Datasheet

SZZA016B
7–240
Basic Design Considerations for Backplanes
Time – ns
1.6
1.4
1.2
1
0.8
0.6
0.4
Voltage – V
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75
Driver (Point A)
Receiver (Point B)
Delta X = 3.75 × 10
–9
ns Delta X = 3.75 × 10
–9
ns
Figure 10. Matched Line Termination
Stripline vs Microstrip Tradeoffs
Table 1 demonstrates the effects of distributed capacitance on various microstrip and stripline
transmission lines used in backplane designs. Er is the dielectric constant and depends on the
material used in the multilayer-backplane printed circuit board. For Table 1 discussion purposes,
the distributed capacitance is fixed at 12 pF/in. Results in the table differ if the distributed
capacitance value is changed. The highlighted stripline Z
o
50-Ω line is closest to the multipoint
distributed-capacitance example discussed previously.
Table 1. Comparison of Backplane Lines (Loaded Backplane, C
d
= 12 pF/in.)
TYPE LINE
Er = 4.5
C
o
(pF/in.)
Z
o
(Ω)
Z
o(eff)
(Ω)
t
pd
(ps/in.)
t
pd(eff)
(ps/in.)
Microstrip 1 140 38.8 140 505
Microstrip 2 70 26.5 140 370
Microstrip 2.8 50 21.7 140 322
Microstrip 4.67 30 15.9 140 265
Stripline 1.29 140 43.6 180 578
Stripline 2.58 70 29.5 180 428
Stripline 3.6 50 24 180 375
Stripline 6 30 17.3 180 312