MAX differential measurement does not match DAQ Assistant output

[metzler]

I am using a USB Compact DAQ AI 9205 and have a differential input running into AI23 and AI31.The signal is run to the AI module with a shielded twisted pair cable.

When using MAX, it shows that my signal is at about 4.4 Vdc.  An external meter agrees with this reading.

When measured using DAQ Assistant in the VI, the output shows -.5 volts.  

Can you tell me what could be causing this?

[AdnanZ]

This might sound really dumb, but are you sure you connected the right channel? Try to start the DAQ Assistant from scratch and see if the configurations match exactly to the setup in MAX. If it's working in MAX, it should work in the DAQ Assistant.

[Dennis_Knutson]

Another mistake could be the terminal configuration in the assistant. Be sure to specifically select differential and not 'Let DAQmx choose'.

[metzler]

thanks for  the suggestions, but nothing changes the differential input. 

If I use MAX, regardless of the input configuration (RSE, NRSE, Differential) I get a 4.1 volt dc signal.  An external meter reads 4.4 Vdc. 

The readings I get with DAQ Assistant are different:

RSE:  1.31 Vdc

NRSE:  1.31 Vdc

Differential:  -.58 Vdc

Is there like the while loop delay that may affect this.  I have 18 signals split after the DAQ Assistant, and this is the only one that is giving me erroneous readings.

[charris]

Metzler:

If only one channel is returning erroneous values, it may be possible that there is a malfunction in hardware. In order to confirm/eliminate the possibility, could you test the suspect input with one of the shipping DAQmx examples?

If you open the Example Finder (Help --> Find Examples), browse to "Hardware Input and Output --> DAQmx --> Analog Measurements --> Voltage. I would recommend trying "cont acq&graph voltage-int clk.vi."

Set it to use the suspect channel and compare the reading with your external meter. If they agree, then there's likely an issue with your code somewhere. You'll need to start simple and build up to your current program's implementation complexity to find where the signal is not being passed correctly.

[metzler]

I tried running the example you recommended.  It came up with the correct voltage, around 4 volts DC. 

I have noticed that the problem only happens when the other channels are also running on the compact daq.  They are a mixture of RSE, NRSE, and Differential inputs, depending on how critical it is to have an accurate measurement.

The last thing I did was move the input from AI23 to AI24, and all my problems went away - it sounds like their is a hardware problem with AI23.

[charris]

So, just to clarify:

When you run the example and only measure AI23, it returns the correct voltage, but if you read multiple channels, AI23 returns the wrong voltage?

It does indeed sound like a hardware problem. If you can reproduce the issue reliably, I'd recommend calling in and getting an RMA number to get it repaired.

Contact NI

[metzler]

Thanks for the advice.  Since I do not have a spare AI modules, I will use AI24 until my spare arrives.