Datasheet

SZZA016B
7–235 Basic Design Considerations for Backplanes
Distributed Capacitance
Figure 3 is a simplified version of Figure 2, where an equivalent capacitive load
(12-pF capacitor) replaces the receivers. It is assumed that the spacing between card slots is
within the rise and fall time of the driver signal, and that all slots are populated with cards.
l = 10 in. or
25.4 cm
Tx
R
TT
A
B
R
TT
d = 1 in. or
2.54 cm
12 pF 12 pF 12 pF 12 pF 12 pF 12 pF 12 pF 12 pF 12 pF 12 pF
V
TT
V
TT
Figure 3. Equivalent Multipoint Application
Total capacitance (C
t
) is calculated by summing all the capacitive components associated with
the transceiver and the connection to the backplane. Figure 4 shows a typical connection
scheme between the backplane stripline and the driving/receiving device on the daughter card.
Point C is the connection to the backplane stripline, while point D is the connection to a
transceiver integrated circuit. Total capacitance (C
t
) at point C is the sum of each of the
elements in the connection chain. Total capacitance then can be distributed uniformly across the
transmission line at an equivalent rate of capacitance per inch (C
d
).
Connector
C
pad1
C
pad2
Via
C
Stub 1 Stub 2
Device
D
Figure 4. Typical Connection Scheme to Backplane
The stub lines (stub 1 and stub 2) are 51-Ω microstrip construction with a characteristic
capacitance of 2.6 pF/in. (102 pF/m). The connection via connects stub 1 to the backplane trace
and has an approximate capacitance of 0.5 pF. Stub 1 is 1/16 in. in length and is connected on
the other end to the surface-mount pad for the connector (C
pad1
), which has a capacitance of
approximately 0.5 pF.
The HSPICE model for the connector used in this example has a connector capacitance (C
con
)
of 0.74 pF. C
pad2
has the same value as C
pad1
and is the surface-mount pad for the connector
on the daughter-card side that connects the connector to the daughter-card stub (stub 2). Stub 2
is 1 in. long and is attached to the other end of the transceiver input/output pin, which has a
typical capacitance (C
io
) of 7 pF.