By Patrick Carrier

Impedance is an important concept in many different realms of engineering.
We often see it in our everyday life, especially if you’ve ever hooked up a home entertainment system.  From 8-ohm speaker wire to 75-ohm coaxial cable, the right impedance is crucial to watching things explode on your TV and making sure they sound good too.

Simply stated, impedance describes a relationship between voltage and current.  For a resistor, that is a pretty simple relationship: V = IR.  For a transmission line, however, the relationship is a bit more complicated, since energy is travelling in fields between the incident and return path, usually a trace and a plane.  The characteristic impedance of a transmission line must be calculated using a field solver, and serves as the basis for signal integrity analysis.  For a signal, trace impedance is targeted to “match” the driver and receiver impedance.
For power, impedance should always be at a minimum.  For DC power delivery, that means low resistance, or as much metal as possible (planes, vias, traces).  For AC power delivery, that means very low-inductance connections to a large number and range of decoupling capacitors.  This is one of the fundamental differences between signal integrity and power integrity analysis. 
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This article first appeared on the Siemens Digital Industries Software blog at