AD6643-250EBZ Analog Devices, AD6643-250EBZ Datasheet - Page 22

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AD6643-250EBZ

Manufacturer Part Number
AD6643-250EBZ
Description
Data Conversion IC Development Tools 11 Bit Dual IF Diversity 3G Receiver
Manufacturer
Analog Devices
Type
ADCr
Series
AD6643r
Datasheet

Specifications of AD6643-250EBZ

Rohs
yes
Product
Evaluation Boards
Tool Is For Evaluation Of
AD6643-250
Interface Type
SPI, USB
Operating Supply Voltage
6 V
Description/function
250 MSPs per channel dual IF receiver
Maximum Operating Temperature
+ 85 C
Minimum Operating Temperature
- 40 C
Operating Supply Current
2 A
Factory Pack Quantity
1
For Use With
HSC-ADC-EVALCZ
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 42), 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 43. 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 42. Differential Input Configuration Using the ADA4930-2
76.8Ω
Figure 43. Differential Transformer-Coupled Configuration
49.9Ω
120Ω
90Ω
0.1µF
ADA4930-2
200Ω
200Ω
2V p-p
33Ω
R1
R1
C1
C2
C2
33Ω
33Ω
R3
15pF
5pF
0.1µF
R3
P
A
ADA4938-2
15pF
R2
R2
15Ω
15Ω
33Ω
Figure 44. Differential Double Balun Input Configuration
VIN+
VIN–
S
VIN–
VIN+
ADC
S
ADC
differential
VCM
AVDD
VCM
0.1µF
0.1µF
P
0.1µF
0.1µF
Rev. C | Page 22 of 40
33Ω
33Ω
0.1µF
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 44). 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 43 and
Figure 44.
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 45 shows an example of the AD8376 driving the AD6643
through a band-pass antialiasing filter.
R1
R1
C2
C2
C1
R3
AD8375
R3
R1 Series
(Ω)
33
15
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

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