Datasheet

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
AD8138, ADA4937-2, ADA4938-2, and ADA4930-2 differential
drivers provide excellent performance and a flexible interface to
the ADC.
The output common-mode voltage of the ADA4930-2 is easily
set with the VCM pin of the AD9613 (see Figure 47), and the
driver can be configured in a Sallen-Key filter topology to
provide band limiting of the input signal.
VIN
76.8Ω
120Ω
0.1µF
0.1µF
200Ω
200Ω
90Ω
AVDD
33Ω
33Ω
33Ω
15Ω
15Ω
5pF
15pF
15pF
ADC
VIN–
VIN+
VCM
ADA4930-2
09637-051
Figure 47. Differential Input Configuration Using the ADA4938-2
For baseband applications where SNR is a key parameter,
differential transformer coupling is the recommended input
configuration. An example is shown in Figure 48. To bia s 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 0.1µF
R1
R1
C1
ADC
VIN+
VIN–
VCM
C2
R2
R3
R2
C2
09637-052
R3
33Ω
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 (see Figure 49). In this
configuration, the input is ac-coupled, and the CML is provided
to each input through a 33 Ω resistor. These resistors compensate
for losses in the input baluns to provide a 50 Ω 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 10
displays 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
each R1, R2, C2, and R3 component shown in Figure 48 and
Figure 49.
An alternative to using a transformer-coupled input at frequencies
in the second Nyquist zone is to use an amplifier with variable
gain. The AD8375 or AD8376 digital variable gain amplifier
(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 (Ω) C1 Differential (pF) R2 Series (Ω) C2 Shunt (pF) R3 Shunt (Ω)
0 to 100 33 8.2 0 15 49.9
100 to 300 15 3.9 0 8.2 49.9
ADC
R1
0.1µF
0.1µF
2V p-p
VIN+
VIN–
VCM
C1
C2
R1
R2
R2
0.1µF
S
0.1µF
C2
33Ω
33Ω
SP
A
P
09637-053
R3
R3
0.1µF
33Ω
Figure 49. Differential Double Balun Input Configuration