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[SOLVED] Electrical Earth & digital ground reference

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siongboon

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Hi,

I am currently facing difficult problem in troubleshooting a digital circuit.
Then I come into this question that I have in mind.

Is it ok to connect the digital ground reference to the
electrical earth that we have in the 3 pin plug for fault protection.
I think it is a design mistake and problem will occur
within a day before the circuit needs to reset.

What is the significant issues/advantages arise if they are connected together.?

Thank you for your guidance.
Siong Boon

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hakeen

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What is your max freq? Theoretically it can work, but depends on several criteria. for example how good your DGND>>CHASSIS>>EARTH wiring? Is it very low impedance and CHASSIS is big enough?

In digital signals you will have many freq components flowing through DGND. The reactance of wiring you have is might affect performance.

Cheers
 

biff44

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The "electrical earth" connection on a wall socket does, eventually, get connected back to a rod in the earth. It also is almost certainly connected to the return on the AC line.

So, I guess the question is, what good might you accomplish if you connected a circuit board ground plane to "electrical earth".

One problem is that most of the circuits in a building also will share that "electrical earth" connection. In a residential house, that would include motors for refrigerators, dishwashers, furnaces, air conditioners, all sorts of other appliances, flourescent lights. And since they are all sharing the "electrical earth" connection, that means that any leakage currents travelling back to earth ground are going to cause ground loops (voltage noise) to be present on the supposed "earth ground".

SO, by connecting the board ground to earth ground, that means that some or all of those building ground loops are going to now be superimposed on your board ground. If it is a stand-alone circuit, then it probably does not care.

However, if it is a circuit that connects to other stuff, then those gound loops can be big enough to cause digital errors. As an example, suppose you have a thermocouple hooked to a copper plumbing pipe in your house, and you run the thermocouple leads back to a controller on a board that is connected to earth ground. The thermocouple signal is in the order of 10's of millivolts, but the copper pipe is probably at zero volts, and the "electrical earth" most likely has 5 volts pk-pk of 60 Hz noise on it. So you are burying your desired signal in a noise that is 500 times bigger! You might get big problems from such a circuit.
 

    siongboon

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siongboon

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Thanks biff44, Thanks hakeen.

Yes biff44. You are answering my question.
The digital circuit is powered up by a AC-DC power supply.
The inputs are digital and is isolated by opto-coupler circuit.
The output is relay interface.
The communication is through Ethernet interface.

The controller on the board is found to hang after a period of time,
but I am unable to reproduced the problem.
I am wondering how, or if possible connecting the circuit reference
to the electrical earth could have any impact on the circuit itself?

Is there any advantage of connecting the circuit to the Earth?
And is this the typical way of connecting.
I learn from the article from the web, that connection is done
through a capacitor, therefore noise can be drain away into the earth.

Should I say that direct connection to the earth have no advantage,
and might in some cases cause problem instead?
So it is better not to connect to the Earth?

Thank you very much.
Siong Boon
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snafflekid

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Your instinct is right, the ground conductor is for fault protection, it is not intended as a reference voltage and may be noisy. Most electronics are going to have the circuit side isolated from the power side.
 

    siongboon

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