Datasheet

7–63
Unloaded Line Impedance
Line Z
o
=
L
o
(pH) =
Line t
pd
(ps) =C
o
(pF) =
Device
C
io
(pF) =
Load Spacing
D1 (in) =
Stub Length
C
d
(pF) =D2 (in) =
Stub
C
o
(pF) =
Connector
C
C
(pF) =
Via
C
via
(pF) =
Effects of C
d
on Impedance (Loaded)
C
t
(pF) =
Connector Pads C
P
(pF) =
Inputs Outputs
/in.
/in.
/in.
/in.50.00
4.10
6.00
0.875
1.0625
2.60
0.70
1.00
0.50
10250.00
205.00
10.96
12.53
t
pd
(ps) = Z
o
(eff) =
412.85 24.8/in.
R
T
Figure 8. Impedance Calculator
The impedance calculator is a spreadsheet that is created using the previous equations to show the effects of distributed
capacitance. The spreadsheet shows that the initial impedance of the 50-Ω backplane trace introduces a delay of 205 ps/in. (see
equation 3) and has a C
o
of 4.1 pF/in. The introduction of backplane loads increases the distributed capacitance (C
d
) to
12.53 pF/in., which increases the propagation delay (t
pd
) to 412.85 ps/in. and reduces the backplane impedance to 24.83 Ω (see
equation 4). The backplane loading is a factor of the input/output capacitance of the driver or receiver (C
io
), stub capacitance,
via capacitance, and connector capacitance. Both ends of the backplane trace are terminated by a stub (A), using a 25-Ω pullup
resistor to the termination voltage (V
TT
= 1.5 V).
5