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Seperating DGND, AGND and Chassis GND in a PCB design

DOV M.
Prodigy 200 points

Hi,

I was wondering what is the prevelant best engineering approach to the GND tree issue.

I know that some favor a "single GND" policy and some favor "split GND".  Things have changed since PCB technology advanced to "GND plane" vs. "GND trace", and noise levels can be kept low with a single low impedance GND plane.

However, I have heard a claim, that on a system level, GND seperation is still required to eliminate interference caused by mixed analog-digital GND currents to other boards in the system. In other words, you can have a situation where your board is working beautifully, but you're "messing up" other boards in the system.

Does any one have any insight on this topic?

 

Thanks,

Dov

 

 

  • 0
    TI__Guru**** 175081 points

    Hi Dov,

     

    I wish there were a simple answer to your question, maybe this topic is something we can address for Tech Day sessions as it comes up fairly often.  From the overall system perspective, the ground scheme needs to be addressed early in the system definition stages.  Many times there are restrictions on what you can do and how you can do it from a regulatory agency standpoint and the products end use - medical equipment for instance has a totally different set of requirements than what you would find in an automotive application.  There are safety standards to consider, EMI/FRI issues to address, high voltage concerns - too many things going on at the system level to try and give you a complete answer on this forum.

    Complex design where multiple engineers are designing different bits and pieces of the system will benefit from a well thought out grounding scheme before they actually start producing hardware so that you don't run into situations where your board works beautifully but knocks out someone else's.  At the board level, when dealing with high performance mixed signal devices, noisy digital signals can kill the analog performance.  We go both ways on our EVM designs - sometimes a solid single ground, sometimes a spit ground - both approaches work so long as you ensure there is clear separation of the analog and digital circuitry.

    Dr. Howard Johnson is probably one of the most authoritative 'experts' in the subject.  There are a variety of resources available on the subject of grounding at the Signal Consulting website - and I believe Dr. Johnson still offers his two day course on grounding in Mixed Signal systems.  Hopefully you can gain a little insight by reading through some these articles on grounding.