Tuesday, September 16, 2008

Trace-to-Trace Capacitors and Inductors

PCB traces are not infinitely thin. They have some finite thickness, as defined by the ounce parameter of the copper clad foil. The higher the number of ounces, the thicker the copper. If two traces run side-by-side, then there will be capacitive and inductive coupling between them as shown below. The formulas for these parasitic effects can be found in transmission line and/or microstrip references, but are too complex for inclusion here.

Signal lines should not be routed parallel to each other, unless transmission line or microstrip
effects are desired. Otherwise, a gap of at least three times the signal trace width should be maintained.

Capacitance between traces in an analog design can become a problem if fixed resistors in the design are large (several MΩ). Capacitance between the inverting and noninverting inputs of an op amp could easily cause oscillation.

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