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3.2 What Is Impedance?

We use the term impedance in everyday language and often confuse the electrical definition with the common usage definition. As we saw earlier, the electrical term impedance has a very precise definition based on the relationship between the current through a device and the voltage across it: Z = V/I. This basic definition applies to any two-terminal device, such as a surface mount resistor, a decoupling capacitor, a lead in a package, or the front connections to a printed circuit-board trace and its return path. When there are more than two terminals, such as in coupled conductors, or between the front and back ends of a transmission line, the definition of impedance is the same, it's just more complex to take into account the additional terminals.

For two-terminal devices, the definition of impedance, as illustrated in Figure 3-2, is simply:

Equation 3-1 where:

• Z = the impedance, measured in Ohms

• V = the voltage across the device, in units of volts

• I = the current through the device, in units of Amps Figure 3-2. The definition of impedance for any two-terminal device showing the current through the component and the voltage across the leads.

For example, if the voltage across a terminating resistor is 5 v and the current through it is 0.1 A, then the impedance of the device must be 5 v/0.1 A = 50 Ohms. No matter what type of device the impedance is referring to, in both the time and the frequency domain, the units of impedance are always in Ohms.

This definition of impedance applies to absolutely all situations, whether in the time domain or the frequency domain, whether for real devices that are measured or for ideal devices that are calculated.

If we always go back to this basic definition, we will never go wrong and oftentimes eliminate many sources of confusion. One aspect of impedance that is often confusing is to think of it only in terms of resistance. As we shall see, the impedance of an ideal resistor-circuit element, with resistance, R, is, in fact, Z = R.

Our intuition of the impedance of a resistor is that a high impedance means less current flow for a fixed voltage. Likewise, a low impedance means a lot of current can flow for the same voltage. This is consistent with the definition that I = V/Z, and applies just as well when the voltage or current is not DC.

In addition to the notion of the impedance of a resistor, the concept of impedance can apply to an ideal capacitor, an ideal inductor, a real-wire bond, a printed circuit trace, or even a pair of connector pins.

There are two special extreme cases of impedance. For a device that is an open, there will be no current flow. If the current through the device for any voltage applied is zero, the impedance is Z = 1 v / 0 A = infinite Ohms. The impedance of an open device is very, very large. When the device is a short, there will be no voltage across it, no matter what the current through it. The impedance of a short is Z = 0 v / 1 A = 0 Ohms. The impedance of a short is always 0 Ohms.