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DGND

The problem is that I already pointed out (hopefully) everything I can contribute from the distance. As I have mentioned, it is simply not possible to confirm some of your questions from the distance. Each and every application is different, and there are just some general rules I can give you from my experience.

Just one example: we have one data acquistion application (with digital control signals, too) which works quite well without much care for ground loops etc. (I know there is at least one in this system but it obviously does not affect signal quality too much so there is no urgent need to change the setup.) In a similar application (among other problems) there was a ground loop which ruined analogue signal quality. In this case, the analog signal was much weaker (less than 1 p.c. of the signal of the first application) and was affected by the ground loop so detailed evaluation of ground loops (and some other problems) was necessary and solved the problem.

In other words, if you just follow some general outlines and avoid the most common mistakes you will probably get acceptable signal quality. Further improvements will need detailed evaluation which is not possible in such a forum.
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Hi Buechsens:

I think I got all your meaning to solve all the questions I have to use DGDN without any problems.

I have two questions left.

1) The parasitic voltage when connecting DGND to the test fixture negative power supply. You said it is hard to tell because of the digital IC in the test fixture something like that. So, there is a dimm light even turing off the power of the test fixture.

Later you found I am using a pull up resistor for the input of the digital IC, not oscillator, so could you please tell what other things cause this parasitic voltages and why stay there, I can not think of it and I don't understand.

 

2) I tried testing the control of the push buttons in LabView to see of the opeation of light in the test fixture opearting correcting. I found there are eight times perfect and two times the light will go back to enable after I tried the test push buttons at last. It is suppose to alway on. Why cause this unproper opeation? I don't understand why it will affect the enable light because I just hit the last test push button, according to schematic, it will not affect this enable push button part. Why causing this problem. You have the experience. Could you please tell me?

Is that because I use a thick wire from CB68LP board to the test fixture and then I use a think wire from CH1-5 to test fixture components? Do you have a clue?

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> Later you found I am using a pull up resistor for the input of the digital IC, not oscillator, so could you please tell what other things cause this parasitic voltages and why stay there, I can not think of it and I don't understand.

There are many reasons for possible parasitic voltage. First you should try to check the nature of the parasitic voltage, i.e. measure it with an oscilloscope ... BUT this oscilloscope MUST have isolated inputs, i.e. do NOT use an oscilloscope which runs directly on mains voltage. A battery-operated oscilloscope is ideal for such applications. If there are any waveforms visible you can go further. I.e. if you see a 50Hz waveform it is pretty likely that the parasitic voltage is caused by some improper earth connection.

Other reasons are: resistance of connectors and terminals etc etc. It is not possible to tell from the distance.

> I tried testing the control of the push buttons in LabView to see of the opeation of light in the test fixture opearting correcting. I found there are eight times perfect and two times the light will go back to enable after I tried the test push buttons at last. It is suppose to alway on. Why cause this unproper opeation? I don't understand why it will affect the enable light because I just hit the last test push button, according to schematic, it will not affect this enable push button part. Why causing this problem. You have the experience. Could you please tell me?

Especially with CMOS components you should be careful about using pull-up resistors. If the pull-up resistor(s) are connected to a different supply than the IC itself (both must have the same GND of course) in case the IC supply is switched off there might be current flowing into the input and feeding other circuitries via the VCC pin of the IC in question. In other words, a voltage (even a signal voltage of sufficient power) applied to the input(s) of a non-supplied IC might be sufficient to 'feed' the whole circuit, but probably not with proper supply voltage. In these cases malfunctions may occur. You should also check for possible oscillations with an oscilloscope. The input resistance should be as high as possible, use a x10 probe which will shift the input impedance to 10MOhms.

> Is that because I use a thick wire from CB68LP board to the test fixture and then I use a think wire from CH1-5 to test fixture components? Do you have a clue?

Using thick wires for GND connections is never a bad approach. I cannot tell you exactly what is going on in your systems. You may try other GND configurations. For example, you can connect all GND lines in one point.

I am afraid this is all I can say from here.
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