# Signal Integrity, Impedance and Electrical Models

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## 3.4 Impedance of an Ideal Resistor in the Time Domain

Each of the four basic circuit elements above has a definition of how voltage and current interact with it. This is different from the impedance of the ideal circuit element.

The relationship between the voltage across and the current through an ideal resistor is:

Equation 3-2

where:

• V = the voltage across the ends of the resistor

• I = the current through the resistor

• R = the resistance of the resistor, in Ohms

An ideal resistor has a voltage across it that increases with the current through it. This definition of the I-V properties of an ideal resistor applies in both the time domain and the frequency domain.

In the time domain, we can apply the definition of the impedance and, using the definition of the ideal element, calculate the impedance of an ideal resistor:

Equation 3-3

This basically says, the impedance is constant and independent of the current or voltage across a resistor. The impedance of a resistor is pretty boring.