Sunday, September 14, 2008

Stray Capacitance

Having stray capacitance is a requirement to create a characteristic impedance for a transmission line. But, a transmission line is not always required – in fact it is often not required within the system but only for external interfacing.

If a transmission line is not required, then this capacitance can be detrimental to the system. It can slow the signals significantly down and also cause zeroes to occur in an amplifier which can lead to oscillations.

To minimize stray capacitance, it is as easy as separating the ground plane away from the signal trace. This can involve increasing th distance on the top layer, and/or removing the ground plane below the signal trace.

Remember, power planes are considered AC grounds and behave exactly
the same as a ground plane. So removing the power planes is as important as removing the ground planes in sensitive areas. This is often referred to as moating.

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