- Describing Signal-Integrity Solutions in Terms of Impedance
- What Is Impedance?
- Real vs. Ideal Circuit Elements
- Impedance of an Ideal Resistor in the Time Domain
- Impedance of an Ideal Capacitor in the Time Domain
- Impedance of an Ideal Inductor in the Time Domain
- Impedance in the Frequency Domain
- Equivalent Electrical Circuit Models
- Circuit Theory and SPICE
- Introduction to Modeling
- The Bottom Line
3.11 The Bottom Line
Impedance is a powerful concept to describe all signal-integrity problems and solutions.
Impedance describes how voltages and currents are related in an interconnect or component. It is fundamentally the ratio of the voltage across a device to the current through it.
Real components that make up the actual hardware are not to be confused with ideal circuit elements that are the mathematical description of an approximation to the real world.
Our goal is to create an ideal circuit model that adequately approximates the impedance of the real physical interconnect or component. There will always be a bandwidth beyond which the model is no longer an accurate description, but simple models can work to surprisingly high bandwidth.
The resistance of an ideal resistor, the capacitance of an ideal capacitor, and the inductance of an ideal inductor are all constant with frequency.
Though impedance has the same definition in the time and frequency domains, the description is simpler and easier to generalize for C and L components in the frequency domain.
The impedance of an ideal R is constant with frequency. The impedance of an ideal capacitor varies as 1/ωC and the impedance of an ideal inductor varies as ωL.
SPICE is a very powerful tool to simulate the impedance of any circuit or the voltage and current waveforms expected in both the time and frequency domains. Every engineer who deals with impedance should have a version of SPICE available to him or her on his or her desktop.
When building equivalent circuit models for real interconnects, it is always important to start with the simplest model possible and build in complexity from there. The simplest starting models are single R, L, C, or T elements. Higher-bandwidth models use combinations of these ideal circuit elements.
Real components can have very simple equivalent circuit models with bandwidths in the GHz range. The only way to know what the bandwidth of a model is, however, is to compare a measurement of the real device to the simulation of the impedance using the ideal circuit model.