kΩ
1kΩ
1kΩ
A
V
DD
ADC
VIN+
VIN–
08540-009
Figure 39. Single-Ended Input Configuration
ADC
R0.1µ
F
0.1µF
2
V p-
p
VCM
C
R
0.1µF
S
0.1µF
25Ω
25Ω
SP
A
P
VIN+
VIN–
08540-010
Figure 40. Differential Double Balun Input Configuration
AD8352
0Ω
0Ω
C
D
R
D
R
G
0.1µF
0.1µF
0.1µF
0.1µF
16
1
2
3
4
5
11
0.1µF
0.1µF
10
14
0.1µF
8, 13
V
CC
200Ω
200Ω
ANALOG INPUT
ANALOG INPUT
R
R
C
ADC
VCM
VIN+
VIN–
08540-011
Figure 41. Differential Input Configuration Using the AD8352