Datasheet

SZZA016B
7–238
Basic Design Considerations for Backplanes
In a previous example (capacitors in Figure 3), C
d
= 12 pF/in. (472 pF/m) and C
o
= 3.5 pF/in.
(138 pF/m) make the C
d
/C
o
ratio = 3.43 and the term
1 )
ǒ
C
d
ńC
o
Ǔ
Ǹ
= 2.1. Figures 6 and 7
reflect the changes in the effective values of the transmission line to be 0.48 times the normal
impedance and 2.1 times the normal propagation delay.
Another way to calculate the new effective impedance and propagation delay is to use
equations 4 and 5 instead of Figures 6 and 7.
The value of the effective impedance is:
Z
o
(
eff
)
+
Z
o
1 )
C
d
C
o
Ǹ
+
51
2.1
+ 24.2 W
The value of the effective t
pd
is:
t
pd
(
eff
)
+ t
pd
1 )
C
d
C
o
Ǹ
+ 178.5(2.1) + 375.6 psńin.
Using equation 2, the new flight time between points A and B in Figure 3 is:
t
flight
+ t
pd
length of line + 375.6 psńin. 10 in. + 3.76 ns
Note that the propagation delay was 1.785 ns in the point-to-point example.
As discussed previously, the optimum termination resistance is equal to the effective impedance,
Z
o(eff)
, of the system so, in this case, the optimum termination resistance, R
TT
, is the same as
Z
o(eff)
which is 24.2 Ω. The optimum termination resistance ensures incident-wave switching
without undershoot or overshoot.
Figure 8 shows the effect on signal integrity in different-terminated conditions. R
TT
should be
less than or equal to Z
o(eff)
for incident-wave switching, optimum signal integrity, and the best
upper noise margin.
Over Termination (R
TT
< Z
o(eff)
)
Matched Termination (R
TT
= Z
o(eff)
)
Under Termination (R
TT
> Z
o(eff)
)
Figure 8. Termination Resistance vs Z
o(eff)
Equation 9 provides all parameters needed to calculate an optimum termination value:
R
TT
+
Z
o
1 )
C
d
C
o
Ǹ
+
Z
o
1 )
C
via
)C
stub1
)C
cpad1
)C
con
)C
cpad2
)C
stub2
)C
io
dC
o
Ǹ
(6)
(7)
(8)
(9)