Ac And Dc Coupling - Xilinx RocketIO User Manual

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AC and DC Coupling

AC coupling (use of DC blocking capacitors in the signal path) should be used in cases where
transceiver differential voltages are compatible, but common mode voltages are not. Some designs
require AC coupling to accommodate hot plug-in, and/or differing power supply voltages at
different transceivers. This is illustrated in
Capacitors of value 0.01 µF in a 0402 (EIA) package are suitable for AC coupling at 3.125 Gb/s
when 8B/10B encoding is used. Different data rates and different encoding schemes may require a
different value.
DC coupling (direct connection) is preferable in cases where RocketIO transceivers are interfaced
with other RocketIO transceivers or other Mindspeed transceivers that have compatible differential
and common mode voltage specifications. Passive components are not required when DC coupling
is used. This is illustrated in
114
W S
1
Trace
H
E = 4.3
r
Reference Plane
Figure 3-13: Microstrip Edge-Coupled Differential Pair
Reference Plane
W S
1
Trace Trace
Dielectric
E = 4.3
r
Reference Plane
Figure 3-14: Stripline Edge-Coupled Differential Pair
TX Z = 100Ω Differential
0
Figure 3-15: AC-Coupled Serial Link
Figure 3-16.
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Chapter 3: Analog Design Considerations
W W = 6.29 mil (0.160 mm)
2
1
W = 6.29 mil (0.160 mm)
2
S = 10 mil (0.254 mm)
Trace
H = 5.0 mil (0.127 mm)
Z = 55.3Ω
01
Z = 55.3Ω
02
Dielectric
Z
0DIFF
W = 3.0 mil (0.076 mm)
1
H
1
W = 3.0 mil (0.076 mm)
2
W S = 6.85 mil (0.174 mm)
2
H = 10.0 mil (0.254 mm)
1
H = 10.0 mil (0.254 mm)
2
Z = 64.8Ω
01
Z = 64.8Ω
02
Z
0DIFF
H
2
Figure 3-15.
C
1
C
2
UG024_23_042503
RocketIO™ Transceiver User Guide
UG024 (v2.3.2) June 24, 2004
= 100Ω
UG024_22_042903
= 100Ω
UG024_22a_042903
RX