Datasheet

SCBA015A
7–8
Fast GTLP Backplanes With the GTLPH1655
C
O
C
L
C
O
C
L
C
O
C
L
L
O
L
O
≈ 6 nH/cm
C
O
≈ 0.6 pF/cm
C
L
≈ 20 pF/2cm
C
tot
= C
O
+ C
L
≈ 10.6 pF/cm
Z
O
= 25 Ω
τ = 25 ns/m
L
O
L
O
d = 2 cm
Figure 2. Physical Relationships on a Bus Line
Table 1. Additional Capacitive Loading of a Bus Line by a Module
CONTRIBUTOR
CAPACITANCE
(pF)
Capacitance of the connector plug ≈5
Capacitance of the feedline from the driver I/O ≈5
Capacitance of the driver I/O ≈10
Capacitive loading from a module (total) ≈20
To illustrate this situation, Figure 3 shows a comparison between the waveform on a line with
that from a load consisting of a lumped capacitance. It can be seen clearly in the diagram on the
left that the length of a line and, therefore, its capacitance, has no influence on the waveform. To
better observe the various loads, the rising edge is shown shifted by 10 ns. In the diagram on
the right, instead of a line, a capacitor having the equivalent total capacitance value has been
connected to the output of the test circuit. In this case, the output edge takes the form of a
capacitor-charging curve. If the two measurement results are compared, it is clear that signals
on a line behave very differently than in the case of a capacitive load. Therefore, an analysis
using transmission-line theory is necessary.