2D U16 vs 1D Wfm in Nchnl Nsamp

[Amanmeet]

Hi,

 I was trying to do a multiple channel data acquisition job using PCI 6229 with BNC 2090A board. After creating the virtual channel and starting the DAQ task when I acquire the signal I am getting a strange behavior. I am giving an input square wave of 1volt and zero volt.

 When I use the 2D U16 Nchnl Nsamp : I am getting spikes of constant magnitude for the zero volt portion, where are it reads the 1 volt portion correctly.

Where as when I use the 1D Wfm Nchnl Nsamp: I get approximately the signal that is input to the system with a square wave behavior and appropriate voltage range.

Please suggest as to what is the difference in the two conditions, and possibly Why am I getting spikes for the zero volt portion.

Regards

Amanmeet Garg

Graduate Student

Simon Fraser University

Burnaby,Canada. 

[Dennis_Knutson]

Do you realize that the U16 is unscaled? You would need to get the polynomial scale with a DAQmx property node and apply it to the raw data.

[Amanmeet]

Thank you Dennis for your prompt response,

Could you elaborate on what do you imply by the polynomial scale.

Because from what understand for the U16 is that it is the property for the A/D conversion that the analog voltage of +- 10 volts is being converted to unsigned 16 bit number.

an example picture or any reference might be useful.

Regards

Amanmeet Garg 

[Dennis_Knutson]

Did you try doing a search? You might have turned up this reference to the DAQmx help.

What it all means is that the U16 values are unscaled and not calibrated.

[Amanmeet · Accepted solution]

Thank you Dennis, 

 Your reference is a good information. I was able to solve my issue by keeping the acquisition as a 16bit signed integer as I am measuring the negative voltages as well.

so in the zero/-ve volt region the system was giving it a value of 65535 in case of unsigned 16 bit sampling. 

I am correcting for the scale and filtering outside in MATLAB and am getting reasonable outputs.

Regards

Amanmeet Garg