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DAQ Assist Reading Wrong Voltage

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I'm using the DAQ Assistant VI to read an analog input voltage from a National Instruments PCI-6221 card. I'm reading the voltage from pin AI0. I supply a voltage directly to this pin from a DC power supply, but the voltage measurement obtained from the DAQ Assistant is incorrect - it seems to be scaled by a factor of about 1/3. For example, if I supply 4 Volts to pin AI0, the DAQ Assist reads 1.43 Volts. I used a multimeter to confirm that the voltage at pin AI0 is in fact 4 Volts, and so I know the problem is with my LabVIEW program and not my power supply.


Here are the steps that lead to my problem:

1. In the block diagram, I insert a DAQ Assistant block.

2. In the Properies of the DAQ Assistant, I select Analog Input->Voltage

3. I select channel ai0

4. I click "test" in order to test the channel

5. The voltage is shown to be 1.43 Volts, even though 4 Volts is being supplied to the pin (this is confirmed with a multimeter).

6. To ensureI click OK to finish configuring the DAQ Assistant. I run the program and plot the voltage. The plot also shows 1.43 Volts.


Does anyone have an idea why this may be occuring. I've spent a good 4 hours trying to diagnose this and haven't found anything.




Abed Alnaif

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Message 1 of 6
The problem could be in your wiring or how you are configuring the acquisition type. There are instructions in the device manual for making connections. You've told how AI0 is wired but not how the reference is wired. In the DAQ Assistant, you choose the same type of terminal configuration (differential, RSE, NRSE, etc.).
Message 2 of 6
I would also suggest opening up test panels in MAX and having a look over there before using DAQ Assistant.
Adnan Zafar
Certified LabVIEW Architect
Coleman Technologies
Message 3 of 6

Thanks so much for your help. I used MAX, and found out that the issue is that I had specified differential voltage, but I should have specified RSE voltage.


However, now I have a different issue: When I apply a voltage to one pin, MAX also shows a voltage on other pins.


For example:

1. I apply a DC voltage of 4V to analog input pin ai0.

2. In MAX->Text Panels..., I select Channel Name = Dev1/ai0 and Input Configuration = RSE

3. I click Start, and the chart shows the correct voltage (4V)

4. I change the Channel Name to Dev1/ai1and Input Configuration = RSE

5. The chart shows a noisy voltage reading between 1.45 and 1.5 V, even though no voltage is applied to pin ai1

6. When I change the voltage on pin ai0 to 2V, the voltage reading on pin ai1 changes to 0.63V

7. Using my multimeter, I confirm that there is in fact 2V on pin ai0 and 0V on pin ai1


Does anyone know why applying a voltage to pin ai0 causes a voltage reading on pin ai1?

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Message 4 of 6
Accepted by topic author Abed Alnaif
That's normal. Do a search for the subject 'ghosting'.
Message 5 of 6
Thanks Dennis! That seems to be the problem!
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Message 6 of 6