PCB Layout question

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digitalDan
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Posted by digitalDan: Tue. Aug 22, 2017 - 01:17 PM
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I'm checking a layout done by a third party and have a question about how the analog ground plane is laid out.  A representation of the layout is shown below.

This is an analog signal ground plane so there are no appreciable currents and the max bandwidth of any circuit serviced by this ground plane is ~15kHz.

My question is whether it is advisable to have the green traces as shown.  I think they should be removed as they create loops.  The overall size of the plane is approximately 2x2 inches (50x50 mm).

All advice and opinions welcome.

Thanks

Dan

This topic has a solution.

Letting the smoke out since 1978

 

 

 

 

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Learning and Information, General Electronics, solved
Last Edited: Wed. Aug 23, 2017 - 10:49 PM
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valusoft
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Posted by valusoft: Tue. Aug 22, 2017 - 01:29 PM
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I don't understand the purpose of those two green traces. They appear to simply tie both sides of the continuous ground plane together.... unless my eyes have failed yet again.

 

If I wanted to decouple/shield those two analogue traces I would have extended the ground plane up the middle of the two analogue traces. But you can call me stupid...

Ross McKenzie, Melbourne Australia

Last Edited: Tue. Aug 22, 2017 - 01:30 PM
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avrcandies
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Posted by avrcandies: Tue. Aug 22, 2017 - 02:17 PM
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Why not change the crossovers so that the blue lines do that instead (top  for 1 inch then bottom for 1 inch then top for 1 inch...) ...then the gnd plane would look mostly solid wherever the blue traces moved to the top.  Was someone trying to divvy upthe plane?  Sometimes that is done to  separate high noise power section from the precision analog (but you have no mention of this situation).

 

see here

http://www.ti.com/lit/an/szza009/szza009.pdf

 

http://cds.linear.com/docs/en/application-note/an136f.pdf

 

might be nice to find the other sections of this seminar---very nice

http://www.ti.com/lit/ml/slyp173/slyp173.pdf

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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frog_jr
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Posted by frog_jr: Tue. Aug 22, 2017 - 03:29 PM
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What are the two traces in the analog plane? Are they analog signals or digital?

What do the vias that are tied by the green across the plane go to? Are they related to some single point GND?

Can the traces in the plane be routed around the vias, making that section solid?

(Lots of questions, lack of details...)

David

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ka7ehk
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Posted by ka7ehk: Tue. Aug 22, 2017 - 03:48 PM
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This style is designed to reduce "slot antenna" effects. Contrary to what you might think, there WILL be currents in that ground plane and, as they flow around the edges of that slot, can radiate at certain preferred frequencies. That is the likely rationale.

 

That said, if this is an analog device with low bandwidth active elements, then it will not do much. If this were analog video, I'd say "good show". On the other hand, what is the cost?

 

Jim

 

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digitalDan
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Posted by digitalDan: Tue. Aug 22, 2017 - 04:01 PM
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So the concern with a layout like this is that this board could radiate EM energy rather than be susceptible to EM radiated by other sources?

Letting the smoke out since 1978

 

 

 

 

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ka7ehk
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Posted by ka7ehk: Tue. Aug 22, 2017 - 04:34 PM
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I'm not sure of the mechanisms, involved, but I do know that antennas are generally "reciprocal" - that is, if they radiate out, they also respond to incoming radiation. In this case, the "response" would be to generate currents in the ground plane. 

 

One of the vexing things that can happen in low frequency devices is rectification of very high frequency signals in various semiconductor junctions. Think BJT audio amplifiers. This can cause unexplained intermittent circuit misbehavior of low frequency circuits. 

 

All that said, if your example layout is representative, the wavelengths involved are quite short and the preferred emissions frequencies would be in the upper hundreds of MHz. Nowadays, when you don't know where or how a board will be used (maybe someone will try close to a TV transmitter or lay a cellphone down next to it), the layout strategy is often "lets take care of every possibility, especially if the incremental manufacturing cost is very low".

 

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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digitalDan
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Posted by digitalDan: Wed. Aug 23, 2017 - 10:48 PM
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Thanks everyone for your input.

@avrcandies thanks for that TI document.  It has a lot of good info and backs up what Jim said in #7.

@ka7ehk thanks, that sets my mind at ease.

 

Letting the smoke out since 1978

 

 

 

 

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