Partitioning and Layout of Mixed-Signal PCBs
Preventing digital logic ground currents from contaminating low-level
analog signals on a mixed-signal PCB can be a difficult problem.
This is especially true on boards with multiple ADCs. Many people
suggest splitting the ground plane in order to isolate the digital
ground from the analog ground. Although the split-plane approach
can be made to work it has many potential problems. The pitfalls,
as well as the benefits, of splitting ground planes are examined.
By understanding how and where high-frequency ground currents flow, and
by using some basic principles of Electromagnetic Compatibility (EMC)
we are able to develop an approach to controlling these currents while
still maintaining a single contiguous ground plane.
This presentation demonstrates that component
and proper PCB topology,
combined with routing discipline,
are the keys to success in laying out a mixed-signal PCB.
Analog voltage distribution, filtering and decoupling are also
discussed along with PCB to chassis connections and the effect of
The presentation is intended for PCB designers, engineers, and
managers who are
involved with the layout of mixed-signal PCBs.
Some Applicable EMC Principles
Lowest Impedance Return Current Path
Microstrip Ground Current Distribution
The Basic Problem
Required Ground Isolation
Minimum Signal Level
A/D Converter Resolution
Methods of Providing Required Isolation
Problems With Split Ground Planes
Acceptable Ways to Pass a Signal Across a Split Plane
A/D Converter Ground Requirements
High Resolution A/D
A/D or D/A Converter Resolution
SNR and Dynamic Range
Stripline/ Asymmetrical Stripline Current
Isolating Analog and Digital Regions
Plane Isolation Guidelines
A/D & D/A Converters
Sampling Clock Jitter
Digital I/O Buffers
Support Circuitry Grounding and Layout
Circuit Ground to Chassis Connections
Common-Mode Ground Currents
PCB Aspect Ratio
Powering of Mixed-Signal PCBs
Filtering Analog Power
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Henry Ott Consultants
48 Baker Road Livingston, NJ 07039
Phone: 973-992-1793, FAX: 973-533-1442
January 30, 2007