nanohenry

The nanohenry is one billionth of a henry, and it is the working unit of high-speed and RF design. A straight length of wire or PCB trace runs roughly 1 nH per millimetre, a bond wire about the same, and the loop inductance from a decoupling capacitor through its vias to the die is the number that decides whether the supply rail holds up during a switching edge. The factor of 1e-9 is exact, being an SI prefix.

Watch out: Inductance belongs to a loop, not to a piece of wire. Quoting nanohenries for a trace only means something once the return path is specified, which is why moving a ground plane cut changes an impedance without anything visible changing on the signal layer.

One nanohenry in every inductance unit

• Mutual Inductance of Coupled Coils
• EMF Induced in a Coupled Coil
• Two Inductors in Series
• Two Inductors in Parallel
• Energy Stored in an Inductor
• Inductive Reactance (X_L = 2πfL)

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