I am sorry. I would like to put everything in one post
5) Here was what you wrote:
>In that case, you meant the current flows through the internal connection between GND of the IC and a randonly selected GND terminal point are in questions. That will generated noise? You said noise is caused by a current changed. According to what you said, when the current flow throung the test fixure, the current is in question and the current is changed, is that correct?
As mentioned, it is hard to tell from the distance where to locate the optimum grounding point. Any IC will have variations in supply current when any of its states (input or output) changes, so the supply current will never be constant. Also, do NOT consider copper tracks, wires etc as a pure ohmic resistance. When there are fast changes in supply current the parasitic inductance comes into play - and this may cause much higher voltage spikes than expected.
What is copper tracks, wires etc as a pure ohmic resistance? Did you mean the copper wire has resistance but it is not pure. It will generate noise.
If so, I am always afraid of a noise that contain in the red lead and the black lead cable. Could you please tell me exactly what you mean here?
>I am sorry. I meant why it MUST take DGND/earth connected to GND of the fixture to complete the path, How can not use take DGND/earth I don't understand. They both seems like a ground.
Yes, this is a very common mistake. You never know where supply current(s) flow in a pc system (it is difficult enough to tell in a self designed system). So you never know whether there will be any voltage shifts between earth and DGND of the PC. And you never know what voltage shifts occur between earth terminals of two different mains outlets.
Just an experiment: Take a DMM, set it to the lowest voltage range and connect it to the DGND terminal of the dig i/o board. Now take the other test tip and wander around the pc metal case (with the pc switched on, of course). You probably will get slightly different readings at any point you touch. Do you still think that DGND and earth of the PC are identical???
Believe me, there is more to grounding and earthing than meets the eye...
These differences in voltage will add (or subtract, in case they are negative) to the low level voltage of the digital IC. As soon as you have more than 0.8V at the low state you are in an illegal region between low and high, and you never know what your system does in this case. (You can avoid quite a bit of trouble by using ICs with Schmitt trigger inputs - many of the boards I designed are cluttered with Schmitt trigger driver IC or at least driver ICs with input hysteresis. Of course you still have to respect proper grounding - once you have noise on a ground line there is no way to filter it since all filters need a ground terminal....)
What did you mean adding a voltage shift to IC low level voltage? Did you mean if you have voltage shift from DGND and pc earh, you will have to add up the voltage shift to IC low level voltage? If so, I don't understand how this voltage shift to the low level voltage of IC in pcb board? I thiough you take DGND as a refernce point. Could you please explain herre?
6) Why DGND will not conflict with the computer earth? Would you pleas explain?
See attached file for what you wrote before. I still have a question that is not solved. Is it if you take DGND as a reference point, then you can solve this question as a solution. Am my understanding correct?
7) Here is what you wrote:
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.
That mean my system is not a high speed system, The digital signal will have the same state during analogue axquisition. I don't need to worry the ground of DGND and AIGND on the CB68LP board for noise cross each other, am I correct?
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.
That mean if I put a 10kohm resistor from AI- to DGND on the CB68LP board, it is not solveing the cross effect of noise in both analog and digital ground, correct? Because the bias resistor is to remove common mode voltage and common mode noise.
I know there is many questions. I hope you can give me a good answer. Otherwise, I can not finish the project. I am waiting for you.