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?