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naperlou
User Rank
Blogger
cuaght by the software
naperlou   8/27/2013 9:23:43 AM
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Cliff, this is an interesting situation.  The engineer was trying to keep track of the different grounds, but the software didn't know that.  What he did made sense, but then again, so did what the software did. 

bob from maine
User Rank
Platinum
Re: cuaght by the software
bob from maine   8/27/2013 10:49:22 AM
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From a design standpoint, separating anlog, digital, and different power supply gnd/returns is vital to the integrity of the circuit and isolation from noise. Measuring voltage on a 12V line requires you connect the scope-probe ground to the 12V return, likewise the 5V and 24V busses. If the designer isolated those lines, there is good reason to believe they should stay isolated. I can't argue with your success, but I'd guess that the problems you were trying to fix still exist and will evidence themselves in degraded performance and/or damaged components.

Nancy Golden
User Rank
Platinum
Potential for Unwanted Ground Loops
Nancy Golden   8/27/2013 1:22:51 PM
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Grounding can be a tricky beast - Avoiding unintentional ground loops is often conceived and avoided at the design level. Looks like that software needs an additional library!

tekochip
User Rank
Platinum
Separate Grounds
tekochip   8/27/2013 6:07:12 PM
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 This can be a bit of a pain with some software and and the typical solution is to create a component that is nothing more than copper attaching the grounds.  I had an engineer that created such a part, and placed the copper joining all the grounds on a new layer.  This was a great idea, but after he left the company the next engineer didn't know that the Gerber files needed to have that extra layer included.  That was a darn expensive mistake.


Cliff Harris
User Rank
Iron
Re: Separate Grounds
Cliff Harris   8/28/2013 3:17:02 AM
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The PC boards were laid out by a consultant. He had almost no communication with the engineer. He autorouted everything and did minimal checking. Understandable, as "time is money".

Cliff Harris
User Rank
Iron
Re: cuaght by the software
Cliff Harris   8/28/2013 3:22:05 AM
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The system shared a common power supply that sourced all the various voltages requied. The grounds definitely needed to be tied together.

Battar
User Rank
Platinum
Could be worse
Battar   8/28/2013 8:43:11 AM
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Many many years ago (when I was young and wore a uniform - never mind what color, thats classified) I had to troubleshoot a fault which was caused when a technician mistakenly swapped the ground and neutral lines on an isolating transformer.

We were testing voltages with a multimeter, and all the voltages checked out (no surprise, the multimeter is floating). It was only when we put a scope on the voltage line that we saw the problem. That taught me a lesson about measuring voltages.

I don't blame the tech who swapped the wires, though. They were old and covered in dust and grime, it wasn't so easy to see the different colors. Only people who never do any work never make mistakes.

bobjengr
User Rank
Platinum
GROUND AROUND THE WORLD
bobjengr   8/28/2013 4:52:29 PM
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Very interesting post.  Some years ago I took on the responsibility of comparing a UL standard for domestic cooking products with a comparable IEC standard describing the same category of appliances.  I was somewhat surprised to find many symbols vary considerably and there is no "universal" standard defining nomenclature.  Even the "caution", "warning", "hazard" symbols are different depending upon application.  Again--good post.

Leigh
User Rank
Silver
Net Ties
Leigh   8/28/2013 7:43:02 PM
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For those of you that use Altium Designer, NetTie is the appropriate object to use.

You create a schematic part of two pins shorted together (with a PCB part with two pads shorted together) then designate the part as a net tie. This allows two nets to be tied with this one part. The PCB then allows the two nets to exist seperately but be tied with the net tie part and then pass DRC checks.

Thinking_J
User Rank
Platinum
Re: cuaght by the software
Thinking_J   8/28/2013 8:22:26 PM
NO RATINGS
garbage in .. garbage out.

Keeping the different grounds isolated from each other is often required for optimum performance. But this generally requires a single point connection for all the return current.
It should be assumed a different ground is indeed "a different ground".

What is the most common resistor value in the world? Zero ohms!  (at least a few years ago, according to a resistor manufacturer)

Used for jumping over traces (reduction of layers) and connecting optional and not so optional (like grounds) wiring nets.

Drives me crazy when a CAD program will allow merging of different nets into a common net without some distinction (like a zero ohm resistor separating them). Otherwise.. why use different names? It just creates confusion... even if it makes design re-use easier (combining pages from different designs when creating a new design).

 

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