Xilinx Spartan-6 FPGA Series Design And Pin Planning Manual

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Xilinx Spartan-6 FPGA Series Design And Pin Planning Manual Manual pdf 43 page image

Spartan-6 FPGA PCB Design and Pin Planning www.xilinx.com 43UG393 (v1.1) April 29, 2010TracesTracesTrace GeometryFor any trace, its characteristic impedance is dependent on its stackup geometry as well asthe trace geometry. In the case of differential traces, the inductive and capacitive couplingbetween the tightly coupled pair also determines the characteristic impedance of thetraces.The impedance of a trace is determined by its inductive and capacitive coupling to nearbyconductors. For example, these conductors can be planes, vias, pads, connectors, and othertraces, including the other closely coupled trace in a differential pair. The substrateproperties, conductor properties, flux linkage area, and distance to a nearby conductordetermine the amount of coupling and hence, the contribution to the final impedance.2D field solvers are necessary in resolving these complex interactions and contribute to thecalculation of the final impedance of the trace. They are also a useful tool to verify existingtrace geometries.Wider traces create a larger cross-sectional area for current to flow and reduce resistivelosses in high-speed interfaces. Use the widest traces that space constraints allow. Becausetrace width tolerances are expressed in absolute terms, a wider trace also minimizes thepercentage variation of the manufactured trace, resulting in tighter impedance controlalong the length of the transmission line.Sometimes, striplines are preferred over microstrips because the reference planes on bothsides of the trace provide radiation shielding. Microstrips are shielded on only one side (bythe reference plane) because they run on the top-most or bottom-most layers, leaving theother side exposed to the environment.For best results, the use of a 2D or 3D field solver is recommended for verification.Trace Characteristic Impedance Design for High-Speed TransceiversBecause the transceivers use differential signaling, the most useful trace configurations aredifferential edge-coupled stripline and differential microstrip. While some backplanes usethe differential broadside-coupled stripline configuration, it is not recommended for10 Gb/s operation, because the P and N vias are asymmetrical and introduce common-mode non-idealities.With few exceptions, 50Ω characteristic impedance (Z0 ) is used for transmission lines inthe channel. In general, when the width/spacing (W/S) ratio is greater than 0.4 (8 mil widetraces with 20 mil separation), coupling between the P and N signals affects the traceimpedance. In this case, the differential traces must be designed to have an odd modeimpedance (Z 0O ) of 50Ω , resulting in a differential impedance (Z DIFF ) of 100Ω , becauseZ DIFF = 2 x Z 0O .The same W/S ratio also must be less than 0.8, otherwise strong coupling between thetraces requires narrower, lossier traces for a Z0O of 50Ω. To clarify, with Z 0O at 50Ω, an evenmode impedance (Z 0E ) of 60Ω or below is desired.Figure 4-1 through Figure 4-4 show example cross sections of differential structures.