Datasheet

SCEA019 - January 2001
7–88 Texas Instruments GTLP Frequently Asked Questions
This illustrates why the termination-resistor values should be lower than the typical 50-Ω
natural transmission-line impedance in multipoint applications. Matching the termination
resistor with the effective trace impedance ensures incident-wave switching and better signal
integrity. Higher-drive (100 mA) devices are offered because termination values lower than 38
Ω often are required.
For example, in a 20-slot, slot pitch = 0.94 inch backplane, the characteristics in Table 1 are
observed.
Table 1. Backplane Characteristics, 20 Slots at 0.94-Inch Slot Pitch
Unloaded (Natural) Embedded
Microstrip Trace
Trace and 20 Empty
Connectors at 0.94 Inch
Fully Loaded With 20
Cards at 0.94 Inch
Z
O
(Ω)
t
pd
(ns/in)
C
O
(pF/in)
Z
O(eff)
(Ω)
t
pd
(ns/in)
Z
O(eff)
(Ω)
t
pd
(ns/in)
Card C
t
(pF)
48 142 2.99 37 183 18 382 15
90 165 1.84 62 239 26 570 16.2
Different stub lengths provide a unique C
t
for each bit on the same daughter card.
Table 1 clearly shows that, in a heavily loaded backplane with the 48-Ω natural Z
O
, the
termination resistance should go as low as 18 Ω, but is limited to 22 Ω by the GTLP high-drive
maximum recommended I
OL
. Increasing the natural Z
O
to 90 Ω can change the termination
resistor to 26 Ω, a value that is within the capacity of the high-drive devices and better
approximates what the medium-drive devices can handle. The disadvantage is that the
backplane time of flight is about 50% higher. Typical card capacitance will be between 12 pF
and 18 pF, depending on device C
io
and stub length, with stub length being most critical. Stub
length always should be less than one inch for best backplane performance. Lower C
t
always
is better.
For best signal integrity, termination resistor R
TT
should equal the loaded-trace impedance
(Z
O(eff)
). A lower value for R
TT
could be used to compensate for possible variations of the
device typical C
io
, up to the maximum C
io
value.