Electrical Properties of Traces
Most of the time, circuit board traces behave ideally. They dissipate
little energy and do not distort the voltage levels that they carry. A
typical trace has a resistance of only 1 Ohm/meter. During such
periods, it suffices to regard them as isolated entities—one-way
connections rather than legs of a circuit.
Driving
electronics
wire
Signal
destination
Characteristic Impedance
Compared to their docility during times of tranquility, traces behave
differently during signal propagation.
Wave-crests encounter much more obstruction along their path.
This momentary resistance is known as the “characteristic
impedance” of a trace—typically somewhere between 20 to 200
Ohms.
V
Region of
Characteristic
Impedance
distance
V
time
Transmission Line Model
For every trace, there is a return path—oftentimes an internal
copper plane. When voltage levels shift, current is drawn by
both legs of the circuit simultaneously.
Positive current
Driving
electronics
Signal
destination
Negative current
Simple Model
R
L
1/G
Z=
C
R' + iwL'
G' + iwC'
Characteristic impedance can be defined in terms of
resistance/length just as easily as actual resistance—
making it a local rather than a global property.
LVDS
Differential signals are sent along two traces instead of one.
That is, each signal has a dedicated return path. This
improvement makes it easier to predict how signals will behave
as they propagate along such traces. An LVDS driver produces
a constant current of 3.5 mA. Signals pass through circuit board
traces and cables before reaching the 100 Ohm termination
resistor. If the characteristic impedance along the entire path is
not kept consistent, the voltage drop at any point will vary and
reflection waves will emerge—causing signal degradation and
additional energy consumption.
V=I Z
time
Termination Resistor
A 100 Ohm resistor dissipates current when voltage levels stabilize.
Characteristic Impedance of an
LVDS Pair
Z = (73.5 Ohms) ln
5.98h
.8w+t
-.96s/h
(1 - .48e
)
1998 National Semiconductor Formula
Z equals 100 Ohms when
s, the separation between the traces = 7 mils
h, the thickness of the board = 12 mils
w, the width of the traces = 10 mils
t, the thickness of the traces = 2.8 mils
LVDS pairs
Trace Lengths
If traces in an LVDS pair have different lengths, one half of the
signal will reach its destination before the other. In such
circumstances, the behavior of the receiving electronics is not
well-defined.
LVDS outputs
Unrelated Note
[L'] = [μ 0 ] = Ns /C
2
1
v=
Z=
εμ
[C'] = [ε 0 ] = C /Nm
2
2
2
1
=
R' + iwL'
G' + iwC'
L'C'
μ0
ε0
= 376.7 Ohms
“impedance of
free space”