CN-0259

Circuit Note

Rev. B | Page 4 of 5

The SFDR performance in this design is determined by two

factors: the amplifier and the ADC interface component values,

as shown in Figure 1. The final SFDR performance numbers

shown in Table 1 and Figure 5 were obtained after optimizing

the filter design to account for the board parasitics and nonideal

components used in the filter design.

Another trade-off that can be made in this particular design is

the ADC full-scale setting. The full-scale ADC differential input

voltage was set for 1.75 V p-p for the data obtained with this design,

which optimizes SFDR. Changing the full-scale input range to

2.0 V p-p yields a small improvement in SNR, but slightly degrades

the SFDR performance. Changing the full-scale input range in

the opposite direction to 1.5 V p-p yields a small improvement

in SFDR but slightly degrades the SNR performance.

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 AD6657A data sheet for further details regarding

common-mode voltages.

Passive Component and PC Board Parasitic

Considerations

The performance of this or any high speed circuit is highly

dependent on proper 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 the MT-031 and MT-101 tutorials for more

detailed information regarding PCB layout for high speed ADCs

and amplifiers.

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-0259 Design Support Package (www.analog.com/

CN0259-DesignSupport) for complete documentation on the

system.

COMMON VARIATIONS

For applications that require less bandwidth and lower power,

the ADL5562 differential amplifier can be used. The ADL5562

has a bandwidth of 3.3 GHz. For even lower power and bandwidth,

the ADA4950-1 could also be used. This device has a 1 GHz

bandwidth and only uses 10 mA of current.

CIRCUIT EVALUATION AND TEST

This circuit uses the EVAL-CN0259-HSCZ 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 circuit's performance. The

EVAL-CN0259-HSCZ board contains the circuit evaluated as

described in this note, and the HSC-ADC-EVALCZ data capture

board is used in conjunction with Visual Analog evaluation

software, as well as the SPI Controller software to properly

control the ADC and capture the data. See the CN0259 Design

Support package for the schematic, BOM, and layout files for

the EVAL-CN0259-HSCZ 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.

LEARN MORE

CN-0259 Design Support Package:

http://www.analog.com/CN0259-DesignSupport

UG-232: Evaluating the AD6642/AD6657 Analog to Digital

Converters

Alex Arrants, 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.

Agilent Technologies, Advanced Design System.

Reeder, Rob, Frequency Domain Response of Switched Capacitor

ADCs, AN-742 Application Note, Analog Devices.

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.

Rarely Asked Questions: Considerations of High-Speed

Converter PCB Design, Part 1: Power and Ground Planes,

Design News, November 2010.

Rarely Asked Questions: Considerations of High-Speed

Converter PCB Design, Part 2: Using Power and Ground

Planes to Your Advantage, Design News, February 2011

Rarely Asked Questions: Considerations of High-Speed

Converter PCB Design, Part 3: The E-Pad Low Down, Design

News, June 2011

Data Sheets and Evaluation Boards

CN-0259 Circuit Evaluation Board (EVAL-CN0259-HSCZ)

Standard Data Capture Platform (HSC-ADC-EVALCZ)

AD6657A Data Sheet

ADL5565 Data Sheet

AD6657A Evaluation Board (AD6657AEBZ)