Different results using NI-9234 (USB) and a NI-4474 (PCI)

[giaulo]

Hi,
I have a NI-9234 (USB) and a NI-4474 (PCI) connected with two accelerometer of the same model in order to acquire the acceleration data from an object (the acquisition is done using the Nidaqmx libraries).

The problem is that when I apply the Fourier transform to pass from time domain representation to the frequency domain (using the GSL - Gnu Scientific Library) I obtain the same peek around 150Hz acquiring with the USB board and around 200Hz with the PCI board.

Why I obtain this gap?  

[AntonisV]

Hello giaulo,

Have you tried to change the two accelerometers between them to see if the measurements are still the same???

Are you acquiring simultaneously the two signals?

[giaulo]

I made the acquisition with the same accelerometer (of course plugging and unplugging it from one board to the other).

I use a sample rate of 20000samps/sec for a half of a second (10000 samps total).

I always obtain the resonance peek (200Hz is the right value) round the 150Hz acquiring with the USB board so I think that it's a USB board problem... 
 

[Brad_K]

Hi giaulo,

I think I see what the problem is. The NI 9234 does not support sampling at 20 kS/s. When you specify a sample rate of 20 kS/s with a NI 9234, DAQmx coerces the sample rate property to the next higher supported rate, which is 25.6 kS/s.

See What Are the Valid Sampling Rates for the NI 9233, NI 9234, and NI 9237? for details as well as a workaround involving resampling. Also, How Is the Actual Scan Rate Determined When I Specify the Scan Rate for My Data Acquisition? shows how to ask DAQmx what the actual sample rate is.

Brad