TLK10031 Datasheet, PDF (137/146 Pages) Texas Instruments – TLK10031 Single-Channel XAUI/10GBASE-KR Transceiver

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TLK10031
SLLSEL3A – JULY 2015 – REVISED AUGUST 2015
10 Layout
10.1 Layout Guidelines
10.1.1 TLK10031 High-Speed Data Path
10.1.1.1 Layout Recommendations for High-Speed Signals
Both “low-speed” side and “high-speed” side serial signals are referred to as “high-speed” signals for the
purpose of this document as they support high data rates. For that reason, care must be taken to realize
them on a printed circuit board with signal integrity. The high-speed data path CML input pins
INA[3:0]P/INA[3:0]N and HSRXAP/HSRXAN, and the CML output pins OUTA[3:0]P/OUTA[3:0]N and
HSTXAP/HSTXAN, have to be connected with loosely-coupled 100-Ω differential transmission lines.
Differential intra-pair skew needs to be minimized to within ±1 mil. Inter-pair (lane-to-lane) skew for the
low-speed signals can be as high as 30 UI. An example of FR-4 printed circuit board (PCB) realization of
such differential transmission lines in microstrip format is shown in Figure 10-1.
Figure 10-1. Differential Microstrip PCB Trace Geometry Example
To avoid impedance discontinuities the high-speed serial signals should be routed on a PCB on either the
top or bottom PCB layers in microstrip format with no vias. If vias are unavoidable, an absolute minimum
number of vias need to be used. The vias should be made to stretch through the entire PCB thickness (as
shown in Figure 10-2) to connect microstrip traces on the top and bottom layers of the PCB so as to leave
no via stubs that can severely impact the performance. If stripline traces are absolutely necessary, and if
via back-drilling is not possible, then the routing layers should be chosen so as to have via stubs that are
shorter than 10 mils.
All unused internal layer via pads on high-speed signal vias should be removed to further improve
impedance matching. On the high-speed side, the HSRXAP/HSRXAN signals are more sensitive to
impedance discontinuities introduced by vias than HSTXAP/HSTXAN signals. For that reason, if only
some of those signals need to be routed with vias, then the latter should be routed with vias and the
former with no vias.
Copyright © 2015, Texas Instruments Incorporated
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