Datasheet
SZZA016B
7–234
Basic Design Considerations for Backplanes
The propagation delay (t
pd
) is the time delay through the transmission line per unit length and is
a function of the natural impedance and characteristic capacitance.
Use equation 1 to calculate propagation delay (t
pd
).
t
pd
+ Z
o
C
o
In this example, t
pd
= 51 Ω × 3.5 pF/in. (138 pF/m) yields 178.5 ps/in. (7038 ps/m or 7.03 ns/m).
The total flight time (t
flight
) is the time required for the signal to propagate down the transmission
line from driver to receiver (time from A to B) and is a function of t
pd
and length of the line.
Use equation 2 to calculate t
flight
.
t
flight
+ t
pd
length of line + 178.5 psńin. 10 in.
or 7.03 nsńm 0.254 m
+ 1.784 ns
In Figure 2, the point-to-point configuration has been changed to a multipoint layout. Eleven
transceivers are placed on the 10-in. transmission line, with 1-in. spacing (d) between each
transceiver. One transceiver is configured as a transmitter (Tx); the other ten are configured as
receivers (Rx). In a multipoint system, any position can assume the role of transmitter, with the
remaining positions acting as receivers, as shown by the transceiver symbol. The 51-Ω stripline
transmission line must be terminated at both ends because the transceiver at either end could
be the driver. The optimum termination resistance (R
TT
) for the multipoint system is less than the
natural transmission-line impedance of 51 Ω, due to the effects of distributed capacitance.
Procedures to calculate the optimum R
TT
and the effects of mismatched R
TT
on signal integrity
are the main focus of this application report.
l = 10 in. or
25.4 cm
Tx
Rx
R
TT
A
B
R
TT
Rx Rx Rx Rx Rx
Rx Rx Rx Rx
d = 1 in. or
2.54 cm
V
TT
V
TT
Figure 2. Multipoint Application
(1)
(2)