Download 2) it seems that I have no ground loop in my set up

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.
) 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
Oooops - I should have said 'DGND and computer earth are THEORETICALLY at the same potential'. Unfortunately in
the real world they are not. This is due to voltage drop in the wires from pc power supply to mainboard. Also, what you
can measure is only DC voltage difference. There may be even heavy voltage spikes between DGND and computer
earth. So in most cases I would NOT recommend to use the computer earth terminal at all. All circuitry of the dig i/o
board is referenced to the DGND terminal of that board, and this terminal should be used as reference terminal for
circuitry like your test fixture which is connected to the dig i/o board.