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Ni SMU Sourcing and Measurement against ground

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Hey guys,

so I used a NI PXIe-4138 SMU for measuring attributes of LEDs that were potential-free. (i.e. measure the voltage for a given current)

But now I need to do this while the cathode of the LED is grounded. (The ground of another PXI card, which in this case is the ground of the PXI bus)

Is this possible or could there be any short circuits or other potentially damaging currents?

 

Thanks in advance,

Eric

 

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Message 1 of 14
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Hello of!

Is the ground of "another PXI card, which in this case is the ground of the PXI bus" the same potential as a ground of your SMU? If yes, it should work properly without any short circuit. 

If not, it would be good to connect those potentials (ground of PXI and SMU) before performing the tests. If not, try to include a resistor (For example 10k) between the LED and the ground of "other PXI", to avoid any problems.

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Message 2 of 14
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Hey Tomasz,

I don't know the inner workings of the SMU so I couldn't tell you which ground is used. Both cards are in the same chassis so they share the ground of the PXI bus if the SMU hardware is directly coupled with it. But I wouldn't know about that.

But either way the SMU terminals I would be using are HI, LO and Sense HI, Sense LO and my problem is that I don't know the potentials of those either.

 

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Message 3 of 14
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Hi again,

If both modules are in the same chassis, you can assume that they have common ground. In this case, connecting cathode of a diode to the ground of PXI and sense LO should not be a problem.

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Message 4 of 14
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Thanks for the response.

So when you only mention "Sense LO", you mean I can only measure? But I also want to source a voltage over the LED. So I need to connect the "LO"-terminal to the cathode and therefore ground as well. Or will that be no problem either?

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Message 5 of 14
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Hi again!

Diode is working only as a passive load in your circut, so it does not matter really if you are measuring it or you include it in Power line, as long as all parameters are good (I mean if current flowing is acceptable for this diode etc.)

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Message 6 of 14
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Yes, that's why I think it could work.

But I am afraid of damaging my PXI SMU by currents flowing from the unknown "LO"-terminal to ground while sourcing. Or in the other direction.

 

That is why I am asking.

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Message 7 of 14
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Hi, 

I think that It will be good to have a schematic of what you want to do, to make things clear. In my opinion, everything should be good, as you have only one common ground of PXI chassis, however I will need a schematic to make sure

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Message 8 of 14
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It would look something like that.

Both cards are powered by the same PXI-Bus in the same PXI-Chassis.

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Message 9 of 14
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Solution
Accepted by topic author ergof

So from what is given here, your ground is exactly the same for everything, it should not be a problem at all

Message 10 of 14
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