Academic Hardware Products (NI-ELVIS, NI-myDAQ)

[Manushak]

Help please!
I send 30 mV voltage to AI channel, then I create task with this parameters:
input terminal configuration -Differential mode.
maximum value - 0,1 V
minimum value -  -0,1 V
When DAQ reads channels, value of AI is 45 mV.
When I change ranges to (-1;1), value of AI is 32 mV(it becomes more correctly). I cant understand why ?. 

[amezam]

Hello Manushak,

When you specify values for minimum and maximum range you’re telling the signal conditioner the range it is going to use to determine the measurement value. What exactly are you measuring? Are you sure you are inputting 30 mV?

Best Regards,

[Manushak]

Hello Amezam
Thank you for replying.
I have gave voltage from VPS, than check it with outside tester. All were ok. But when I measured it by AI channel, I saw the problem. when I increase the ranges, the measurment was correctly. Than I saw the next, when I decreased the sample rate from 400ks to 100ks the measurment was more correct.

[amezam]

Hello Manushak,

Have you tried calibrating this device? This behavior is not right at all, if you calibrate it and you keep seen these unusual behaviors, it would be better if you send it for repair.

Best Regards

[charlesy]

Hello Manushak,

You mentioned that you are generating a very low voltage (30mV) from the VPS and see a sample-rate dependency on your results. Since the VPS cannot reliably output that low, I assume you are using a voltage divider to drop the VPS output to 30 mV. If so, what is the resistance of your divider? The analog inputs on NI ELVIS are multiplexed to a single ADC. If you are measuring more than one channel in a task, it sequentially switches between them and there is some settling delay proportional to the source impedance. For a description of this behavior, see the section titled, "Multichannel Scanning Considerations" in the NI ELVIS II manual: http://www.ni.com/pdf/manuals/374629c.pdf

If this is your situation, you could decrease the impedance of your divider, measure slower, or measure just one channel in your task. If that's not practical for some reason, you could use a unity-gain buffer on the output of your divider.

I hope this is helpful.

Charles

[Manushak]

Hello Amezam.
I didnt try calibrating of my device. but I saw, when i connected the second channel without changing daq configuration, then the results of the first is changed.

[Manushak]

Hello Charles
Thank you for replying.
I have to measure 3 or more channels. But when I configure daq for 1 channel, and connect the second channel in circuit, the measurement is changed.

[amezam]

Hello Manushak, 

What is exactly what you see? How is the measurement changed?

Best Regards,