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发表于 2009-5-25 12:25:10
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大家可以参考INTEL SPEC
AC Coupling Capacitors :
The PCI Express specification requires that each lane of a PCI Express link be AC
coupled between the driver and receiver. The specification allows for the actual AC
coupling capacitors to be located either on or off the die. However, it is anticipated that
in most cases the AC coupling will be separate from the die and in the form of discrete
capacitors on the motherboard itself. Use the following guidelines for AC coupling
capacitors:
• The Intel 82801GBM PCI Express expansion interface requires external AC coupling
capacitors.
• The AC capacitors may be placed anywhere on the line between the Intel
82801GBM and the PCI Express device/connector.
• Locate capacitors at the same linear location along the signals of a given
differential pair.
— No more than a 5-mil (0.127 mm) delta should exist between the differential
signals on the trace segments on either side of the AC coupling capacitor.
— There is no requirement to length match the routing from the ICH to the AC
coupling capacitors between the differential pairs.
• Place the capacitors as close to each other as possible per DFM rules for each
differential pair.
• While the 0603 size capacitors are acceptable, size 0402 capacitors are strongly
encouraged.
— The smaller package size reduces the series inductance.
— The smaller package size reduces the overall board area needed to place the
capacitors.
• Pad sizes for each of the capacitors should be the minimum allowed per DFM to
minimize parasitics.
• The same capacitor package size should be used for each signal in a differential
pair.
• C-packs are not recommended for PCI Express AC coupling capacitor purposes.
VNA measurements have shown that the loss across the channel due to crosstalk is
great and causes a severe decrease in margin. Also, layout studies show only a
small decrease in board area is needed to place the 0504, x2, C-packs compared
with 0402 discrete capacitors (less than 10%).
• The spec requires that a value anywhere between a minimum of 75 nF and a
maximum of 200 nF be used for each of the capacitors.
— 0.1 uF capacitors are a good choice because they are a common value and
generally present in a typical bill of materials.
— Tolerance values of the capacitors are irrelevant as long as the overall range of
tolerances falls within the spec minimum/maximum values of 75nF – 200nF.
• The “breakout” into and out of the capacitors should be symmetrical for both signal
lines in a differential pair. Minimize this trace route in order to maximize the
amount of coupling between the signals within the pair.
• Routing studies have shown that the AC coupling capacitors can be place in less
than 0.3 square inches (194 square). |
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