Changing gain on PXI-6221

[Oleks]

Hello all,

I apologize in advance for a newbee question. I recently started working on force measurements in fluid mechanics lab. We  got a 3-axial load cell with 0.5 mV/V rated output per channel, which I hooked up to SG24, which sits in SC-2345, connected to PXI-6221.

As far as I understand, after signal exits SG24 unit, it is amplified to 500mV, and feeded to PXI card, which in turn have 4 possible gains of 0.2V, 1V, 5V, 10V. To keep the resolution as fine as possible I would be interested to pick a +/- 200mV gain, which in turns will clip my signal by  more then half but will keep  a resolution ~ 6.1uV, I am fine about not having full range, my expected forces will fall into shorter range.

The problem is that I can't find neither from manual nor from MAX a way how to actually change the gain. If I open MAX>NI-DAQmx Devices>PXI 6221, I don't see any option to change input voltage. I can see +/- 10V in the Test Panel, but if I change it to +/-200m it resets to +/-10 after I close and reopen it again.

Can somebody please help on this issue?

Thanks in advance

[Chris_W.]

Hi Oleks,

There certainly aren't any silly questions, we all have to start somewhere. But, like you mentioned there are 4 possible input ranges for the PXI-6221, which gives you certain resolutions based on the range you choose. When you change that value in the Test Panel it will, like you said, only remian that way until you close the test panel. When you re-open it the values return to their default value of ±10V. Eventually you will be using either a pre-created MAX DAQmx task or just creating the task in LabVIEW. In either case you can input the range you'd like and then save the task, which will hold your range values for your next use. 

The test panels are really just for use in troubleshooting and making sure your setup is working on a very basic level. The easiest way for you to see the functionality you'd like right now is, in MAX, right click on Data Neighborhood under the Configuration window and select "Create New>>NI-DAQmx Task>>Acquire Signals>>Analog Input>>Voltage." Then select your 6221 and the analog input you're using. Now, in the signal input range parameters you can put in any of the four ranges that you would like and when you save that task, those values will save with it. 

If you're curious about how this is done in LabVIEW you can take a look at some of the analog input shipping examples that you can find by going to "Help>>Find Examples..." and searching for "analog input" or just browsing to the following file path: Hardware Input and Output>>DAQmx>>Analog Measurements>>Voltage. This should get you going. 

Chris W

High Speed Products Group

Message Edited by Chris W. on 11-24-2008 04:40 PM

[Oleks]

Hello Chris,

Thank you for the reply. One thing I was wandering about is that my signal is scaled within global channel(according to the load cell calibration) to +/- 500mV(0.5mV/V , 10V gain from SCC-SG24, and 100 gain from SCC-SG24). But if I use global channel constant in Labview, should I wire it to DaQ task in which PXI-6221 voltage was adjusted say to +/- 200mV, and that will give me resolution I am looking for?

[Chris_W.]

Hi Oleks,

You got it! When you are using that task in LabVIEW, you simply have to change the maximum and minimum values on the Create DAQmx Chanel VI and it will change your input range, and therefore change your resolution.

Chris

[Oleks · Accepted solution]

Hi Chris,

You made my day ;))

Thanks a bunch!