Datasheet

SZZA016B
7–236
Basic Design Considerations for Backplanes
Two different printed circuit board (PCB) transmission lines are shown in Figure 5. Basically,
microstrip resides on the top of the PCB, whereas the stripline is imbedded within the PCB
layers. A microstrip is faster due to the less inherent capacitance, but a stripline exhibits better
signal integrity because the reference planes shield the conductor from damaging EMI fields.
Other performance differences are discussed later in this application report.
Metal Reference
Plane
Dielectric
Transmission Line
Trace
Microstrip Stripline
Metal Reference
Plane
Dielectric
Metal Reference
Plane
Figure 5. Typical PCB Transmission Lines
The capacitance of the via, pads, and stubs can be calculated based on the dimensions and
type of traces. www.ultracad.com provides an excellent capacitance calculator with background
information. The C
io
of the transceiver and the connector capacitance can be obtained from the
manufacturer’s specification sheet.
The capacitance in this chain is summed as:
C
t
+ 12 pF
C
t
+ C
via
) C
stub1
) C
cpad1
) C
con
) C
cpad2
) C
stub2
) C
io
Where:
C
via
= capacitance of via = 0.5 pF
C
stub1
= capacitance of stub 1 = 0.0625 × 2.6 = 0.16 pF
C
cpad1
= capacitance of C
pad1
= C
pad2
= 0.5 pF
C
cpad2
= capacitance of C
pad2
= C
pad2
= 0.5 pF
C
stub2
= capacitance of stub 2 = 1 × 2.6 = 2.6 pF
C
con
= capacitance of connector = 0.74 pF
C
io
= input/output capacitance of device = 7 pF
The total capacitance (C
t
) of 12 pF is placed at point C on the backplane for every transceiver.
More than half of C
t
is the transceiver typical input/output pin capacitance. This is why
backplane designers require low-capacitive ICs to optimize performance in high-frequency
backplanes.
With all the slots filled, the 10-in. transmission line has 11 12-pF capacitors distributed at 1-in.
intervals. The distributed capacitance (C
d
) affects both the propagation delay and the
characteristic impedance of the stripline transmission line, which results in a new effective
impedance, Z
o(eff)
, and a new effective propagation delay, t
pd(eff)
. The distributed capacitance
equals the total capacitance divided by the separation, or C
d
= C
t
/d. In our example,
C
d
= 12 pF/1 in. or 12 pF/in. (472 pF/m). The new effective impedance, Z
o(eff)
, and effective
propagation delay, t
pd(eff)
, can be calculated using equations 4 and 5.
Z
o
(
eff
)
+
Z
o
1 )
C
d
C
o
Ǹ
t
pd
(
eff
)
+ t
pd
1 )
C
d
C
o
Ǹ
(3)
(4)
(5)