Unintended Effect
Unintended electrical effects arise from the physical proximity of conductive paths on a circuit board. Parasitic trace capacitance occurs when two copper traces are separated by an insulating dielectric. The coupling is only relevant at high frequencies where the impedance of the capacitance becomes low enough to pass a signal.
Signal Interference
Coupled energy between lines creates crosstalk in high speed digital circuits. The parasitic trace capacitance acts as a low pass filter and slows down the rise times of pulses. In precision analog designs, it can lead to instability or oscillation in amplifier stages.
Ground planes are used to provide a predictable return path and minimize the interaction between signal lines.
Layout Optimization
Software tools are used to extract these values from the board layout and simulate the impact on signal integrity. Designers reduce the effect by increasing the spacing between traces or using a dielectric with a lower permittivity. Every parasitic trace capacitance is a function of the trace area and the distance between the conductors.
Critical paths are often routed as differential pairs to improve noise immunity.
Frequency Response
At high frequencies, even a few picofarads can greatly alter the impedance of a circuit. The parasitic trace capacitance is measured during the prototype phase to verify that the board meets its performance specifications.
