Controlled impedance is the maintenance of some specified tolerance in the characteristic impedance of an interconnect line (transmission line) that is used to connect different devices on a circuit. Controlled impedance is often a design consideration for high-speed digital or high-frequency analog circuits. This guide is intended to be used by circuit designers, packaging engineers, printed board fabricators, and procurement personnel. Enhancements within Revision A include expanded data for logic families, updated equations for unbalanced lines, new equations for balanced line configurations, and detailed comparisons between capacitive lines and transmission lines, including propagation within transmission lines. 53 pages. Released March 2004.