Multifunction DAQ

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trying to measure differential input not reading 0V

We have an FFT program written to use differential input on AI1 and ground. I'm currently using an M series DAQ card with a KW mono.

We have a single SCB-68 that works correctly. It is a Rev. 2 if it matters. We wanted to move the FFT program to work on another machine as well as the original, but I cannot get any other SCBs to give me correct readings. I have tried SCB-68 Rev 1's, Rev 2's, and two brand new SCB-68a's. Whenever I connect these devices, turn on the mono with nothing in it, and pull up daqmx, the RSE reading is something besides 0V (with the SCB that works, the ai1 reading is almost exactly 0V). Sometimes it is only a little low or high, but some of them go as high as a steady 2V with nothing in the measuring field. I have tried two different computers, two different KW monos and the only setups that worked were the ones where I used our original SCB-68.


It doesn't look like any modifications were made to the SCB itself. I noticed there is a "self-calibrate" option in daqmx, but even with brand new SCB-68a's, it fails. The knowledgebase article I read on the error says I need to do a manual calibration, but we don't have a unit that can supply the precise voltage it looks like would be required. Do you know if calibration is even the right thing to look for or am I mistaken? If it is likely that calibration of the SCB is required, is there any way to calibrate it that doesn't require me buying a multi-thousand dollar unit?


Thank you,

Mike Rodenbaugh

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Message 1 of 8



The SCB-68 is just a cable and connector. The actual analog to digital conversion is done in the device which is connected to the SCB-68 at the computer end. That device is likely where the problem is.


For differential connections you do not use AI1 and ground. That is a connection for single ended measurements. On many devices differential inputs use two AI connections with a difference of 8 in the channel numbers such as AI0 and AI8, AI1 and AI9, ...


Please tell us what DAQ device you have in the computer.



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Apologies, I misspoke. It's just between AI1 and ground. To be specific, our standard connection is pins 33 and 67.


The "good" SCB-68 has worked on both PCI-6251 and PCIe-6351 on two different computers. Any other SCB-68 I try doesn't work as the Ai1 to AiGrnd doesn't stay at a constant 0V when I hook up a KW Mono.


Thank you,


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Message 3 of 8
How have you set the switches on the scb? Do you get 0 volts if you ground ai0?
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Message 4 of 8

It doesn't look like any of the analog inputs are giving 0 volts.


S1: left

S2: left

S5: down

S4, down

S3, down


I tried switching fuses from the "good" scb-68 to one of the others and that didn't make a difference, either.

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Message 5 of 8
I have idea what you actually have connected. Please just short you analog input to ground so that you have a valid test.
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Message 6 of 8

So when I said the SCBs I was trying were "all the same", what I meant was "i'm an idiot." When I went to wire up the SCBs to test today, I noticed that someone had soldered resistors along the left-hand side. We currently have these on channels B and D (ground).

I guess now I'm wondering, since we are only using Ai1 as our input, do we need all those extra resistors for all the other channels? Or can I remove some of them to stick onto other boards? (I attached pictures this time!)

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Message 7 of 8
Bias resistors as explained in the advanced user manual:

It looks like you are using b,d,f, and g. If you are not using a particular channel I believe you can safely remove the unused resistors.
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