AM3874 datasheet(331/361 Pages) TI

331 / 361 page

AM3874, AM3871

www.ti.com

SPRS695C – SEPTEMBER 2011 – REVISED DECEMBER 2013

In general, closely coupled differential signal traces are not an advantage on PCBs. When differential

signals are closely coupled, tight spacing and width control is necessary. Very small width and spacing

variations affect impedance dramatically, so tight impedance control can be more problematic to maintain

in production.

Loosely coupled PCB differential signals make impedance control much easier. Wider traces and spacing

make obstacle avoidance easier, and trace width variations do not affect impedance as much; therefore, it

is easier to maintain an accurate impedance over the length of the signal. The wider traces also show

reduced skin effect and, therefore, often result in better signal integrity.

Table 8-98 shows the routing specifications for the PCIe data signals.

Table 8-98. PCIe Routing Specifications

PARAMETER

MIN

TYP

MAX

UNIT

PCIe signal trace length

10(1)

Inches

Differential pair trace matching

10(2)

Mils

Number of stubs allowed on PCIe traces(3)

Stubs

TX/RX pair differential impedance

100

120

TX/RX single ended impedance

Pad size of vias on PCIe trace

25(4)

Mils

Hole size of vias on PCIe trace

Mils

Number of vias on each PCIe trace

Vias(5)

PCIe differential pair to any other trace spacing

2*DS(6)

(1)

Beyond this, signal integrity may suffer.

(2)

For example, RXP0 within 10 Mils of RXN0.

(3)

In-line pads may be used for probing.

(4)

35-Mil antipad max recommended.

(5)

Vias must be used in pairs with their distance minimized.

(6)

DS = differential spacing of the PCIe traces.

Peripheral Information and Timings

331

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