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Rule of thumb: Impact of adjacent trace on impedance

04 Mar 2015  | Eric Bogatin

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Figure 2 shows the calculated characteristic impedance of the microstrip as the spacing between the signal line and adjacent metal decreases:


Figure 2: The calculated characteristic impedance of the microstrip as the spacing between the signal line and adjacent metal decreases:


Note the vertical axis scale is 1Ω per division. When the spacing to the adjacent metal is farther than three line widths – 15 mils – there is no impact.

Even when the spacing is twice the line width away from either side, the microstrip's impedance is reduced by only 0.27Ω, which is 0.5%, and this is from metal on both sides of the surface trace.

As the adjacent metal comes closer, the fringe fields are more sensitive to proximity, and at a spacing equal to the line width, for the 50Ω line, the impact on the line's impedance is less than 2Ω, or 4%. If there was metal on just one side of the signal line, the impedance would be decreased by less than 2%.

This is the origin of the rule of thumb:

If the spacing to adjacent metal is more than one line width, the impact on the line impedance from metal on one side will be less than 2%.

Keep in mind though, the impact on the characteristic impedance from adjacent metal is just one metric of performance. There are much bigger problems from routing adjacent metal to a surface trace, like resonant coupling and far end crosstalk.

Now you try it:

1. Should you worry about the impact on the impedance of a 50Ω microstrip signal line when there is an adjacent trace two line widths away?

2. Suppose the impedance of the microstrip line were to increase by moving the bottom plane farther away. Is the influence of adjacent coplanar metal increased or decreased?


About the author
Eric Bogatin is Signal Integrity Evangelist at Teledyne LeCroy.


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