Datasheet

SZZA016B
7–253 Basic Design Considerations for Backplanes
Table 4. Theoretical vs Actual V
OL
Measurements
R
TT
THEORETICAL VALUES ACTUAL VALUES
R
TT
(Ω)
V
OL
SLOT 2
(V)
V
OL
SLOT 20
(V)
DIFFERENCE
(mV)
DIFFERENCE
(mV)
25 0.260 0.380 121 120
33 0.207 0.305 98 96
38 0.184 0.271 87
50 0.145 0.215 69 72
Data from Table 4 also implies that there is a theoretical maximum length of a backplane. As the
length increases, the V
OL
difference increases and reaches a point where the highest V
OL
value
is within the predetermined noise margin allotted to the system by the designer.
Conclusion
Good backplane designs should follow the design rules in this application report and account for
the capacitive loading effects on a backplane transmission line to obtain better signal integrity
and achieve incident-wave switching. Minimizing the distributed capacitance on the backplane
transmission line is desirable and can be accomplished by using transceiver devices with low
C
io
, selecting connectors with low capacitance, using higher natural trace impedance (balanced
against backplane time of flight), and by keeping stub lengths short.
The backplane integrated-circuit drive strength must be selected based on the fully loaded
characteristic impedance of the backplane and should be able to maintain the minimum required
V
OL
levels along the entire length of the backplane without exceeding the recommended
maximum low-level output current limitations.