AD6642 Analog Devices, AD6642 Datasheet - Page 17

• Manufacturer Part Number: AD6642
• Description: Dual IF Receiver
• Manufacturer: Analog Devices

Differential Input Configurations
Optimum performance is achieved when driving the AD6642
in a differential input configuration. For baseband applications,
the AD8138, ADA4937-2, and
provide excellent performance and a flexible interface to the ADC.
The output common-mode voltage of the ADA4938-2 is easily
set with the VCMx pin of the AD6642 (see Figure 30), 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 31. 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 30. Differential Input Configuration Using the ADA4938-2
76.8Ω
Figure 31. Differential Transformer-Coupled Configuration
49.9Ω
120Ω
90Ω
0.1µF
ADA4938-2
200Ω
200Ω
ANALOG INPUT
ANALOG INPUT
2V p-p
R1
R1
ADA4938-2
C
33Ω
33Ω
C2
C1
C2
D
15pF
5pF
0.1µF
P
R
0.1µF
15pF
A
D
0.1µF
R2
R2
15Ω
15Ω
R
differential drivers
Figure 33. Differential Input Configuration Using the AD8352
0Ω
0Ω
G
Figure 32. Differential Double Balun Input Configuration
VIN+
VIN–
S
16
VIN–
VIN+
1
2
3
4
5
ADC
S
ADC
AD8352
VCM
AVDD
VCM
V
CC
8, 13
14
0.1µF
P
10
Rev. A | Page 17 of 32
11
0.1µF
0.1µF
0.1µF
0.1µF
33Ω
33Ω
200Ω
200Ω
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 AD6642. For applications in
which SNR is a key parameter, differential double balun coupling
is the recommended input configuration (see Figure 32). 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 and may need to be reduced
or removed. Table 10 lists recommended values to set the RC
network. At higher input frequencies, good performance can be
achieved by using a ferrite bead in series with a resistor and
removing the capacitors. However, these values are dependent
on the input signal and should be used only as a starting guide.
Table 10. Example RC Network
Frequency
Range
(MHz)
0 to 100
100 to 200
100 to 300
1
An alternative to using a transformer-coupled input at frequencies
in the second Nyquist zone is to use the
(see Figure 33). For more information, see the AD8352 data sheet.
0.1µF
In this configuration, R1 is a ferrite bead with a value of 10 Ω @ 100 MHz.
0.1µF
0.1µF
R1
R1
R
R
C2
C1
C2
C
R1 Series
(Each)
33 Ω
10 Ω
10 Ω
R2
R2
1
VIN+
VIN–
VIN+
VIN–
ADC
C1 Differential
5 pF
5 pF
Remove
ADC
VCM
VCM
AD8352
R2 Series
(Each)
15 Ω
10 Ω
66 Ω
differential driver
AD6642
C2 Shunt
(Each)
15 pF
10 pF
Remove