D2A scale factor change on PCI-6731. Mine changed by 20%

[Jeff Gilkey]

I am using a PCI-6731 board

The scale factor for the D2A on the board changes.  Sometimes as much as 20%.

It is stable for a day, but if I use it a month later, I notice the change.

The board is being used in a MATLAB/SIMULINK/XPC Target application and the board is supported by MATLAB.

The board works fine, except for this change in scalefactor.

[J_Thomas]

Hi Jeff,

Welcome to the NI discussion forums. I have read your post and I am bit confused about what you mean by “scale factor.”  Would you mind elaborating a bit more about what you mean?

• Are you talking about a custom scale that you have created to scale your measurement in Matlab?
• Are you talking about a DAQmx Custom scale?
• Are you referring to the gain of the PCI-6731? The PCI-6731 has a single input range and thus a fixed amplifier gain.
• Are you seeing this scale factor on your analog outputs? Digital I/O?
• Can you provide an example about the behavior you are seeing?
• Where is your computer setup? Has the device been calibrated recently?

Any additional information you can provide would be helpful.

Jared T.

[Jeff Gilkey]

I meant the gain instead of scale factor.

I command the PCI-6731 D2A to output 4 volts, and I measure the output on digital voltmeter and read 5 volts.

I have noticed this before, last month I would command an output 4 volts, and it would output 4.2 volts.

I checked the calibration on the voltmeter, its good.

( I have rounded these numbers, but you get the idea)

I am commading the PCI-6731 using an MATLAB/SIMULINK/XPC Target application

Thanks.

Jeff

[J_Thomas]

Hi Jeff,

Thanks for clarifying the behavior that you are seeing.  One thing I would suggest would be to try changing the output voltage level using Measurement & Automation Explorer (MAX).  When you installed the NI-DAQmx driver for the device to use with SIMULINK it should have installed Measurement & Automation Explorer (MAX).  MAX is a tool that can communicate directly with the hardware and is useful for testing and debugging.

You can open MAX by going to Start >> Programs >> National Instruments >> Measurement & Automation. On the left hand side you can then navigate to My System >> Devices and Interfaces >> NI-DAQmx Devices and you should then see an entry for your device.  Right-click on the entry for your device and select Test Panels.  You can then use the Analog Output tab to update the voltage being sent out on each channel and verify with a multi-meter if the voltage is what you expect.

 

 

It is possible that we may have grounding or signal connection issue.  How do you have your analog output channels physically wired?

Jared T.

Message Edited by J_Thomas on 06-25-2008 11:34 AM