Section 13: Integrating DDS-based Hardware into a System Environment

Section 13: Integrating DDS-based Hardware into a System Environment
Section 13. Integrating DDS-based Hardware into a System Environment
Note - Please refer to Section 7 in the Analog Devices “High Speed Hardware Design
Techniques” seminar book for complete detailed information on best practices for mixedsignal system implementation strategies, PCB layout, and power supply recommendations.
Please visit the Analog Devices website at www.analog.com for information on obtaining the
seminar handout.
DDS devices are examples of mixed-signal technology, which means they contain both digital
and analog signals. High-frequency mixed-signal devices present a particular challenge in system
implementation and PCB design. The combination of high-speed clocks and (up to) 12-bit data
path resolutions in the sampled output, require a high degree of system and PCB layout
consideration in order to protect the integrity of the output signal. Some common problems with
DDS-based synthesizers that can be attributed to poor system implementation are:
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Excessive system clock feedthrough
Degraded wideband spur performance
Jitter in the output signal
Degraded narrowband spur performance due to widening of the “skirt” around the
fundamental
Elevated noise floor in the output spectrum
Ground plane
One of the fundamental requirements in implementing a successful high-speed mixed-signal
PCB design is to incorporate large, low-impedance ground planes. Low-impedance ground plans
acts as a return-path for high-frequency analog and digital signals and reduces the overall level
of EMI/RFI emissions. DDS devices typically provide ground connections separately labeled as
“digital ground” and “analog ground”. It is generally desirable to connect these analog and
digital ground pins to a single massive ground plane within the PCB. All IC ground pins should
be soldered directly to their corresponding ground planes to minimize series inductance. The
IC’s power supply pins should be decoupled to the ground plane, as close as possible to the chip,
with low-inductance ceramic surface-mount capacitors. If through-hole capacitors must be used,
their leads should be less than 1mm in length and Ferrite beads may be employed to compensate
for parasitic resonance. Multi-layer PCBs are recommended for “motherboard” designs, with
individual layers utilized as a dedicated analog and digital ground planes.
Power supplies
Most DDS devices will also have multiple supply pins labeled as “analog supply” and “digital
supply” which identifies the nature of the stage that the individual pin is powering. It is
recommended that separate analog and digital power supplies be used to power the associated
pins. The preferred analog power supply is a linear supply with ripple limited to <1%. Digital
supplies are typically switching supplies with output ripple in the range of up to 5%. It is
common practice to utilize 3-terminal regulators to supply the voltage for mixed-signal devices
that require non-standards voltage supplies for their analog sections.
Copyright  1999 Analog Devices, Inc.
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