DGND

[nyborn]

 Digital systems are immune to noisy signals in a certain range and will tolerate grounding schemes which are not optimized, but it is better to avoid trouble just from the beginning.

Here you mend digtial circuit component in fixture and CB68LP can slove ground problem and noise problem? Am my understanding correct?

What do you mean by better to avoid trouble just from the beginning? Could you pls describe?

BTW in analogue data acquisition systems you can ruin signal integrity completely by wrong grounding. It is never a fault to take as much care of grounding digital signals as you would do with analogue signals

Are you talking about analog and digital signal in one board as CB68LP? need to take care the ground of the analog signal

If so, how can I reslove the same ground issue?

[nyborn]

Here is  a garph to describe my case:

 First: the operation of my test fixture is like:

 1) press "enable" button using digital I/O in Labview

  The current will flow from my fixture to NI5112 ground through BNC cable. BNC cable is a hard ground. It means most or all currents will select to go to this ground.

   I would like to ask when i press enable button, the current will go from p.0.1 to DGND, then to my fixture ground. I have a hard ground. Do you think the current will then go to the BNC cable in this opeation. I am not sure in this situation. Could you please tell me? Then the AI+ and AI- will be measuing in CB68LP board. Do you think the analog part in CB68LP will not affect the DGND (pin18) on the CB68LP board. They are the same wire that can not seperated. Are there any noise affect each other or the board (CB68LP) itselt automatically stop digital part first then go to the analog part.. Could you tell me?

[Buechsenschuetz]

>How come you don't afraid of the ground loop issue?

You did not mention any other GND connections between i/o board and test fixture, so I assume the one I suggested will be the only one. If there are other GND connections of course you should not establish a second one.

>Are you saying itis much better to setup like the diagram you verify in the previous mail? If so, what other methods could I do? I could not think of.

In the diagram I verified you connected the DGND to a random point of your test fixture. To avoid any noise generated by supply currents in general it is better to use the neg terminal of the power supply.

>You are talling about different ground (earth) here.

Usually designers avoid to connect earth and GND. Anyhow, in all personal computers I have encoutered there is such a connection. If your system happens to have a connection between GND and earth, too (I hope it does not), voltage drops along the mains supply may cause severe problems.


> How come the digital circuit components can not solve this problem? Even the fixture has no earth connected, only digital I/O lines connected to earth.

Of course if your test fixture does not have an earth connection to GND you don't have to fear any hazzle with that issue.

> If the the earth (computer) has a very large ground 4.5V and the fixture has 0.3V. How come the digital components impendance can not solve this problem

This is a very common mistake. The high level voltage (4.5V) will not be severely affected but the low level voltage (0.3V) may be affected by differential voltages between seperate earth connections. A colleague currently is involved into a severe data transmission problem in an old building with poor mains power lines. Some systems work OK with the setup, but a new system failed from time to time - very probably due to problems with earth connection. We are getting somewhat off-topic now but for long data transmission lines it is better to use either fiber-optical connections (with suitable converters) or a non-GND dependent differential transmission protocoll such as RS485.

[Buechsenschuetz]

>Here you mend digtial circuit component in fixture and CB68LP can slove ground problem and noise problem? Am my understanding correct?
>What do you mean by better to avoid trouble just from the beginning? Could you pls describe?

I am not sure about your background with digital data transmission, but I have some experience in this field, and I would strongly recommend to use the DGND terminal of the i/o board as GND connection to your test fixture. Maybe this is NOT the very best method but I think most others will be worse.

>Are you talking about analog and digital signal in one board as CB68LP? need to take care the ground of the analog signal

Properly grounding analogue signals is even more difficult than with digital signals, and more problems arise if you have to make GND connections for both analogue AND digital signals in the same setup. Anyhow, this is an issue which cannot be easily discussed from the distance. Proper grounding analogue signals sometimes even involves analysis of your power supply and wiring setup etc. I am neither a consultant nor a free-lance engineer, in some cases such a person is needed for doing it right.

[nyborn]

Hi Buechsens:

I would like to make it simple. I have the following questions that need your direct answer, otherwise I don't understand and I can not finish the project. Would you please explain to me?

On the Original setting you simply connected the digital input you wanted to test to BOTH dig output AND DGND. This, of course, will short the output (what do you mean by shorting the output? Becasue I know the picture of this setup, I can not get what you meant. Could you please describe to me or draw a simple picture) , and no test signal is generated. (I believe if you expalin to me about shorting the output, I may see why you said the test signal is not generated)

The 'Setting is right' seems to be correct. However, I would suggest to connect DGND directly to the neg terminal of the 9VDC power supply. This will avoid noise generated by excessive long and winding GND connections. (Did you mean the lead cable wire from the groud of the push buttons in the fixture to the DGND will contain noise?  How come the wire put in the negative terminal of the 9V DC using the same wire will have no noise?  Would you pls explain?

The third setting will only work (and probably not very good) if there is parasitic GND connection via the mains supply earth (here you meant computer ground connected to the earth, right?). Similar to a supply circuit (supply circuit means a signal source with a power supply and a resistor in a close loop circuit, right) , a signal circuit must always be closed, i.e. both need a return path. In this setting the return path is open (you meant if the DGND is not connected to the ground of the fixture, it is connected to the computer earth. It is not a return path. Why? Becaure the ground of the fixture and the DGND is connected right now, why computer ground can replace for a reture path in ths case. Would you pleaase explain? I don't understand.and you very probably will never get proper test signals ( I think you have the experience because I am exactly having this appearance.)