and gain). However, the peaking, which often occurs in the
bandwidth response, can be minimized by varying R
A
and R
KB
.
The value of R
A
also affects SNR performance. Larger values,
while reducing the bandwidth peaking, tend to slightly increase
the SNR because of the higher signal level required to drive the
ADC full scale.
Select the R
KB
series resistor on the ADC inputs to minimize
distortion caused by any residual charge injection from the
internal sampling capacitor within the ADC. Increasing this
resistor also tends to reduce bandwidth peaking.
However, increasing R
KB
increases signal attenuation, and the
amplifier must drive a larger signal to fill the ADC input range.
For optimizing center frequency, pass-band characteristics, the
series capacitor, C
AAF2
, can be varied by a small amount.
Normally, the ADC input termination resistor, R
TADC
, is selected
to make the net ADC input impedance between 200 Ω and 400 Ω,
which is typical of most amplifier characteristic load values. Using
too high or too low a value can have an adverse effect on the
linearity of the amplifier.
Balancing these trade-offs can be somewhat difficult. In this
design, each parameter was given equal weight; therefore, the
values chosen are representative of the interface performance
for all the design characteristics. In some designs, different values
can be chosen to optimize SFDR, SNR, or input drive level,
depending on system requirements.
The SFDR performance in this design is determined by two
factors: the amplifier and ADC interface component values, as
shown in Figure 1.
Note that the signal in this design is ac-coupled with the 0.1 µF
capacitors to block the common-mode voltages between the
amplifier, its termination resistors, and the ADC inputs. Refer to
the
AD9642
data sheet for further details regarding common-
mode voltages.
Circuit Note
Passive Component and PC Board Parasitic Considerations
The performance of this or any high speed circuit is highly
dependent on proper printed circuit board (PCB) layout. This
includes, but is not limited to, power supply bypassing, controlled
impedance lines (where required), component placement, signal
routing, and power and ground planes. See Tutorial
MT-031
and
Tutorial
MT-101
for more detailed information regarding PCB
layout for high speed ADCs and amplifiers. In addition, see the
CN-0227
and the
CN-0238.
Use low parasitic surface-mount capacitors, inductors, and resistors
for the passive components in the filter. The inductors chosen
are from the Coilcraft 0603CS series. The surface-mount capacitors
used in the filter are 5%, C0G, 0402 type for stability and accuracy.
See the
CN-0279 Design Support Package
for the complete
documentation on the system.
COMMON VARIATIONS
The
AD9643
is a dual version of the
AD9642.
For lower power and bandwidth, the
ADA4950-1
and/or
ADL5561/ADL5562
can also be used. These devices are pin
compatible with the other singles previously listed.
CIRCUIT EVALUATION AND TEST
This circuit uses a modified
AD9642-250EBZ
circuit board and
the
HSC-ADC-EVALCZ
FPGA-based data capture board. The
two boards have mating high speed connectors, allowing for the
quick setup and evaluation of the performance of the circuit. The
modified
AD9642-250EBZ
board contains the circuit evaluated
as described in this note, and the
HSC-ADC-EVALCZ
data
capture board is used in conjunction with VisualAnalog® evaluation
software, as well as the SPI Controller software to properly control
the ADC and capture the data. See
User Guide UG-386
for the
schematics, BOM, and layout for the
AD9642-250EBZ
board.
The
readme.txt
file in the
CN-0279 Design Support Package
describes the modifications made to the standard
AD9642-250EBZ
board.
Application Note AN-835
contains complete details on
how to set up the hardware and software to run the tests described
in this circuit note.
Rev. 0 | Page 4 of 5
Circuit Note
LEARN MORE
CN-0279 Design Support Package:
http://www.analog.com/CN0279-DesignSupport
UG-386 User Guide, Evaluating the AD9642/AD9634/AD6672
Analog-to-Digital Converters
Arrants, Alex, Brad Brannon and Rob Reeder, AN-835
Application Note,
Understanding High Speed ADC Testing
and Evaluation,
Analog Devices.
Ardizzoni, John.
A Practical Guide to High-Speed Printed-
Circuit-Board Layout, Analog Dialogue
39-09, September 2005.
MT-031 Tutorial,
Grounding Data Converters and Solving the
Mystery of “AGND” and “DGND”,
Analog Devices.
MT-101 Tutorial,
Decoupling Techniques,
Analog Devices.
Quite Universal Circuit Simulator
Nuhertz Technologies, Filter Free Filter Design Program
Reeder, Rob,
Achieve CM Convergence between Amps and ADCs,
Electronic Design, July 2010.
Reeder, Rob,
Mine These High-Speed ADC Layout Nuggets For
Design Gold,
Electronic Design, September 15, 2011.
CN-0279
Rarely Asked Questions:
Considerations of High-Speed
Converter PCB Design, Part 1: Power and Ground Planes,
November 2010.
Rarely Asked Questions:
Considerations of High-Speed
Converter PCB Design, Part 2: Using Power and Ground
Planes to Your Advantage,
February 2011.
Rarely Asked Questions:
Considerations of High-Speed Converter
PCB Design, Part 3: The E-Pad Low Down,
June 2011.
Data Sheets and Evaluation Boards
AD9642 Data Sheet
ADL5565 Data Sheet
Circuit Evaluation Board (AD9642-250EBZ)
Standard Data Capture Platform (HSC-ADC-EVALCZ)
REVISION HISTORY
7/12—Revision 0: Initial Version
(Continued from first page) Circuits from the Lab circuits are intended only for use with Analog Devices products and are the intellectual property of Analog Devices or its licensors. While you
may use the Circuits from the Lab circuits in the design of your product, no other license is granted by implication or otherwise under any patents or other intellectual property by
application or use of the Circuits from the Lab circuits. Information furnished by Analog Devices is believed to be accurate and reliable. However, Circuits from the Lab circuits are supplied
"as is" and without warranties of any kind, express, implied, or statutory including, but not limited to, any implied warranty of merchantability, noninfringement or fitness for a particular
purpose and no responsibility is assumed by Analog Devices for their use, nor for any infringements of patents or other rights of third parties that may result from their use. Analog Devices
reserves the right to change any Circuits from the Lab circuits at any time without notice but is under no obligation to do so.
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