ni-5114 scope interfaced to NI 2576 relay multiplexter and to a test signal via Coax, noise issues

[malanis]

Hi,

We are using a system connected as follows and have some questions about the best method to interface to our Design Under Test.

DUT(Test Point[n])  --------> 6ft Coax Cable  ------>96pin connector(interface board) --> NI 2576 ----> NI 5114

Note1: the 96pin interface board and the NI equipment share the same ground plane.

Note2: We have tried with changing the NI5144 scope setting to 50ohm and 1M ohm but don't seem to see a difference on our potential noise issue.

The questions are:

1) What is the best place to ground the shield of our coax cable, the DUT side (to board ground) or to the 96 pin side?

2) We currently have the shield connected to the 96pin side and it is bringing in some potential noise to our DUT test Point signals,  we have added a 100 ohm series resistor  close Testpoint and it helps resolve the issue, however it is not acceptable to have this resistor in the cable harness per our client and we are trying to find an additional option to solve this potential noise issue.   

Any help and guidance on this is appreciated.

Regards,

Miguel

[CoffeeBreak]

Hi Miguel,

After a little research on my end it looks like you may have run into a problem of attempting to drive capacitive loads that your DUT cannot handle; thus, your amplification can turn into essentially an oscillator.

1.) You mentioned that the 96 pin device and the NI hardware is connected to the same ground plane, but how is your DUT grounded? I believe you said board ground, but is that the same ground as the 96 pin connector?

2.) Could you provide the part/product number for the coax cable you are using? Are you able to test a shorter coax cable to see if the same noise is introduced? I ask because it seems that adding the 100 ohm resistor employs the out-of-loop compensation method to solve the capacitive load drive problem.

Does your DUT have a capacitive load drive capability specification?

[malanis]

Hi Sam,

Thanks for the support.

Here are the answers to your follow up questions.

1) The DUT ground is shared with the NI Hardware through a different interface pin, so essentially yes the same as the 96 pin connector.

2) The coax cable part # is the Belden "83284 Coax - 50 Ohm Coax", it appears that it has a capacitance value of 174pF for 6ft, we have tried temporarily removing the shield connection at the NI HW and the issue goes away.

We also found out that our DUT has a very low capacitive load specification, output of some OPAmps with <10pF spec, so that could be the culprit as well.

Regards,

Miguel

[CoffeeBreak · Accepted solution]

Miguel,

1.)  As long as you are not introducing any ground loops in your system you should be fine for either implementation for the coax ground. I did want to mention it is important that your DUT is not referenced to power ground also because this may introduce ground loops in your system.

2.)  As for the culprit of the noise it is the combination of the two specifications you listed. The op-amps in your DUT output are specified for less than 1/10 the capacitance of the coax cable. This is the origin of the noise you are seeing because the op amp is incapable of driving the capacitive load and in turn causes instability from the amplifier.

There are a couple methods you could use to rectify this situation, and here is a resource that describes some of these solutions.  

[malanis]

Thanks for all the help Sam.

regards,

miguel

[CoffeeBreak]

Miguel,

Glad to help and good luck with the rest of your application.