NI9219 - Analog voltage measurement not to scale

[RobDekker]

I am using the NI 9219 analog input module to measure a 0-10V input.  When the input is 10V (checked with a multimeter), labview is measuring 5V.  I am using the sample code "Cont Acq&Graph Voltage - Write Data to File (TDMS) to collect the data.

What parameter do I have to set to get the proper scale?

[John_P1]

Hi Rob,

You may need to set the Maximum Value input of DAQmx Create Channel as shown below:

 

The example should have this as an input on the front panel already, so the issue could be something else.  After you verify that the range is set correctly, I would like to find out the following information so we can better diagnose the problem:

1. Can you measure everything accurately up to 5V, and then everything above is "railed" at 5V?  Or, is your measured signal consistently below the actual amount?
2. What are you using as a signal source?
3. Which terminals are you connected to on the 9219?

This information should help us figure out what exactly is happening.  Thanks for posting!

-John

Message Edited by John P on 04-20-2009 10:45 AM

[RobDekker]

I tried changing the max and min values.  I used +/- 60, +/-15, and 0-10.  Changing these values did not affect the measured values.

The measured value is consistently less than the actual amount.  It starts off being off by a factor of 1.6 at 0 and changes to a factor of 1.55 near 10. ie. measure about 6.2V at 10V

The voltage source is an LVDT (output of 0-10V)

I connected to terminal 4 (HI) and 5 (Low) on Connector 0.

thanks for your help

[John_P1]

Hi Rob,

Thanks for the update.  A couple other questions:

1. Could you link the spec sheet of your LVDT?  What are you using for excitation?  Is the LVDT grounded or floating?
2. Could you try measuring another known voltage source with the 9219?
3. Could you try measuring the LVDT (or other voltage source) on one of the other connectors?  The 9219 has one ADC per channel, so this would help determine if the problem is with the ADC.

I'm not sure what you mean by "a factor of 1.6 at 0".  Is there an offset amount when measuring 0 volts or just a gain error?

-John

[RobDekker]

Sorry that it took me so long to reply. I got sent to a customers site for 2 months. I switched over to channel 1 from channel 0 and that solved my scaling problem. The ADC on channel 0 must have a problem with it. Is there anyway I can fix it? thanks for your help.

[John_P1]

Hi Rob,

I hope the visit went well.  You will likely have to contact NI by phone to arrange for a repair.  You can choose to speak with an engineer to help troubleshoot the issue, although it sounds like the problem is indeed likely with the ADC on channel 0.

-John