Nulling a transducer in NI-DAQmx

[MVF]

My coding experience has been with Traditional NI-DAQ but I am looking at writing for a device that isn't supported and so I'm 'moving up' to NI-DAQmx using the ANSI C library. The applications I've written in the past have catered for transducers that have a voltage offset that drifts slowly with time and changing environment and so a calibration would be a function of (V - Voff) where Voff is the voltage offset that drifts and that can be measured in quiescent conditions and subtracted before applying the calibration. The channels and scales as seen in MAX look great and would be very useful and provide a great deal of functionality saving on coding and software development had they allowed for the nulling of a transducer as described above. As they don't, unless I'm missing a trick, they are all but useless. (There is a way of compensating if the calibration is linear but it involves carrying another variable and modifying the calibration equation each time the transducer is nulled.) Am I being dense or do I have to ignore the functionality of NI-DAQmx and work with voltages directly?

[Tony_G]

Hi,

Have you seen the Auto Zero Mode option for the device in MAX?  Auto Zero Mode is a procedure for eliminating offsets generated by an amplifier stage for example.  In Auto Zero Mode, NI-DAQ shorts the input channel to ground and subtracts the obtained measurement from all subsequent samples.  It is basically a quick calibration technique that takes place at the beginning of a scan.

Auto Zero Mode can be configured very easily in MAX when using DAQmx.  To do this, you simply create a Global Channel that is reading the desired physical channel. Once the Global Channel has been created, you can set the Auto Zero Mode by highlighting your channel under My System>>Data Neighborhood>>NI-DAQmx Global Channels in MAX. The Device tab can then found between the Settings and Calibration tabs in the window on the right. This is where the Auto Zero Mode can be set.

There are three options:

• None—Do not perform auto zero.
• Once—Perform auto zero at the start of every acquisition.
• Every Sample—Perform auto zero at the start of every sample.

Best regards,

TonyG

[MVF]

Thanks for the reply. I haven't seen the Auto Zero Mode option for a device in MAX. I guess the auto zero option is device dependant and isn't available for any of the devices I have. In any case, from your description, it would appear that it compensates for the drift in the device itself rather than a transducer. Given that NI recognises that offsets exist in the amplifier chain and describes transducer offsets in the description of pressure measurement on the web site it's a shame that they're not provided for in NI-DAQmx, at least as far as I can see. As I said before it means that for me most of the functionality of NI-DAQmx and MAX is useless.