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Accuracy question for PCI 6229

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We are using PCI 6229 and have the following wiring: AO1 output is connected to the AI6 input, AO0 output is connected to the AI7, Both AI6 and AI7 are configured and wired as differential channels. I checked the wiring, grounding, specs on the accuracy, used accuracy calculator on NI web site.

We have a small app which writes some values to the AO channels, which in turn go directly to the AI channels.

The app writes values every 1000 msec, the reading occurs every 50 msec.

 

Here is the problem. For the first pair (AO1->AI6) the values are quite close, but for the second pair on the same terminal (AO0->AI7) are quite different. And the smaller the surplied voltage to AO, the bigger the difference.

 

I have read that the accuracy gets worse when the voltage gets smaller, but why accuracy is so different for channels on the same board?

 

Below are some values.

 

AO1                AI6                   AO0                AI7

0.011719        0.011477        0.007812        0.006868
0.011719        0.011464        0.007812        0.006855
0.011719        0.011503        0.007812        0.006855
0.011719        0.011509        0.007812        0.006797
0.011719        0.011490        0.007812        0.006823
0.011719        0.011477        0.007812        0.006907
0.011719        0.011516        0.007812        0.006823
0.011719        0.011522        0.007812        0.006810
0.015625        0.012691        0.011719        0.008088
0.015625        0.015488        0.011719        0.010854
0.015625        0.015566        0.011719        0.010919
0.015625        0.015514        0.011719        0.010834
0.015625        0.015521        0.011719        0.010867
0.015625        0.015527        0.011719        0.010873
0.015625        0.015482        0.011719        0.010854
0.015625        0.015508        0.011719        0.010880
0.015625        0.015495        0.011719        0.010899
0.015625        0.015514        0.011719        0.010860
0.015625        0.015462        0.011719        0.010821
0.015625        0.015514        0.011719        0.010821
0.015625        0.015547        0.011719        0.010828

 

Thanks in advance.

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We actually did the following experiment: connected one AO channel to both AI channels, then another AO channel to both AI channels.

Turned out the AO1 has lesser error than AO0.

 

Any idea why and how to fix it?

 

Thanks.

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The absolute accuracy of the Analog Output is 3,230 uV and of the Analog Input is 112 uV for a combined accuracy of 3342 uV (from pg 4 and 5 of 6229 specifications)

 

http://www.ni.com/pdf/manuals/371290g.pdf

 

So it appears that your measurements are within specified accuracy as you are seeing a disparity of roughly 1000 uV.  If you are in need of better accuracy you could try calibrating the card, or creating a clibration curve and manually handling the offset.

Doug Farrell
Solutions Marketing - Automotive
National Instruments

National Instruments Automotive Solutions
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OK, but why one AO channel would be better than the other?

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Solution
Accepted by topic author vera

Hello vera,

 

Our M Series cards have a seperate Digital-to-Analog converter for each AO channel.  As a result, each AO channel will have slightly different properties and inaccuracies, though they will all conform to the specifications.

 

Regards,

Seth B.
Principal Test Engineer | National Instruments
Certified LabVIEW Architect
Certified TestStand Architect
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