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zilog 30th August 2006 11:54 AM

inductance of SMD 2W milliohm resistor
I am trying to choose between using 47 mOhm SMD resistors (2W) and corresponding current sensing shunts for overcurrent sensing in a class d output stage. Shunts have very low inductance (basically just 20 mm of straight wire), but what is the inductance of the typical SMD resistor (how are they internally constructed)?

I have bad experience when it comes to inductance in axial 47 mOhm "non-inductive" resistors, so I want to know if my design is forced to use expensive shunts, or if I can manage with resistors without too much sacrifice in inductance.

sawreyrw 30th August 2006 10:06 PM

The inductance of wire over a grond plane should be well under 20 nH/inch, so the inductance of a 2W SMD is negligable. (I can't imagine this is a wirewound resistor.)

jackinnj 30th August 2006 11:10 PM

and don't forget that the impedance of the inductive part is swamped by the d.c. resistance and that the PCB material becomes conductive around a few hundred MegaHertz due to parasitic capacitance :)

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