Termination; Ac And Dc Coupling; Figure 4-27: Transmit Termination; Figure 4-28: Receive Termination - Xilinx RocketIO X User Manual

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Termination

The RocketIO X transceiver implements on-chip 50Ω termination in both the transmitter
(TXP/TXN) and receiver (RXP/RXN). The output driver and termination are powered by VTTX at
1.5V. This configuration uses a CML approach with 50Ω to TXP and TXN as shown in
The receiver termination supply (VTRX) is the center tap of differential termination to RXP
and RXN as shown in
high-side, low-side, and differential (floating or active). This configuration supports
receiver termination compatible to Virtex-II Pro devices, using a CML (high-side)
termination to an active supply of 1.8V – 2.5V. For DC coupling of two Virtex-II Pro X
devices, a 1.5V CML termination for VTRX is recommended.

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
transceiver has on-chip AC coupling caps in the receiver after the receive termination, thus
supporting a wide common mode input range (0.25V – 2.5V). For common mode voltages
outside of this range, external AC coupling should be used. The on-chip AC coupling
120
Chapter 4: Analog Design Considerations
50Ω

Figure 4-27: Transmit Termination

Figure 4-28. This supports multiple termination styles, including
VTRX
50Ω
50Ω
RXP
RXN

Figure 4-28: Receive Termination

www.xilinx.com
1-800-255-7778
VTTX (1.5V)
50Ω
TXP
TXN
ug035_ch4_22_092303
x700_04_022103
Figure 4-29. The RocketIO X
RocketIO™ X Transceiver User Guide
UG035 (v1.5) November 22, 2004
Figure 4-27.