NI 9207 Voltage below expected value

[DylanP]

I am trying to record a voltage using the NI 9207 voltage/current card. When I connect the voltage to the card and have it displayed in LabView, it shows the voltage lower than the expected value. I should see the voltage around 2.5V and instead see the voltage at 2.2V. I used DAQExpress to check the voltage and with the ADC Timing Mode being set to Automatic, the voltage is appears to be oscillating between 0 and 2.5V. The voltage is a direct current and should remain constant. When I set the ADC Timing Mode to High Resolution, the voltage became 2.2V like I saw in LabView. I have tested my voltage with a multimeter to ensure that it is accurate. I also removed any other sensors connected to the NI 9207 to remove any interference and tested multiple ports on the NI 9207 to ensure all were the same. Lastly, I checked other slots on the chassis to ensure that all were the same. The error occurred with every configuration.

Any help is greatly appreciated.

[santo_13]

If your concern is about the hardware measuring the signal incorrect, the best way to check is to connect a 1.5 AA cell to the AI of the card and make a measurement, if the reading is good, then it is an indication that the hardware is good.

Please provide more information about where does the signal come from, what is the output impedance of the signal source, exact wiring, etc.,

The most likely issue would be you're using an incorrect configuration while reading or you have wired things wrong. A hardware malfunction or inaccurate reading (out of calibration) is highly unlikely.

[JÞB]

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@santo_13 wrote:

If your concern is about the hardware measuring the signal incorrect, the best way to check is to connect a 1.5 AA cell to the AI of the card 

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BAD IDEA! this could actually damage the device.  

BETTER METHOD! Run the device self test.  If it doesn't pass its bad

Now back to our OP's question.

The 9207 has 8 differential Voltage inputs with a <1GOhm input impedance.  This is not likely to load down a 2.5V signal that is properly connected to pairs AI0 and AI8 through AI7 and AI15.  

The Device also has 8 differential Current inputs.  These have 85Ohm shunts on each leg.  If you connected them to the same input you likely would load down your voltage source.

Then you mentioned that you are seeing some "Oscillation between 0 and 2.5V"  That begs the question about what the common mode voltage is.  Each Voltage Input must remain within 10.2V of AGND or, you'll see some weird readings.   Oh, you ARE seeing weird readings!  

Baring in mind that your handheld Fluke doesn't care about common mode voltage right up to the point where it's case and lead resistances break down and YOU become a common mode path to earth ground (@300V or so hand to foot.)  Who said DC can't be measured in "Hurts?"

What are the characteristics of the voltage source?