EMC Question of the Week: November 11, 2019
The most important parameter affecting the inductance of a ground strap is its
- length
- width
- thickness
- permeability
Answer
The best answer is "a". Of course, inductance is a property of current loops; and the contribution of a ground strap to a loop inductance will depend on many factors. Nevertheless, in practical applications where the loop is relatively open, the effective inductance of the ground strap will be approximately proportional to its length. The width is a secondary contributor, with the effective inductance (in typical geometries) varying inversely with the natural log of the width. The thickness typically has very little effect on the inductance when the width is much greater than the thickness. The permeability is also negligible, especially at high frequencies where the currents flow on the surface of the strap and the internal inductance of the strap approaches zero.
A general rule-of-thumb for estimating the contribution of one segment of a relatively open wire loop is that the inductance is "on the order of 1 nH/mm." This estimate neglects all parameters of the ground strap except its length. Another, slightly more accurate means for estimating the branch inductance of a relatively open loop is,
where L and W are the length and width of the strap in centimeters, respectively.
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