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

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From: Martyn Gaudion (Martyn@gaudionm.freeserve.co.uk)
Date: Thu May 11 2000 - 14:41:23 PDT


Mike,

Thanks for the feedback especially the comments
that you feel length match is more critical than impedance
imbalance between the lines

Kind regards
Martyn Gaudion

At 12:30 10/05/00 -0600, you 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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