How to lay out ground planes to eliminate EMI and injection of noise into analog circuitry
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Ground Plane and Related Layout Techniques to Eliminate Noise, Interference and EMI Larry Miller PhD www.linkedin.com/in/ lrmiller miller@elect-design. com Copyright 2014 Larry Miller
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Shows you how to lay out ground planes to prevent EMI and coupling if power supply switcher converter noise into sensitive analog circuitry.
Transcript of How to lay out ground planes to eliminate EMI and injection of noise into analog circuitry
Ground Plane and Related Layout Techniques to Eliminate Noise, Interference and EMI
Larry Miller PhDwww.linkedin.com/in/[email protected]
Copyright 2014 Larry Miller
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Basic configuration: two analog stages and switching power supply
Triangles indicate analog sections
Dark gray is ground plane
Even with recommended load capacitor, a typical switching converter’s output has 10 to 30 millivolts peak to peak noise in the band of very roughly 200 KHz to 1 GHz.
Copyright 2014 Larry Miller
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Second configuration: Three distributed LC filtering stages added
3 distributed LC stages added.Noise reduction, excluding ground-plane AC currents and magnetic field coupling, is typically 33 to 100 to 1 in voltage per stage if L and C of sufficient size used.Thus the power supply noise at the stages is expected to be less than one microvolt peak to peak
Copyright 2014 Larry Miller
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Second configuration: showing AC current paths in the ground plane
The high frequency current flowing within the analog sections create voltage drops across the ground plane within the analog sections, and thereby inject converter switching noise into the analog circuitry.
Red curved arrows show high frequency current flows in the ground plane generated by the switching converter.The ground-plane flow between converter to first filter cap is most intense near cap. and converter but exists elsewhere as shown by arrows.
Copyright 2014 Larry Miller
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Third configuration: moats around analog sections and switching converter
Wide light lines are gaps (moats) in dark grey ground plane isolating the two analog section and the converter section.
They interrupt high frequency currents in the ground plane.
There is one ground bridge interrupting the moat for each isolated section.
All signals travel over this bridge, as shown.Signals must not cross a moat since they could inappropriately carry the same high frequency current that the moat interrupted.
There must be only one bridge per section since two bridges would allow high frequency ground currents to flow through isolated area from one bridge to the other.
Copyright 2014 Larry Miller
A grounded metal can should be added around each isolated section. This prevents electromagnetic coupling and near-field magnetic and electrostatic coupling between isolated sections.
The example may be generalized to more isolated sections with excellent results.
The following slide is a working implementation of these concepts.
Grounded Metal Can
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Actual BoardThe upper centered rectangle is a grounded land that the can attaches to. There is a moat associated with the land and a bridge at bottom center of the land, where all signals and power traces cross.
There are numerous other isolated sections as well, with a variety of A to D converters and switching power converters.
In operation the board shows no interference on its analog and digitized signals, despite the presence of several switching converters.
