Ac And Dc Coupling - Xilinx RocketIO User Manual

Product Not Recommended for New Designs
PCB Design Requirements

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
RocketIO™ Transceiver User Guide
UG024 (v3.0) February 22, 2007
W S
1
Trace
H
E = 4.3
r
Reference Plane
Figure 3-13: Microstrip Edge-Coupled Differential Pair
Reference Plane
W S W
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
www.xilinx.com
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 = 100Ω
0DIFF
W = 3.0 mil (0.076 mm)
1
H
1
W = 3.0 mil (0.076 mm)
2
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 = 100Ω
0DIFF
H
2
Figure
C
1
RX
C
2
UG024_23_042503
Figure
R
UG024_22_042903
UG024_22a_042903
3-15.
3-16.
117