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To split or not to split, that is the question.

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corrado33

New Member
First post! I'll try to make it a good one.

I've gotten into the habit of designing PCBs for my electronic projects instead of trying to shove them onto a project board. However, that's brought up a lot of other things I have to learn.

One thing I've seen discussed many times, with much support on both sides is the separation of ground planes by analog/digital or by high speed/low speed/analog circuits.

Notice, I said separation and not splitting. The hot topic I've seen is that some people think physically splitting the ground planes with "bridges" that the traces run over is the best way to do things. While other people think that a single ground plane, with the circuits divided by an imaginary line, is a better idea. I've seen tons of support for both methods, with papers and research to back them up.

The point is to keep high speed digital current (and current return) away from the analog circuitry.

I've read the following PDF which seems to support the single ground plane idea. What I want to know is... what do you do? I'm sure anyone that answers this thread has a ton more knowledge than me, so i just wanted to hear some ideas.

EDIT: Yes, I did get the PDF from a thread on this exact subject on this site.
 

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ericgibbs

Well-Known Member
Most Helpful Member
hi,
Do you have a circuit diagram you could post and a simple description of its function.

Reason for asking, is that often the 'best' layout method depends upon the actual circuit.
 

crutschow

Well-Known Member
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The idea, of course, is to keep the digital ground currents from contaminating the analog ground. As Eric states the best method to do this somewhat depends on the circuit.

If there are no high speed digital signals going to the analog side of the circuit, then a physically split plane with a single-point connection may be best.

If there are some high-speed digital signals going to the analog side, such as a D/A or A/D converter, or you are using a microprocessor with an A/D input then a well laid-out circuit with a single plane may be better. But some A/D converters have separate pins for digital and analog ground which would allow the separation of the ground planes if desired. So you have to look at the circuit and it's application.
 

ronv

Well-Known Member
Most Helpful Member
I can remember one tough bug caused by a split plane. It was many years ago in a big mainframe tape drive. Sometimes when one of the big vacuum pump motors would turn off on an adjacent drive it would cause a ground shift between the 2 planes and write a glitch on the tape. Probably not so bad today but this was before storage scopes and logic analyzers.
 
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