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DGND

>Why are you so afraid of isolation of digital and analog part using opto isolator when measuring high accuracy? Could you please tell me?

I am NOT afraid of doing this, remember that this was my recommendation. Only thing to consider are cost and assembly of a digital isolation. With opto-isolators like HCPL2630 it is no big problem.

>If I put DGND to 7805 digital power supply ground, this is not relevant to my problem, is that right? That is what I think

The GND pin of 7805 should be at GND potential, however, it depends on pcb layout or wiring of the test fixture which is the best point to connect GND. But your method is a good starting point.

As mentioned, in your case, some issues will probably not be very important. If you have high frequency digital signals and/or analogue outputs proper grounding is much more important than in rather slow systems.

Sorry, I do not have a diagram concerning the 1k resistor in the connection to NI5112. I do not have experience with that board and don't have a manual (and don't have the time to check it). Usually NI documentation is very comprehensive, and you should find hints and explanations for setting up the analogue output. You can also post a question to the forum 'signal conditioning'.
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Hi Buechsens:

I am organised my question in order. I am sorry. I did not get my meaning across to you last time. So, I write it clear this time. I hope you can get it and answer my question directly if you can.

1) Here was what you wrote in you previous mail:

     As mentioned, in your case, some issues will probably not be very important. If you have high frequency digital signals and/or analogue outputs proper   grounding is much more important than in rather slow systems. :

     That means my case is a very general case, not like having high frequency digital signals and/or analogue outputs, proper grounding is much more important than in rather slow systems.

      My setup as far as you see is correct from my understanding? Is that right?

2)   Attached is my graph, I put R1 between the negative terminal of the power supply in the fixture and AIGND of the I/O board. You told me the definition of ground loop is the circle of the ground. Do you think this picture is also a ground loop?
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Message 52 of 63
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3)  Here is what you wrote in the previous mail

     The GND pin of 7805 should be at GND potential, however, it depends on pcb layout or wiring of the test fixture which is the best point to connect GND. But your method is a good starting point.

       PCB layout or wiring of the test fixture tell the GND pin of 7805 the same wire as the negative power supply of the test fixture.  You said it depends on pcb layout or wiring of the test fixture which is the best point to connect GND.  To my understanding, you mean the negative terminal of power supply in the test fixture is the best point. But I don't understand why you said my method is a good starting point. I am very confuse.

4) Here is what you wrote:

  I am NOT afraid of doing this, remember that this was my recommendation. Only thing to consider are cost and assembly of a digital isolation. With opto-isolators like HCPL2630 it is no big problem.
   

   I am sorry. What I mean is why you will use optic isolator to seperate digital and analog signal for measuring high accuracy, but when the measurment is low accuracy, I don't need to use this method. Why high accuracy need optic islatior, but low accuracy does not need? What is the reason inside it?Is it because digital signal will affect analog signal and analog signal will affect digital signal because of the fast transcient current spike in digital ground  and the slow transcient current spike in analog ground. Could you please tell me? I know it is not very important issue. but my next question is the CB68LP board.Becasue DGND is connected to CB68LP board and AI+ and AI- is connected to CB68LP board. If the digital transcient current spike is affecting analog ground and the analog ground is affecting digital ground. I am thinking if I put 10K bias resistor from AI- to DGND. Do you think it will solve this problem? It is probably not becasue of the same wire of DGND and AIGND in CB68LP board, right? Am I correct?

Could I please open another quires becasue I just start working this?

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Message 53 of 63
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Message 54 of 63
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Hi Buechsens:

I found anther definiton of ground loop. I would like you to verify if it is.

Here is what you wrote:

 you measure a voltage between floating GND (unconnected) (that mean no wire from DGND pin of the CB68LP board to the negative terminal of the power supply of the test fixture, right?) and pc earth there MUST be a ground loop somewhere.(It seems I am right, if so, I did it and it was exactly what you said. Thanks for your explaination. (Note that DMMs with high input impedance will display random values, usually in the range of hundreds of mV, when connected to an open circuit

1) If my understanding is correct, that is the ground loop is also occured between two different ground which is not close circuit. One ground is not the ground signal source which is my floating GND and another ground is the DGND which is not connected to floating ground. Then you take DMM, you get random value in 200MV range. Am my understanding correct for anthter ground loop?

2) Becasue the negative of my power supply in the test fixture is not directly connected other ground in my test fixture, if you look at the block diagram, it is connected to the power switch (on/off). So where should I put my DGND wire connected as you see in this case? I need your opinion. Maybe my perperctive is wrong. It should be in the negative terminla of the power supply in the test fixture

3) How come I use DMM, I apply current and voltage 9V DC to my test fixture. The common ground was 0V before the power tun on. After the power turn, how come there is a voltage as 0.9mV between different component ground on the PCB board. Coould you please answer to me this question.

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Message 55 of 63
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1) You always said take DGND as a reference point. Is that mean you take this point as 0V. Then, everthing else measure to the ground is refer to 0V. Is that what you meant?

That way you don't have to care 0.7mV, 0.8mVor0.9mV happened somewhere in the chasis. Everything is measured and refered to the DGND, Am I correct.

If so, what see the picture is test fixtuer with ground (no earth) connected to 6250 and 5112 board with DGND, AIGND (same wire with one point of the wire in the fixture. That is like a return path, a completer circuit, you don't need to worry the differet ground (earth) of pc and the fixture ground because you take DGND as a reference point. Am my understanding correct.? If you can, I hope I can get direct answer.

 

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Message 56 of 63
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I'll try to answer your bunch of questions.

1. It is correct that galvanic isolation (using opto isolators) is more important in high speed systems where there might be heavy transients on the GND lines. If you have more or less static digital signals (i.e. signals which have the same state during your analogue acquisition) this problem is much less important.

2. It is not possible to say which is the optimum point to connect DGND to your test fixture. Sometimes it is even a matter of trial and error. As I mentioned I would start with connecting the GND pin of the voltage regulator to DGND. You should check your analogue signal when trying different DGND connections and select the one which produces the smallest amount of noise and/or base line shift.

3. I don't know exactly what's the reason for the voltage shift of 0.7 to 0.9V between different points of the DGND line. There are many things which might be the reason: leakage current of capacitors, rising bias current of integrated circuits etc.

4. The bias resistor(s) on the analog inputs have little to do with ground loops. Anyhow, you should always connect them as close to AGND and A+/A- respectively as possible, as any noise and voltage shifts on the AGND line will affect analogue signal offset.
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Message 57 of 63
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Hi Buechsenschuetz:

 

I am sorry. I just tried to test the voltage between power swich (on/off) and my power supply negative terminal. I understand how it work not.

I make a mistake. When the power swithc is on, then everything in my block diagram ground are connected to together, I already test it by DMM diode

mode. So, if I connected it to negative power supply terminal, it work with optimum result and minimum troubles.

But then I suggest to put the DGND to digital power supply ground (7805) in the fixture. I did not understand what you said about it? Is this idea no better than the negative terminal of the power supply in my setup? Would you please verify with me this time

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Message 58 of 63
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Hi Buechsens:

 

I haven't heard from you. I would like to know if my understanding of some of the questions is correct. Would you pleae send it to all of my questions when you have time and you think I already know what I am doing?


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Message 59 of 63
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Hi Buechsens:

 

I have not heard anything from you yet. What is happening? Would you please email me back?


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