AD6643 Analog Devices, AD6643 Datasheet - Page 20

• Manufacturer Part Number: AD6643
• Description: Dual IF Receiver
• Manufacturer: Analog Devices

AD6643
decoupling capacitor close to the VCM pin to minimize series
resistance and inductance between the device and this capacitor.
Differential Input Configurations
Optimum performance is achieved by driving the AD6643 in a
differential input configuration. For baseband applications, the
AD8138, ADA4937-2, ADA4930-2, and
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 AD6643 (see Figure 28), 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, as shown in Figure 29. To bias the analog input,
the VCM voltage can be connected to the center tap of the
secondary winding of the transformer.
VIN
2V p-p
0.1µF
Figure 28. Differential Input Configuration Using the ADA4930-2
76.8Ω
Figure 29. Differential Transformer-Coupled Configuration
49.9Ω
120Ω
90Ω
0.1µF
ADA4930-2
200Ω
200Ω
2V p-p
33Ω
R1
R1
C2
C2
C1
33Ω
33Ω
R3
15pF
5pF
0.1µF
R3
P
A
ADA4938-2
15pF
R2
R2
15Ω
15Ω
33Ω
Figure 30. Differential Double Balun Input Configuration
VIN+
VIN–
S
VIN–
VIN+
ADC
S
differential
ADC
VCM
AVDD
VCM
0.1µF
0.1µF
P
0.1µF
0.1µF
Rev. A | Page 20 of 36
33Ω
33Ω
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
the true SNR performance of the AD6643. 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 10 lists
recommended values to set the RC network for different input
frequency ranges. However, because these values are dependent
on the input signal and bandwidth, they are to be used as a
starting guide only. Note that the values given in Table 10 are
for each R1, R2, C2, and R3 component shown in Figure 29 and
Figure 30.
Table 10. Example RC Network
Frequency
Range
(MHz)
0 to 100
100 to 300
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) provide good performance for driving the AD6643.
Figure 31 shows an example of the AD8376 driving the AD6643
through a band-pass antialiasing filter.
0.1µF
R1
R1
C2
C2
C1
R3
AD8375
R1 Series
(Ω)
33
15
R3
R2
R2
33Ω
or
VIN+
VIN–
AD8376
C1
Differential
(pF)
8.2
3.9
ADC
VCM
digital variable gain amplifiers
0.1µF
R2 Series
(Ω)
0
0
Data Sheet
C2
Shunt
(pF)
15
8.2
R3
Shunt
(Ω)
49.9
49.9