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DGND

>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.
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Message 21 of 63
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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
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Message 22 of 63
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>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.
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Message 23 of 63
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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?

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Message 24 of 63
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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.
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Message 25 of 63
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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.

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Message 26 of 63
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Hi Buechsens:

The following is what you just said:
 
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.
 
At last, you said only one. Did you mean only one wire from DGND to computer ground earth (ground signal source), am I correct? I need your direct answer.

 

2) it seems that I have no ground loop in my set up. But why my fixure negative terminal is 0V from the 9V DC source, now it is connected to DGND (negative terminal of computer earth). I was not sure the DGND voltage. SInce DGND is connected to the negative terminal of 9V DV power supply, I belive it is 0V. am I correct? Pls verify me here.

If the answer is yes, then DGND will not conflict with the computer earth? Would you please give a good explaination so that it make sense.

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Message 27 of 63
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Hi Buechsens:
 
I think I should give a little bit information. I think it will be much easier to solve the questions.
 
As you know setup is from 5V digital power supply in the fixture, then go to CH2-1, then go to p0.0, then LabVIEW push button and then DGND, and then you said go to negative terminal of power supply 9V DC.
Then it will start to flow current to UUT after pressing the "enable" push button in LabVIEW. So , the sequence is changed in this situation. First, the negative power supply ground (9V DC) , then search for BNC cable (hard ground). Is that information that I am thinking is corret? That is relate to the two questions previously
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Message 28 of 63
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>At last, you said only one. Did you mean only one wire from DGND to computer ground earth (ground signal source), am I correct? I need your direct answer.

The short answer: Yes.
The long version: Using a second wire (or a secondary connection to DGND) will create a loop, i.e. an inductivity with 1 winding. Any magnetic field around will generate a voltage in this loop. Since one point of this loop is the reference point for the control signal, the control signal voltage will shift. As mentioned before, this is less dramatic in digital setups which usually can tolerate several 100mV's of noise. But when measuring analogue signals in the sub-mV-range it's a different story...

2) it seems that I have no ground loop in my set up. But why my fixure negative terminal is 0V from the 9V DC source, now it is connected to DGND (negative terminal of computer earth). I was not sure the DGND voltage. SInce DGND is connected to the negative terminal of 9V DV power supply, I belive it is 0V. am I correct? Pls verify me here.

Voltages are relative, and you always have to define the reference point for your measurement (or your assumptions). In your system, the reference point for all voltages is the neg terminal of the power supply which - by definition - has 0V with respect to this point. If you have a direct non-isolated connection between DGND of the i/o board and the neg power supply terminal the voltage of DGND will be 0V with respect to neg supply voltage terminal, too.

>If the answer is yes, then DGND will not conflict with the computer earth?

You are lucky, since you have a floating GND on your test fixture, and this is almost ideal. Since there is no voltage source between floating GND and earth there will never be any voltage between DGND and earth, Or, more precisely (since DGND is connected to earth in your pc and on the same potential), there will be no compensation current flowing into (or out of) the DGND terminal.

If you don't want to be loaded with theory, you can skip the following chapter: Things would be MUCH different if there would be a voltage source between your floating GND and earth. This does not happen too often, but it might be the case in battery-operated high voltage UPS's (uninterruptable power supplies). In many cases the battery string of these systems IS earthed, and may have a potential (voltage) of several hundred volts with respect to earth (and lotta power behind it, remember the cranking power of your car battery - and UPS batteries can be MUCH stronger). You can imagine what will happen if you make incorrect GND connections in these cases.
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Message 29 of 63
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Hi Buechsens:
 
You forgot to answer the question of current flowing from CH2-1 or CH2-2 to BNC cable ( hard ground)  to 5112. Actually, am I thinking it correctly?  
 
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.

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Message 30 of 63
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