Ac And Dc Coupling - Xilinx Virtex-4 RocketIO User Manual

Chapter 6: Analog and Board Design Considerations
high side, low side, and differential (floating or active). This configuration supports
receiver termination compatible to Virtex-II Pro RocketIO transceiver, using a CML (high-
side) termination to an active supply of 1.8V – 2.5V. For DC coupling of two Virtex-4
devices or with a Virtex-II Pro X device, a 1.5V CML termination for V
recommended.

AC and DC Coupling

AC coupling (use of DC blocking capacitors in the signal path) should be used in cases
where MGT differential voltages are compatible, but common mode voltages are not. Some
designs require AC coupling to accommodate hot plug-in, differing power supply voltages
at different MGTs, and/or the need to interface with other vendors' devices. This is
illustrated in
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.
Figure 6-11
•
•
Capacitors of value 0.01 µF in a 0402 package are suitable for AC coupling up to 6.5 Gb/s
when 8B/10B encoding is used. Different data rates and different encoding schemes can
require a different value.
The on-chip AC coupling supports DC-balanced data for the entire range of data rates
(622 Mb/s – 6.5 Gb/s). DC-balanced coding ensures that the coupling capacitor does not
charge up or down, thus leaving the common mode voltage at an optimal value.
172
VTRX
50Ω
RXP
RXN
Figure 6-10: Simplified Receive Termination Diagram
Figure 6-11. The MGT has on-chip AC coupling caps in the receiver after the
shows the three possible configurations for AC coupling:
On-chip AC coupling caps with V
signal amplitude for long trace lengths. 1.2V can be used for shorter channels to
simplify power supply circuits (preferred for Virtex-4 transceivers).
External AC coupling caps are required when the receiver common-mode input is less
than 0V or greater than 2.5V, or the single-ended peak voltage is greater than 2.5V. In
this scenario, there are two options for V
♦
The internal AC-coupling cap is enabled, and V
2.5V (recommended).
♦
The internal AC-coupling cap is bypassed, and V
www.xilinx.com
50Ω
= V = 1.2V or 1.5V. 1.5V provides the largest
TTX TRX
:
TRX
can be set to a value of 0V to
TRX
can be left floating.
TRX
Virtex-4 RocketIO MGT User Guide
UG076 (v4.1) November 2, 2008
R
is
TRX
ug076_01_111405