The Bridge - Issue 2, 2022 - 15

Feature
Return Path Discontinuities and
Common Return Path Issues
Dr. Michael Cracraft, IEEE Senior Member, Rose-Hulman Institute of Technology, Gamma Theta 1998
Abstract
Perusing any circuit design schematic is evidence
enough to conclude that signal paths are essential
in a design. The schematic tells a layout designer
exactly which pins to connect. Then, there are
the supply voltage and reference connections. A
schematic does not convey that these supply and
reference connections have physical size, nor does
the schematic direct what the references should
look like in the final design.
Herein lies a nonobvious design challenge in circuit
layout. What happens to currents beyond the
voltage and ground symbols on the schematic?
From the schematic perspective, current is drawn
from the voltage rails and deposited in the ground.
However, the current must return to its source-
complete the loop. This paper will discuss the
hidden part of the current path and how it can
hinder or break a design.
I. INTRODUCTION
Two related issues occur when the return path
is neglected: discontinuities and crosstalk. These
two may lead to a host of signal integrity (SI) and
electromagnetic compatibility (EMC) issues [1].
Unfortunately, the signal path receives most of the
attention, and the return path issues go unnoticed
until there is an issue. If the return path is noticed
at all, that is a positive sign that the designer is
cognizant of its importance. In many situations, the
return path may be checked only after every other
avenue of debugging has been exhausted.
Return paths are not clearly defined like signal paths.
The signal and return paths can be compared to the
difference between SI and EMC. SI typically concerns
delivering a quality, robust signal from a transmitter
to a receiver. EMC requires checking everywhere else
that the stray energy from that signal may go.
The signal path on a " printed circuit board (PCB) "
and in most systems is a clearly defined path that can
be traced from start to finish. The return path is more
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The Bridge - Issue 2, 2022

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