Datasheet

759
Medium- and high-performance backplanes can be modeled as a distributed load. This is because performance drives a
multidrop architecture, where the capacitance is distributed over the length of the backplane. To design an optimized medium-
or high-performance backplane, a few concepts must be understood. These include the characteristic impedance of the
backplane, (Z
o
), the characteristic delay per unit length (τ
o
), and the reflection coefficient (ρ), defined as the ratio of the
amplitude of the reflected wave to the incident wave.
Figure 3 shows the transmission line as a distributed inductance and capacitance. The backplane driver charges the capacitance
and is delayed by the inductance along the line. The signal sees the line as a characteristic impedance, given as:
Z
o
+
(
L
o
ńC
o
)
Ǹ
Where:
L
o
, C
o
= distributed inductance and capacitance per unit length
The current flowing into the transmission line is of the form:
I + V
in
ńZ
o
The transition time or the time it takes for the signal to travel along the transmission line is:
t
o
+
(
L
o
ńC
o
)
Ǹ
The intrinsic per-unit delay along the line is multiplied by the distance to give the overall delay across the line.
I
a
I
b
I
c
I
d
I
e
L
o
L
o
L
o
L
o
Figure 3. Transmission Line
The connectors on the backplane connect the backplane traces to branch transmission lines called stubs. These stubs are the
communication ports between the backplane and the plug-in modules. These stubs, which have inductance and capacitance,
change the overall impedance of the transmission line, and affect the signals that feed into the plug-in modules. This lumped
capacitance changes the impedance and delay constants along the line by the following relationships:
Z
L
+ Z
o
ń
(
1 )C
d
ńC
o
)
Ǹ
t
d
+ t
o
(
1 )C
d
C
o
)
Ǹ
Where:
C
d
= added capacitance per unit length
C
o
= intrinsic capacitance (as defined previously)
A point on the backplane where the impedance changes is called a discontinuity. A discontinuity on a backplane can occur if
the drivers are placed too far from the backplane, there is improper termination, or the driver and receiver characteristics are
not properly matched. At each point where a voltage wave that travels down the backplane meets a discontinuity, some of the
signal is reflected, while the rest is transmitted along the backplane. The reflection coefficient determines the amount of signal
that is reflected and is defined as the ratio of the reflected wave to the incident wave.
Figures 4 and 5 show the effects described above by using the GTL16622A to drive lumped and distributed loads, respectively.
The lumped load consists of 25 to 1.5 V, 30 pF to GND, whereas, the backplane (distributed load) consists of 16 slots
separated by 0.875 in. Each load is approximately 14 pF.
(1)
(2)
(3)
(4)
(5)