Datasheet

SZZA016B
7–245 Basic Design Considerations for Backplanes
4.9
4.8
4.6
4.5
4.4
5.3
4.7
0204060
t
5.1
5
5.2
80 100
Stub Z
o
– Ω
(flight)
– ns
Rt = 24 Ω
Conn. Z
o
= 50 Ω
Rt = Calc.
Conn. Z
o
= 50 Ω
Rt = 24 Ω
Conn. Z
o
= 25 Ω
Rt = Calc.
Conn. Z
o
= 25 Ω
Figure 16. System Flight Time vs Stub Impedance
Figure 16 indicates that a range of stub impedances produces a minimum system flight time.
Lowest flight times are observed between 35 Ω and 50 Ω.
Higher-impedance stubs have a larger value of inductance that results in gradually longer stub
delays and flight times.
Lower-impedance stubs have larger values of capacitance that result in increased distributed
capacitance on the backplane. This increases the effective propagation delay and also
increases flight time much more dramatically than the higher-impedance stubs.
A design goal would be to have the stub Z
o
between 35 Ω and 65 Ω for optimum performance.
Distributed Capacitance – Theory to Practice
The effect of distributed capacitance was observed in the GTLP evaluation module (EVM). The
EVM is a 17.9-in., 48-bit, 20-slot stripline backplane with slot-to-slot spacing (slot pitch) of
0.94 in. and removable terminations. The backplane is divided into six groups, with each group
having eight traces. Group 1 is 20 slots long, while group 6 is only 2 slots long. The other groups
are 16, 12, 8, and 4 slots long. Group 1, bit 1 was used for this theory-to-practice evaluation.
The natural trace impedance (Z
o
) was planned to be 55 Ω for all, but two traces in group 1,
including the trace used for this experiment, were higher due to missing reference planes
between them. Available termination-resistor values are 25 Ω, 33 Ω, 38 Ω, and 50 Ω. Data was
taken at 23-MHz, 50-MHz, and 87-MHz clock frequencies. The SN74GTLPH1655 high-drive
transceiver was used for this experiment and was operated in the latched mode where data
frequency is one-half of clock frequency. The data waveforms are shown. The driver in all cases
is in slot 1; the waveforms shown were obtained directly from the backplane connector pin of the
receiver slot under test.