NEEDS 2006 workshop Advanced Topics in EMC Design Tim Williams Elmac Services C o n s u l t a n c y a n d t r a i n i n g i n e l e c t r o m a g n e t i c c o m p a t i b i l i t y email timwelmaccouk web wwwelmaccouk phone 44 01929 558279 Issue 1 The ground plane to split or not to split Outline The ground plane Why split the GP What are the disadvantages of split GPs What happens if you dont split 1 2 1 The purpose of the ground plane via connections directly to ground plane top ......
NEEDS 2006 workshop
Advanced Topics in EMC Design
Tim Williams
Elmac Services
Consultancy and training in electromagnetic compatibility
e-mail timw@elmac.co.uk web www.elmac.co.uk
phone +44 (0)1929 558279
1
Issue 1: The ground plane
– to split or not to split?
Outline
• The ground plane
• Why split the GP?
• What are the disadvantages of split GPs?
• What happens if you don’t split?
2
1
The purpose of the ground plane
via connections directly to ground plane
top track/component layer
ground plane
power plane
bottom track/component layer
side view
connections to ground plane minimise total loop area for each track
3
Voltages developed across the GP
element
inductance L
G
di/dt
circulating currents di/dt
in ground plane
V
N
V
N
= -L
G
· di/dt
total noise voltage =
ΣV
N
• maximum inductance L
G
is at the edges of the ground plane, so ...
• don’t cross the edges, and keep them free of high di/dt return currents
4
2
The effect of a discontinuity in I
ret
No split: high mutual inductance,
low ground inductance
I
ret
I
ret
Extra return path adds ground inductance
I
ret
V
N
I
ret
generates noise across radiating dipole
5
Advice to split
Example
“...This layout separates the analog and digital
ground planes with a 2 to 3 mm gap and
connects them at one point beneath the codec
with a single 3 to 4 mm wide link. ... The link
between the planes, as close to the codec as
possible, prevents any potential difference due
to ESD or fault currents. Without the link, these
currents could flow through the codec’s
substrate degrading performance. You should
try to avoid running any digital or analog signal
traces across the gap between the digital and
analog planes…”
“… During PCB development, you may find it
useful to provide removable links between the
ground planes in several PCB locations, to
permit debugging and testing for ground
isolation…”
– Analog Devices AN-404
6
3
Why separate A & D ground planes?
• high frequency digital noise currents in DGND might
pollute sensitive wideband circuits on AGND
– common impedance coupling is a threat if there are mixed
ground connections
• most application notes advise it
– guards against inadequate layout practices in either analogue
or digital section
• “we’ve always done it this way”
• if properly done, it gives good internal signal integrity,
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