Datasheet

SZZA016B
7–244
Basic Design Considerations for Backplanes
The termination was calculated for each stub length, using equation 9. The capacitance of the
different stub lengths changed the distributed capacitance on the backplane. Figure 14 shows
that as stub length increases, stub delay increases and driver rise time (slew rate) decreases.
Figure 15 shows the effect of stub length on termination resistance, and demonstrates that
longer stub lengths result in a lower optimum R
TT
when everything else is held constant.
23
22
21
20
0.25 0.5 0.75 1 1.25 1.5
Termination Resistance –
24
25
Stub Length – in.
26
1.75 2
Ω
Figure 15. Effect of Stub Length on Termination Resistance at S1
In all three cases, minimum stub lengths are desired because they result in the best stub
propagation delay, rise time, and termination resistance. A stub-length design goal is 1 in.,
or less.
Figure 16 shows the results of simulations of flight time in a backplane. Various stub
impedances with fixed stub lengths of 1 in., coupled with a fixed 25-Ω or 50-Ω connector and
termination resistors fixed at 24 Ω, or calculated based on stub impedance, were modeled into
the system shown in Figure 2. The driver’s rise time (20% to 80%) was set to 1.5 ns. The
measurements show the delay between the driver and the receiver located at opposite ends of
the 10-in. transmission line.