AD9613
Data Sheet
Rev. C | Page 24 of 36
Differential Input Configurations
Optimum performance is achieved while driving the AD9613
in a differential input configuration. For baseband applications, the
drivers provide excellent performance and a flexible interface to
the ADC.
The output common-mode voltage of the
ADA4930-2 is easily
driver can be configured in a Sallen-Key filter topology to
provide band limiting of the input signal.
VIN
76.8
120
0.1F
200
90
AVDD
33
15
5pF
15pF
ADC
VIN–
VIN+
VCM
ADA4930-2
09637-
051
Figure 47. Differential Input Configuration Using th
e ADA4938-2For baseband applications where SNR is a key parameter,
differential transformer coupling is the recommended input
configuration. An example is shown in
Figure 48. 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
R1
C1
ADC
VIN+
VIN–
VCM
C2
R2
R3
R2
C2
09637-
052
R3
33
Figure 48. Differential Transformer-Coupled Configuration
The signal characteristics must be considered when selecting
a transformer. Most RF transformers saturate at frequencies
below a few megahertz. 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 AD9613. For applications where
SNR is a key parameter, differential double balun coupling is
the recommended input configuration (se
e Figure 49). In this
configuration, the input is ac-coupled, and the CML is provided
to each input through a 33 resistor. These resistors compensate
for losses in the input baluns to provide a 50 impedance to
the driver.
In the double balun and transformer configurations, the value
of the input capacitors and resistors is dependent on the input
frequency and source impedance. Based on these parameters,
the value of the input resistors and capacitors may need to be
adjusted or some components may need to be removed
. Table 10displays recommended values to set the RC network for different
input frequency ranges. However, these values are dependent
on the input signal and bandwidth and should be used only as
a starting guide. Note that the values given in
Table 10 are for
An alternative to using a transformer-coupled input at frequencies
in the second Nyquist zone is to use an amplifier with variable
(DVGAs) provides good performance for driving the AD9613.
Figure 50 shows an example of the AD8376 driving the AD9613
through a band-pass antialiasing filter.
Table 10. Example RC Network
Frequency Range (MHz)
R1 Series ()
C1 Differential (pF)
R2 Series ()
C2 Shunt (pF)
R3 Shunt ()
0 to 100
33
8.2
0
15
49.9
100 to 300
15
3.9
0
8.2
49.9
ADC
R1
0.1F
2V p-p
VIN+
VIN–
VCM
C1
C2
R1
R2
0.1F
S
0.1F
C2
33
S
PA
P
09637-
053
R3
0.1F
33
Figure 49. Differential Double Balun Input Configuration