In this paper, we’ll define the difference between characteristic and instantaneous impedance and explore the essential principles of impedance and design-controlled impedance using transmission lines as an example. We’ll investigate the impedance of two of the most common transmission lines, striplines and microstrips, using an analysis tool to explain how the different parameters of each transmission line type affect time delays and characteristic impedance. The output impedance of the driver is the first impedance a signal sees as it propagates from the source. This paper equips the reader with an easy way to derive the output impedance. Finally, the parasitics of the package leads and the on-die capacitance can also have a great effect on the signal quality, especially at high speeds. In order to understand their effect, those features are extracted from the IBIS model.
signal integrity, impedance, Mentor Graphics, PCB, EDA