Datasheet

SCEA019 - January 2001
Texas Instruments GTLP Frequently Asked Questions 7–87
The capacitance in the chain illustrated in Figure 8 is summed as follows:
C
via
= capacitance of via = 0.5 pF
C
stub1
= capacitance of Stub1 = 0.0625 inch × 2.6 pF/inch = 0.16 pF
C
cpad1
= capacitance of C
pad1
= 0.5 pF
C
con
= capacitance of connector = 0.74 pF
C
cpad2
= capacitance of C
pad2
= 0.5 pF
C
stub2
= capacitance of Stub2 = 1 inch × 2.6 pF/inch = 2.6 pF
C
io
= typical input/output capacitance of device = 7 pF
C
t
= C
via
+ C
stub1
+ C
cpad1
+ C
con
+ C
cpad2
+ C
stub2
+ C
io
C
t
= 0.5 + 0.16 + 0.5 + 0.74 + 0.5 + 2.6 + 7
C
t
= 12 pF
This total capacitance (C
t
) of 12 pF is placed at point C on the backplane for every
transceiver. With all the slots filled, the 10-inch transmission line has ten 12-pF capacitors and
one transmitter (12 pF) distributed at 1-inch intervals.
Total capacitance then can be distributed uniformly across the transmission line at an
equivalent rate of capacitance per inch (C
d
). The total capacitance per distance is the
distributed capacitance. The higher the total card capacitance and the closer the card spacing
(slot pitch) the heavier the backplane loading.
The distributed capacitance equals the total capacitance divided by the separation, or
C
d
= C
t
/d. In our example, C
d
= 12 pF per 1 inch or 472 pF per meter. The new effective trace
impedance Z
O(eff)
and effective propagation delay (t
pd(eff)
) can be calculated using the
following equations. C
o
is the characteristic capacitance, which is dependent on Z
O
and is
fixed.
()
()
o
C
d
C+1
O
Z=
effO
Z
() ( )
odpdpd
CC+1×t=efft
The distributed capacitance (C
d
) affects both the propagation delay and the characteristic
impedance of the transmission line. A larger C
d
(higher C
t
and/or smaller d) results in lower
effective trace impedance (Z
O(eff)
) and a higher effective propagation delay (t
pd(eff)
).