AD6649 Analog Devices, AD6649 Datasheet - Page 18

• Manufacturer Part Number: AD6649
• Description: IF Diversity Receiver
• Manufacturer: Analog Devices

AD6649
Differential Input Configurations
Optimum performance is achieved while driving the AD6649
in a differential input configuration. For baseband applications,
the AD8138, ADA4937-2, ADA4938-2, and
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 AD6649 (see Figure 27), and the
driver can be configured in a Sallen-Key filter topology to
provide band-limiting of the input signal.
For baseband applications where SNR is a key parameter,
differential transformer coupling is the recommended input
configuration. An example is shown in Figure 28. To bias the
analog input, the VCM voltage can be connected to the center
tap of the secondary winding of the transformer.
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.
VIN
2V p-p
0.1µF
Figure 27. Differential Input Configuration Using the ADA4930-2
76.8Ω
Figure 28. Differential Transformer-Coupled Configuration
49.9Ω
120Ω
90Ω
0.1µF
ADA4930-2
200Ω
200Ω
2V p-p
R1
R1
33Ω
33Ω
C1
C2
C2
R3
R3
15pF
5pF
0.1µF
P
15pF
A
R2
R2
15Ω
15Ω
33Ω
33Ω
ADA4930-2
Figure 30. Differential Double Balun Input Configuration
VIN+
VIN–
S
VIN–
VIN+
0.1µF
ADC
ADC
S
VCM
VCM
0.1µF
P
0.1µF
0.1µF
Rev. A | Page 18 of 40
33Ω
33Ω
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 AD6649. For applications where
SNR is a key parameter, differential double balun coupling is
the recommended input configuration (see Figure 30). 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 fre-
quency 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 9
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 9 are for each
R1, R2, C2, and R3 component shown in Figure 28 and Figure 30.
Table 9. Example RC Network
Frequency
Range
(MHz)
0 to 100
100 to 250
An alternative to using a transformer-coupled input at frequencies
in the second Nyquist zone is to use an amplifier with variable
gain. The
(DVGAs) provides good performance for driving the AD6649.
Figure 29 shows an example of the AD8376 driving the AD6649
through a band-pass antialiasing filter.
NOTES
1. ALL INDUCTORS ARE COILCRAFT
2. FILTER VALUES SHOWN ARE FOR A 20MHz BANDWIDTH FILTER
0.1µF
AD8376
WITH THE EXCEPTION OF THE 1µH CHOKE INDUCTORS (COILCRAFT 0603LS).
CENTERED AT 140MHz.
Figure 29. Differential Input Configuration Using the AD8376
R1
R1
C2
C2
C1
R3
R3
1µH
1µH
AD8375
R1
Series
(Ω)
33
15
1000pF
1000pF
R2
R2
VPOS
1nF
33Ω
or
180nH
301Ω
180nH
VIN+
VIN–
AD8376
C1
Differential
(pF)
8.2
3.9
ADC
0.1µF
5.1pF
220nH
220nH
®
VCM
0603CS COMPONENTS
digital variable gain amplifier
3.9pF
165Ω
165Ω
R2
Series
(Ω)
0
0
VCM
15pF
1nF
68nH
Data Sheet
C2
Shunt
(pF)
15
8.2
2.5kΩ║2pF
AD6649
R3
Shunt
(Ω)
49.9
49.9