Loopback-test with USB 6008 result in odd voltages

[vdBerg]

I am using the a new NI USB 6008 DAQ. When connecting the Ao0 & GND with Ai0 + & - respectively and measure in the test-panel whether the before set voltage for the output appear at the input the result differ systematically from what I would expect.

- the device passed the self-test
- I am using the analogue-side
- channels are set correctly
- output is set to DC, 0 min & 5 max, the voltage itself is set to a value between 0 and 5
- input is set to 0 to 5 volt again, differential measuring, single mode
The whole procedure is described also here: http://techteach.no/tekdok/usb6009/#test

Result:
For every 0,5 V the output rises the input increases by ~0,14 V

This is what I get, on the left the generated output (V), on the right the displayed input (V).
0     -     1
0,5   -   1,14
1     -    1,28
1,5   -   1,43
2   -   1,57
2,5    -   1,71
3   -    1,85
3,5   -   1,99
4   -   2,14
4,5   -   2,28
5   - 2,42

No scaling was defined whatsoever. When measuring the generated output with an external device the correct numbers appear.

Any help or explanation would be greatly appreciated as I am just starting with these matters.

[Chris_Rake]

Can you run the same test again, but connect AO0 to AI0 and run the channel in single-ended mode?  Also perform the same experiment with AO1.  This can be done with the test panels in MAX.  Also, if you could check the output of both AO0 and AO1 with a DMM it will help narrow down the issue.

[vdBerg]

First of all thank you for your answer... I performed the tests requested. Results:

Ao0 (V) -> Ai0 (V), measuring in RSE:

0 -> 1
0,5 -> 1,15
1 -> 1,29
1,5 -> 1,43
2 -> 1,58
3 -> 1,85
4 -> 2,14
5 -> 2,42

... so same thing for this, also when connecting Ao1 to Ai0...

But the next one was interesting:

Ao1 (V) -> Ai1 (V), in RSE:

0 -> 0,01
0,5 -> 0,51
1 -> 1,01
1,5 -> 1,5
2 -> 2,01
3 -> 3,01
4 -> 4,01
5 -> 5

With this 0,01 tollerance I can totally live, so this Channel Ai1 seems to work all right which is good news for me. Concerning the output of the DoX: As I said they both are fine when using a DMM as well as an oscilloscope.

I went on checking Ai2 to 7 in RSE which turned out to be working fine. This applies to Ai1, Ai2 and Ai3 +/- in differential mode respectivly.
To sum this up obviously only Ai0 seems not to work as expected, be it in RSE or in diff-mode.

Is there an explanation for this? Thanks in advance again... 🙂

[Chris_Rake]

Based on your experiment it seems that AI0 has been damaged.  I will forward this thread to a Product Support Engineer who can assist in arranging for repair.

[vdBerg]

Thank you.

[vdBerg]

Just to end this: I sent the device back to the NI Service and got a replacement, which works fine. Thank you all again and have a nice weekend. 🙂