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Max and Min limits don't affect analog input of NI 9239

Hello,

when I set Max Input Limit and Min Input Limit for Analog input measurement with NI 9239, it seems that it has no real influence to measured data. For example, when I set limits to +-3V and I generate sine signal with amplitude 5V to AI0 of NI 9239, I would expect that the measured signal will be cut into range +-3V, however it isn't.

 

Am I missing something?

Any ideas would be highly appreciated.

Best regards,

Martin

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Hi Martin,

 

I assume you're referring to the Min/Max input limits on the DAQmx Create Virtual Channel function.  Is that correct?

 

If so, those inputs are used to determine the gain setting of the device's Programmable Gain Input Amplifier (PGIA), if applicable.  For example, if the device has an input range of either +/- 1V or +/- 10V, setting the min/max to +/- 0.5V would select the +/- 1V input range and a corresponding 10x gain (to ensure we still use the full +/- 10V input range of the ADC) on the PGIA.

 

However, the 9239 only has a single input range, +/- 10V.  Therefore, setting the Min/Max on that module does not (and should not) have any effect.

Cody A.
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Hi Cody,

 

thanks a lot for your reply. Now, I understand the whole behavior a little bit more.

Could I have two additional questions?
Is there available any table which contains all possible gain settings for each individual device (not only cDAQ cards but PXI cards as well)? I just would like to know on which of our analog input cards is this gain settable.
For particular devices, is it possible to get list of all possible gains programmatically (e.g via Property Node)? For me it is a little bit illogical to allow measurement operator to set Min/Max Inputs from continous interval if there are few discrete ranges if any. Instead of this, I would like to let him choose between few available ranges for that particular device.

Thank you very much for any help.


Martin

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Hello Martin,

 

as Cody already responded, the Max and Min Input limits are used to set proper gain of the amplifier on the device so the ADC uses its range as effectively as possible. However, the gains are already preset for each device, this is the reason why the input range set in LabVIEW is sometimes rounded to the nearest possible higher value of input range for the device.

 

I am attaching two links to our knowledge base, which might help you as well.

How Do I Set the Analog Input Gain for My DAQ Device?

http://digital.ni.com/public.nsf/allkb/E69C47B0BB063B548625794A004B8354

Why Are My Analog Input Channels Being Clipped at Different Values Than I Set in LabVIEW?

http://digital.ni.com/public.nsf/allkb/DD2B6FF2BFFF216C86256E23007A2532

 

Best regards,

 

Vít Jandásek
National Instruments
Applications Engineer

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Concerning your further questions:

- You can refer to the data sheet of each particular module, they contain the possible gain settings.

- Using property node, you can actually get the input range, which was chosen by the device. The property node, which can do this is DAQmx Channel > Analog Input > Max/Min Value. You can create eg. an enum for each particular device to allow only the discrete values to be chosen.

 

Vít Jandásek
National Instruments
Applications Engineer

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