Triaxial Cables for PXIe-4139, 4135, and Connections

[Anne_Z]

I will be using PXIe-4139 for a series of low current (sub-pA) source and measurement. We don't have a NI SA-413T adapter yet.  Also we planned adding a PXIe-4135 in the not-too-far future. So the questions below apply to both 4139 and 4135.

Here are my questions:

1. When I looked up triaxial cables, there are 3 slots and 2 slots types. However, there is no mention in the specs. How does one select correct triaxial cable?

2. In our setup, current path is from SMU to a box with banana connectors. The cable connection might include triax cable, triax-to-bnc connector, and bnc-to-banana connector. That seems like a lot of pieces, non-triax setup needs only one cable. This would add unwanted resistance and capacitance to the current path. I wonder if there are better approaches.

How do you set up using triax cable? Pros and cons?

Appreciate your inputs.

Anne

[paigec]

Hi Anne,

1. What are the BNC/triaxial cables you are choosing from/referring to? The specs for the adapter refer to Triaxial (MIL-STD-348A) as the type to use.

2. I can understand the concern with a number of cables. What are you connecting to/what is your application? Maybe there is another way to connect than the banana interface to minimize the number of cables.

[Anne_Z]

Hi, paigec

Thank you very much for your response! Regarding the 2 questions:

1. I searched more, just figured out that the number of slots means number of side openings on a male connector, similar thing describes lugs on a female connector.

2. I checked triax to alligator cables. This would be a better option. I can unscrew top piece of the box with banana connection, and apply alligator clips to inside nodes. Then I need only one cable.

Thank you!

[paigec]

You're welcome! And great, sounds like you've got it sorted!

[Now_With_Underscores]

You mentioned you're going to use the PXIe-4139 and/or PXIe-4135 for "sub-pA" sourcing and measurements.  Keep in mind that while both of these devices have a resolution in the fA range, the guaranteed uncertainty is at best 40 pA on the PXIe-4139 & 5 pA on the PXIe-4135.

For conceptual reference, 5 pA equals 0.000000000005 coulombs per second, which is just 31,207,509 electrons flowing through the wire's cross section per second.  "sub-pA" seems to imply you're trying to accurately measure down to 100 fA, which is just 624,000 lonely electrons wandering down an empty path, fraught with the perils of ever-present leakage current, stray EMI pickup, solar flares, and more.  At currents this low, it becomes difficult to "count" the number of electrons without introducing a bunch of error.

Even though the PXIe-4135 is definitely a low-current measuring machine, it still might not be able to accurately measure currents as low as you've proposed.  Or maybe I've misinterpreted what you meant by "sub-pA", in which case I trust your judgment... just want to make sure you know what you're getting into when attempting to measure currents this low.  In my career, I've taken current measurements that literally required me to leave the room, but you're not going to need to be that ridiculous with the PXIe-4135, as its noise-induced uncertainty is several orders of magnitude higher than the static discharge a well-grounded test board would pick up from walking on carpet.

On the other hand, if you're just looking for precision (and not accuracy), then the PXIe-4135 &/or PXIe-4139 should fit your application.  In that case, the PXIe-4135 can resolve differences as little as 62,415 electrons per second, which is hands down insane, so kudos to the hardware team for their efforts making that possible.

[Anne_Z]

Thanks, John

You made an excellent point. Because the accuracy level is lower, we plan to measure currents covering bigger range, then calculate delta, in place of direct measurement.

Anne

[Now_With_Underscores]

Sweet!  Happy electron wrangling!

[尧尧]

Hi John, 

    I test the MOS and BJT by two 4135 SMUs, but I found the test method of  "hardware-timed two channel voltage sweep" is different  Keithley 4200, that is when I set the current limit is 10 mA, the off-state of current is 220nA.  when I set the current limit is 10 nA, the off-state of current is pA-level, but I could not get the entire the Id-Vg curve if I set this current limit. 

    Keithley 4200 could auto adjust the output resolution of current, from low to high, like I set the current limit is 100 mA, the off-state of current is pA-level, and the test value reach the max resolution of current, step to the higher resolution, until the current limit.

     Do you have the same problem, and how do you solve this problem.

regards,

Zhang