NI 9237 FPGA Offset Calibration

[StevenA]

I'm working with an NI 9237 in LV FPGA 8.2.1. NI-RIO 2.4 and I would like some explanation of what the offset calibration does.  I would expect you to be able to connect a strain gauge bridge and with the offset calibration you could null out any offsets you may have when the bridge is at rest (not strained).  However, all it seems to do is give you a zero value and when you disable the offset cal it goes back to the same offset it had before.  Does it not have software controlled trimming pots like the SCXI 1520?

[Eli_S]

Hi Steve,

The offset you are getting from the calibration node is NOT for offsetting the actual measurement but rather this offset is used for converting the binary data to nominal (meaningful) data.

For more information about how calibration works please search for the following in the LabVIEW help:

“NI 9237”

“Converting and Calibrating CompactRIO Analog Input Values”

Additionally, please take a look at the following example about offsetting your actual measurement (zero-out an unloaded bridge)

 http://zone.ni.com/devzone/cda/epd/p/id/5816

Lastly, in the LabVIEW example finder (Help »  Find Examples…), browse to Hardware Input and Output » CompactRIO » Module Specific » Analog Input » NI 9237 Getting Started.lvproj

[StevenA]

Eli,

Thanks for the response.  However, I'm using the calibrated values (Fixed Point) output of the 9237 not the binary values.  My question is more a hardware question in wanting to verify exactly what the module is doing when you switch the Offset calibration to True.  I thought I had read somewhere that the only thing the module does is disconnect the AI lines from the bridge and short them together.  Is this true?

[Eli_S]

Yes you are correct. When you set the Offset Cal Enable to true, the module shorts the AI lines to ground and subtracts the value measured so that the end result is zero.

For more information please refer to this knowledgebase article:

http://digital.ni.com/public.nsf/allkb/C167C8A59C4C6DE886257402006AF374?OpenDocument

[StevenA]

It seems if you are using the calibrated outputs, then the offset cal feature is meaningless.  I'm dissapointed that this module cannot do a real offset calibration. like the SCXI module. 

[Eli_S]

I haven't tired it out myself, but it seems like the offset cal. feature should do the same wether you are using the binary or the calibrated data. Did you experience something different?

[StevenA]

I think you are right, they should give the same values.  However, I wish the module would allow you balance the bridge in hardware... removing offsets to allow the full range of the module.  It's very common to have pre-strained gauges that you want to call zero strain.  With this module you can do that in software, but you pay the price of having your measurements not centered at zero volts.  In the case of a pre-strained bridges, the Offset Cal feature is basically of no use.

[Eli_S]

I see what you mean. Yes, the offset cal in the NI 9237 is done in software not hardware.