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Measuring current using a PXIe-1071 chassis, PXIe 4141 SMU with NI 2501 and TB-2606

Hi there,

I am trying to source and sink current across a couple of LEDs with a common input and separate outputs using a PXIe-4141 SMU and the 4 method.  I apply a voltage to both of the LED's outputs using the SMU and use the switch and matrix to choose which output sinks, so which LED I measure across. 

When I use the SMU by its self and just manually connect the output I want, the circuit works perfectly (sources and measures 20mA across both LEDs). However, when I try to use the switch and test box, the current I get out is limited to less than half of the current I have tried to apply, even when I just use the NI-DC Power soft front panel and not my software. 

I am thinking there may be a fault with the way I have actually connected my circuit to the Switch (circuit design is not a strength of mine) so I will share which connections I have used. 

I have tried two different methods, both attempting to put 20mA across with voltage limited to 10V which should me far more than enough for an LED.


1st (achieves the highest current of 8.5mA)

Sense Hi and Output Hi connected directly from SMU to the common input on the circuit.

!st LED output connected to c4+ and c4- on matrix.

2nd LED output connected t0 c5+ and c5- on matrix.

Sense Low from SMU connected to R3+ on matrix

Output Low from SMU connected to R3- on matrix

R3+ and R3- also connected to chassis ground.

I then connect R3 to c4 and c5 using the NI switch soft front panel and try to put 20mA though using NI-DCPower soft from panel.


2nd (only gets 2mA through but the circuit seems more logical to me)

Common input is connected to c4+ and c5+ which will then connect to R3+.

Sense Hi and output Hi from SMU are then physically connected to R3+ using the soft from panel

Output of 1st LED is connected to c4- and output of 2nd LED connected to c5- which are then connected to R3- using the soft front panel.

R3- is then physically connected to the SMU sense low and output low and the chassis ground.


Any help or tips would be greatly appreciated! 

I can also share my program, but as I said before, the problem is also seen in the soft front panels so I'm thinking its more of a hardware issue.


Thank you in advance! 

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Did you take into account the multiplexer resistance as mentioned in the specifications of PXI 2501
PXI 2501 SpecificationsPXI 2501 Specifications

 Looks like the total path resistance can be in kOhms.


This additional resistance from the FETs of the Multiplexer would be behaving as a series current limiting resistors for your LEDs. Please share the simplified circuit diagram of the setup.

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