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Sherlock Ohms

Ground Is Not Ground the World Around

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J.Lombard
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Iron
Ground is NOT Common
J.Lombard   8/29/2013 4:54:31 PM
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The term "ground" should be used more carefully. In our equipment, we distinguish between "ground" and "DC common" for good reason. Ground refers to chassis or "safety ground" to take errant AC voltage to ground instead of through operator's bodies.

The mixing of the two can have unintended effects, as was discovered in a few equipment failure instances we encountered. In addition to the outright failures, signal integrity (noise) issues become significant, especially in the analog sensor world 

Before the advent non-volitile solid-state (flash) memory, many systems used battery-backed SRAM to hold system data. Remember computer CMOS settings and batteries? Our system held calibration data in BB SRAM. The CPU (single-board computer) had its DC common rail tied to the mounting pads on the PCB. We mounted the board (solidly) to the equipment metal baseplate and chassis with metal stand-offs. Guess where safety ground was tied to? After several equipment returns with "Calibration Lost" as the failure mode, we determined that any (AC?) bounce in the ground plane was exceeding the 1.5V SRAM voltage and erasing the data. Nylon stand-offs anyone?

Recently, we discovered that more than one of the major industrial LCD manufactures tie the metal frame (chassis) of the LCD to DC Common. When asked about this, they replied, "To reduce EMI". This may be fine to make the display pass emission standards, but is just plain wrong from an electronic design point of view. Industrial equipment (the intended market) is often housed in metal enclosures tied to AC ground. This is not throw-away consumer equipment in a plastic (non-grounded) case!

This one disturbed the non-volitile flash memory, causing the CPU configuration data to be corrupted. The same ground path was demonstrated with dramatic results when the power switch manufacturer changed their design (another whole story - maybe for Made By Monkeys). The switch failure allowed the AC voltage in the switch to come in contact with the 12V DC conductor for the LED indicator light in the switch. The AC chose the path to ground through the 12V rail to the display, through the electronics to the DC common, directly to AC chassis ground because the two were connected. Several 1000's of dollars worth of electronics was reduced to paperweight status in the blink of an eye.

Lesson for the day: Ground and common are not the same, and never the twain shall meet!  

  

mkost
User Rank
Iron
Re: cuaght by the software
mkost   8/29/2013 1:33:21 PM
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I often use a 0 ohm resister to connect grounds so that there is a single point ground and it doesn't get lost in the software or someone else picks up the design.

tekochip
User Rank
Platinum
Re: cuaght by the software
tekochip   8/29/2013 11:52:06 AM
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I've used jumpers and 0 Ohm resistors too.  You're right, I believe the resistor is the best choice.

William K.
User Rank
Platinum
Re: cuaght by the software
William K.   8/28/2013 10:18:34 PM
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The problems with PCB layout people are the mostly due to the fact that many of them have no electrical understanding at all, and a larger portion certainly have no grasp of circuit theory. The result is that in addition to being given a circuit drawing they need to be given a grounds drawing, and sometimes they need to be checked on every 2 or 3 hours. My solution to the ground traces needing to have a "star point" was to create a three terminal device that was a 3-way jumper wire, when the boards were built. That was good for diagnostics as well, since the grounds could be separated in serching for shorted circuits.

Thinking_J
User Rank
Platinum
Re: cuaght by the software
Thinking_J   8/28/2013 8:44:52 PM
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Related thoughts..

Q:What does an engineer do?

Ans: create documentation for others to build with.

 

Q: What is the purpose of a schematic?

Ans: Several purposes..

To convey the electrical connections... (netlist)

To show what the circuit is intended to do. (over view)

An indication of how the circuit should work (for those going to trouble shoot later)...
perhaps some example signals?

But it is a lousy tool for telling a pcb designer how to layout a board. (separate these ground paths during layout, please, keep this area clear of xxx, etc...) Adding enough notations, rules, etc.. to provide ALL the required info, would only create a hopelessly un-readable , confusing document - in conflict with the schematics other purposes.

So... it shouldn't try.

 

Thinking_J
User Rank
Platinum
Re: cuaght by the software
Thinking_J   8/28/2013 8:22:26 PM
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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).

 

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.

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.

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.

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.

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