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Grounding of PXI 5412 and PXI 5124 used in conjunction.



I'm a civil engineering student looking to measure the resistivity of various samples of clay. I'm using a NI PXI 5412 signal generator in conjunction with an NI PXI 5124 oscilloscope. I've split the co-axial cable coming from the signal generator to make the proposed circuit detailed in figure 1 below. In order to calculate the resistivity of the clay, I plan to use the oscilloscope to measure the potential across the clay, similarly by splitting the oscilloscope co-axial cable as also detailed in figure 1.


The trouble I have is, even when my circuit isn't complete as detailed in figure 2, if I generate a signal with the function generator and connect the central co-axial wire from the oscilloscope to the central coaxial wire of the function generator, I read on the oscilloscope exactly the signal I am generating. Even with the circuit complete, no matter where I connect the outer wire from the oscilloscope to on the circuit the reading on the oscilloscope does not change.


Is this proposed circuit feasible? I am a novice in electronics, is it something to do with the grounding of the function generator and oscilloscope and if so is there a way to change this? Any information would be helpful.


Regards Chris






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Message 1 of 3

Hi Karl,


Welcome to the forums!


With the devices that you have there shouldn't really be any grouding issues with your current setup. Do you have a vague idea of what the resistance of the clay should be? Could it be that you have the circuit operating currently in a range that the clay is not going to affect it? Have you tried replacing the clay with another resistor which you know the resistance of so that you can calculate what the values should be? I would recommend doing this so that you can see if the circuit itself works and then you can replace it with the clay to test the resistance of it.


Kind Regards

David B
Applications Engineer
National Instruments UK
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Message 2 of 3

Hi David, thanks for the reply.


Yes, I've replaced my clay with a variable resistor for now. I don't understand why it doesn't seem to matter whether my circuit is complete or not though? Neglecting the clay and resistor for now, if I generate 2 volts with my signal generator and connect the central wire from my split coaxial cable to the central wire of my split oscilloscope cable I can read the two Volts I am generating perfectly. Even if the outer wire from my function generator coaxial and outer wire form my oscilloscope aren't connected to anything. Again, when I put my resistor in series with my variable resistor, it doesn't matter if I don't complete the circuit by connecting them to the outer wire from the coaxial cable of the function generator, I can still take readings using the central wire from the oscilloscope coaxial. It's as if the outer wires of both the function generator and oscilloscope are redundant. Does this make any sense?


Regards Chris

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