Points to be Noted: Crosstalk and Noise from GND Lines

Until now, we have explained inductors, ferrite beads, and common mode filters as noise countermeasures using inductors. In this article, we describe points to be noted relating mainly to PCB layout.

Crosstalk

In “What is Crosstalk?” it was explained that crosstalk results when stray capacitances and mutual inductances between wiring on a board cause noise to be coupled with other, adjacent board wiring. Below are shown examples of measures for dealing with crosstalk through LC filter pattern layouts and component arrangement.

In the layout on the left an LC filter is inserted in the VCC line, but because the wiring after the filter is close to unfiltered wiring that includes noise, in this example noise coupling due to crosstalk results in a reduced filtering effect. The example on the right addresses this problem: by using a layout that is not close to a line containing noise, such noise coupling can be kept as small as possible.

Noise from GND Lines

This is an example of the occurrence of noise due to the way the capacitors before and after the inductor used in a Π filter are grounded. In the example on the left, noise from GND is propagated through a capacitor, as indicated by the arrow, and may escape to outside the filter.

In such a case, a method in which the parasitic inductance of vias is utilized, providing connections to the GND plane through the vias so as not to allow noise to be transmitted directly, may be effective.

As often noted, in switching power supply circuits the board layout is highly important. Here know-how relating to board layouts is essential. In the Tech Web “DC-DC Converter PCB Layout” article, the basics of board layout are explained; this article should be consulted as well.