Two signals from one analog i/p channel using Analog Input block of Data Acquisition Toolbox

https://www.dropbox.com/sh/28hkrkt8voe5e3y/yPr35z1QqK

I am using the Analog Input block of Data Acquisition Toolbox of Simulink and am seeing two values for one channel. I am using a NI USB-6212 mass determination device. Data is collected at 200 Hz and I have tried using different modes (Differential / SRE / NRSE) but in vain. I am using sample-based signal type.

In the two figures on dropbox link, the top sub-plot is raw data (in V), middle one is data after removing dc-offset and passing through a low pass filter (12 Hz) and last sub-plot is after applying an appropriate gain to see the response (I am interested in measuring angular displacement, so the last sup-plot is in deg).

I understand that there is noise in the system (which is a separate problem and I am working on it) but nonetheless I could not understand as why should I see two signals even though I have selected only one channel. May be I am not clear as how this block interprets the incoming signal.

I double checked the signal in NI's Measurement and Automation Explorer and see only one signal as I expect (with noise of similar magnitude).

Any help will be greatly appreciated.

Thanks Rahul

Answer 1

[Shankar Subramanian]

Hi Rahul,

The two values that you see are because the Block Size is set to 2 for NI devices in clocked acquisition mode. Block size is a parameter on the mask as well. You cannot set a value less than 2. You can connect the output of AI block to drop one value out of it before your processing blocks run.

HTH,

Shankar

Answer 2

[Rahul]

Thanks