Re: [SI-LIST] : Match of differential pairs

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From: Ritchey Lee (leeritchey@earthlink.net)
Date: Mon May 15 2000 - 10:51:33 PDT


With regard to the length matching comments below, the National LVDS specification allows 300 pSEC of length difference
from all causes between members of a pair. This translates to about 1.8 inches in FR-4. Requiring length matching of 10
mils is excessivley tight and not achievable if one adds up all of the places that length can be different. Specifing a
length matching requirement of +- 0.5 inches is well within the LVDS specification and makes PCB layout much more
tractable.

Lee

Mike Saunders wrote:

> Martyn,
>
> Some comments regarding allowable differential pair mismatch:
>
> 1. Placing the diff. pairs on the stripline layer is a good selection
> since control of line width is easier to maintain on internal layers.
>
> 2. Manufacturing deviations should average out (concerning trace width)
> along the length of the line. Therefore, by maintaining consistent
> spacing, the slight impedance fluctuations along the line should be minimal.
>
> 3. What the National app. note does not say specifically is that for
> differential pairs, a good match consists of several elements: consistent
> trace width/spacing, equal number of vias (if any), trace length matching
> (including trace segments which are defined by via transistions), keeping
> the overall trace length as short as possible and routing over a continuous
> plane when possible. If all of these guidelines are followed, common mode
> noise should be minimal.
>
> 4. The actual trace length matching tolerance depends on your timing
> budget. Usually I try to increase overall desing margin where it is easily
> maintained, so I will generally specify a trace length mismatch of +/- 10
> MILs maximum. Since flight time is about 1nS per 6" of trace, a 0.010"
> mismatch in overall trace length should only produce a mismatch at the
> receiver of around 1.6pS. This minimal amount of skew should be within
> most timing budgets. Otherwise, specify a tighter mismatch tolerance.
> **It is more important to match the trace lengths than to maintain
> consistent trace widths or trace spacing.**
>
> 5. I don't know what your exact impedance requirements are, but an example
> using USB diff. pairs would be as follows:
> Target diff. Zo: 90ohms
> Single trace Zo: ~45 - 50 ohms
>
> Required stackup & trace specs to meet these requirements: 4 MIL trace
> using 2 oz. Cu, 6 MIL space,
> 4 MIL dielectric height above 2 oz. Cu.
>
> First, specify your trace width, then adjust the dielectric height and Cu
> thickness to get the correct single-ended impedance.
> Then, adjust the trace spacing to give the target differential impedance.
>
> 6. If your stackup allows it, try to keep the trace-to-trace seperation
> smaller than the dielectric height. This will do wonders for common mode
> noise rejection, as each trace will specifically reference its pair rather
> than the plane.
>
> 7. Favor shorter and fatter traces if real estate area allows it. This
> will cut down on both trace capacitance and trace inductance.
>
> 8. Maintain sufficient spacing between the diff. pairs and any other
> signal groups, especially clocks and other high-speed signals. Also, keep
> diff. pairs at least 2h away from any board edges or apertures.
>
> Hope some of this helps...
>
> --Mike
>
> At 11:46 PM 5/9/2000 +0100, you wrote:
> >Hi everyone,
> >
> >Can anyone help on the following subject? My customers
> >are in the PCB manufacturing industry and are increasingly
> >called to build differential striplines, however on fine line
> >boards it is very hard to make an exact match of the signal
> >pair.
> >
> >This obviously has effects on cmrr, emc etc, but I cannot find
> >any documents that define how much mismatch is acceptable on
> >a differential pair.
> >
> >National have a good app note on LVDS for example but when
> >matching is discussed it simply says good match is necessary.
> >
> >This was easy on 7 or 8 mil line width, much less so at 3 to 4 mil.
> >
> >Look forward to your input
> >
> >Kind regards
> >Martyn Gaudion
> >martyn.gaudion@polar.co.uk
> >www.polarinst.com
> >
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