dc offset

[Gautam Dee]

Hi everyone

I am using a cRIO 9014 with NI 9239. The analog signal from the 9239 card has a DC offset of 4-5mv which is way too much for my application. I have a cDAQ 9172 USB chassis and the card works fine with it, offset is around 6-7 micro volts with it.

Is it a problem with the rio chassis ? Can anyone tell me the reason for the offset and how I can get rid of it ?

Thank you

Tamanna 

[Andrew_E]

Hi Tamanna,

There shouldn't be an offsett between the two chasis. I would suggest, if you haven't already, try the input module in different slots of the cRIO chasis and see if you still have the offsett. Also, do you see this as a persistent problem with other modules in the cRIO chasis?

Regards,

[Bassett_Hound · Accepted solution]

Tamanna,

Andrew is correct that there should not be an offset between a cDAQ Chassis or cRIO Chassis.  I don't think trying the module in different slots will show much of a different if you were already able to read correct values on a cDAQ Chassis.  I would take a look into running the example program for the 9239 if you haven't already.  From the short description in your first post it sounds like your using un-scaled and calibrate binary data instead of scaled and calibrated Fixed Point data out of the IO node.  If your using binary data you'll also want to make sure your using the Binary to Nominal.vi found here:

C:\Program Files\National Instruments\[LabVIEW Version]\examples\CompactRIO\Shared

If your still seeing the offset after the above then post some screen shots or code and also look to make sure the COM and the AI Inputs are connected correctly when in the cRIO chassis.

Hope that helps,

Bassett

[Gautam Dee]

Thanks Bassett

I guess the problem was that I was using raw data and not converting into engineering units. The example helped and I am trying to pick up DC offset and LSB weight of each channel in my FPGA code. I am using NI RIO 2.4.1, so caliberaterd data can be got directly, but is it not that I would lose precision by using fixed point data ?(precision is very important in my application)  and when i change my properties of my module to pick up caliberated data, I get a zero signal even if i connet a 12v signal. Cant figure out why this is happening.

Thank you

Tamanna 

[JMota]

You shouldn't be getting a zero signal. Did you update the data type of the indicators to be the Fixed Point data type that the IO Node returns (instead of the integer data type the Raw mode uses)?  

Using Calibrated data will still meet the accuracy specifications of the module and should be very close in precision to using raw data and calibrating yourself. You might get a bit more accuracy by doing the calibration yourself, but I'm not that sure about it. Maybe somebody else can comment on it.

[Gautam Dee]

What do you mean by indicators ? do you mean the conversion when i am trying to get data out of the FIFO ?

[JMota]

I made an assumption that doesn't apply to your code (that you were accessing the analog data through an indicator on the FPGA VI). What you need to make sure is that if there is any cast going on for the data you are writing to the FIFO, make sure it is safe. Depending on what version of LV you are using, you may or may not have to cast the fixed point data to an integer.

A screenshot of your code would help here. 

[Gautam Dee]

Hi

I am not doing any type casting in my FPGA code but I certainely am in a RT target vi where I am reading the data from the FIFO. The code looks something like the picture attached.

Thanks

Tamanna 

[Bassett_Hound]

Tamanna,

If your using Fixed Point data for your IO in your FPGA application, then you'll want to make sure the data comes out of the DMA FIFO as fixed point as well.  If your using LabVIEW 8.5.x then there is an additional step you need to perform.  The following KnowledgeBases will help you out.

How Can I Transfer My Fixed-Point Data Using a FIFO or Memory in LabVIEW 8.5.x?
Using Fixed-Point Data with CompactRIO and LabVIEW FPGA

If your using LabVIEW 8.6 you would want to configure the FPGA FIFO to use Fixed Point Data Type.  If your not using Fixed Point then you might also want to post and image of your FPGA application as it will better help everyone understand what your doing.

Thanks,

Bassett

[Gautam Dee]

Thanks  Bassett

That was exactly what I needed to do. Thanks a lot. 🙂

--tamanna