Datasheet
SZZA016B
7–243
Basic Design Considerations for Backplanes
2.5
2
1.5
1
0.5
0
S4
S3
S2
S1
25
51
75
Rise Time – ns
Stub Z
o
– Ω
S4 = Rx (end)
S3 = B/P (end)
S2 = B/P (start)
S1 = Driver
Figure 13. Rise Time vs Stub Z
o
at Various Points on the Backplane
The higher-impedance stubs (higher inductance) produce a faster driver rise time (higher slew
rate) and, therefore, faster rise times at all points along the backplane. This shows that system
slew rate is dependent on both the device slew rate and the stub impedance. If the system rings,
a lower stub Z
o
can alleviate the problem, because it would slow the rise time to a more
manageable value. Figure 14 shows the effects of stub length on stub delay and driver rise time.
Stub impedance is fixed at 51 Ω, and only stub lengths and R
TT
were changed.
0.8
0.4
0.2
0
0.25 0.5 0.75 1 1.25 1.5
Time – ns
1.2
1.4
Stub Length – in.
1.6
1.75 2
1
0.6
S1 – Stub Delay
S2 – Driver Rise Time
Figure 14. Effects of Stub Length on Stub Delay and Rise Time