Biomedical Engineering Reference
In-Depth Information
W
t
Surface Track
Layer 1
{
t
t
PCB
dielectric
Layer 2
{
buried
track
h
Layer 3
{
Ground
Plane
Layer 4
{
V
cc
Plane
Layer 5
{
Layer 6
{
Figure 4.35
The transmission line impedance of a stripline PCB track is affected by its position relative to the ground or power plane as
well as by its geometry and by the dielectric constants of the board and the surrounding medium.
load impedance
Z
l
with a distorted version of the pulse, and presenting an e
ff
ective delay
τ
. On a typical surface PCB track, the pulse conduction velocity is approximately
0.15 ns/in.
0.06 ns/cm, so
τ
0.15
l
t
(ns) represents the total delay caused by a track of
length
l
t
(measured in inches).
If the load impedance does not match the track's impedance perfectly, a part of the
arriving signal will be re
fl
ected back into the transmission line. In general, pulse re
fl
ection
occurs whenever transmission lines with di
erent impedances are interconnected or when
a discontinuity occurs in a single transmission line. For a connection between two trans-
mission lines of impedances
Z
0
and
Z
1
, the re
ff
fl
ected voltage
V
r
is related to the incident
voltage
V
i
through
Z
Z
Z
Z
1
1
0
0
V
i
V
r
The ratio
Γ
V
r
/
V
i
, called the
re
fl
ection coe
cient
, describes what portion of the pulse
incident from
Z
0
on
Z
1
will be re
,
V
i
, and
V
r
are usually complex
quantities because they deal with both the magnitude and phase of the signals that travel
along transmission lines.
Using tracks buried within the PCB between the power and ground planes is an
fl
ected back into
Z
0
.
Γ
e
ective way of controlling RFI and ESD threats. In contrast with one- or two-sided
PCBs, multilayered PCBs with ground and power planes keep ground impedances and
loop lengths su
ff
cant levels of
RFI. Typically, PCBs with signal tracks sandwiched between ground and power planes
are a full order of magnitude less sensitive to RFI than are well-designed two-sided
boards.
For nonmedical equipment, the power and ground planes are usually grounded to the
chassis. In medical equipment, however, it is usually the most sensitive circuits that need
to
ciently low to avoid circuit tracks from picking up signi
fi
float, making it impossible to apply classical EMC techniques for proper signal-path
decoupling and shielding. For
fl
floating circuits that end up being sensitive to EMI, try to
form a capacitive grounding path between the power and ground planes of the PCB and
fl
Search WWH ::
Custom Search