DGND

[Buechsenschuetz]

>But they go beep in diode mode. What does it mean? COuld you pls explain? I don't understand

Most instruments apply a certain current to the DUT (device under test) in diode test mode, and will beep if the voltage across the DUT is lower than a certain value. I.e., this is everything but a precise measurement. You could check with an ohmmeter but you will need a very precise one... and it does NOT tell you about parasitic inductance in the ground path.

>(i don't want to out of my topic, my topic is if I put wire from DGND to computer ground, I mean not connect DGND to test fixture ground, why can't it set this up to complete the circuit, could you please explain?)

The simple reason is that any signal needs a return path. So you have to connect the DGND terminal of the i/o board to a suitable ground terminal of the test fixture. You cannot switch on a bulb with a single wire only, and you cannot apply a test signal with one wire only.

Similar, you need a second wire to transfer the i/o signals to your test fixture. Since output of the dig i/o boards is with respect to DGND, and input of the test fixture is with respect to test fixture GND (or neg supply) these two points MUST be connected, and the connection should be as directly as possible.

You should not make any extra connection between DGND of the i/o board and the computer GND/earth. Board designers in general have good reasons to put their ground path the way they did. Anything different from the ground path designed on the board may (but does not necessary) disturb the grounding scheme and generate noise.

>Please give your answer directly. It is much easier

I hope this was sufficiently direct.

[nyborn]

Hi:

You did not answer the following question to me directly, I need the direct answer.

So, in conclusion, it is better to put a DGND (earth) to the negative terminal of the power supply, since wire has a noise, IC have variations that cause noise, in queston to serch ground. Once you put it in the negative terminal of the power supply, it is optimum to reduce troubles, am I correct?

Am I rightt? It seems I am right, correct?

2) I am thinking when CH2-1 go to p0.0, then go to push button in computer Labview, I press the push button in LabView to close the negative wire to DGND, you said the designer of the board will make DGND connected to the computer earth, right?

The test fixture ground is also the ground. The computer is also the earth ground. Is that becasue they are in different device. Therefore. if the +5V from the test fixture to the CH2-1, the from CH2-1 to p0.0. After that p0.0 to the push button in LabVIEW, then the pushbutton in LABVIEW to the computer ground (earth). It wiil not be the close circuit because the computer ground in this path is different ground of the device. Am my perpertive right, now? I need to connect DGND to the negative terminal of the fixture power supply, right? I need you to give me direct answer which is either yes or no. That  would be much easier

[Buechsenschuetz]

>So, in conclusion, it is better to put a DGND (earth) to the negative terminal of the power supply, since wire has a noise, IC have variations that cause noise, in queston to serch ground. Once you put it in the negative terminal of the power supply, it is optimum to reduce troubles, am I correct?

At least in most cases it is a good starting point. If your setup works OK you should use this method.




> It wiil not be the close circuit because the computer ground in this path is different ground of the device. Am my perpertive right, now?

Exactly. At least as long as the neg power supply of your test fixture is NOT connected to earth. But hardly anyone will do so, so you probably will have no trouble with that.

> I need to connect DGND to the negative terminal of the fixture power supply, right?

Yes.

> I need you to give me direct answer which is either yes or no. That  would be much easier

Sorry for some long and winding reasoning. Maybe it's my fault that I do not only enjoy people doing it the right way but I also enjoy it when they know WHY they are doing this.

[nyborn]

Hi:

Am I confuse here? Because I have a fixture which is connected to 9V DC power supply WITHOUT earth. The CB68LP and 6250 is connected to the computer. The computer has an earth ground. Once I connected DGND from CB68LP to the fixture, the gounds are all connected together. In that case, can you consider the ground has a ground loop? I read the tutorial. The link is http://zone.ni.com/devzone/conceptd.nsf/webmain/177A8B29FEDC0F5886256FA90083C0F8

It said the ground loop is the potential difference in two individual ground signal source. In my case, one of my test fixture is a floating source. So, I think it is not a ground loop. Is that right?

I need to verify with you. If not, could you please explain?

[Buechsenschuetz]

Your assumption is absolutely correct. What you have on your test fixture is a floating GND. So you can hook up this test fixture to anything which has floating or earthed ground WITHOUT creating a ground loop. At least as you use only ONE (I said ONE) wire for the ground connection.

If you have to connect an analogue signal, too, things will become more complex and probably too complex to discuss them from the distance.

[nyborn]

Hi Buechsens:

I would like to give you another question so that you don't have answer on at a time. I think it will not good.

The question is the following

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?

Again I need your direct answer in both questions.