PCI 6251 AI FIFO size

[Jason2010]

Hi,

PCI 6251 M series DAQ card, AI FIFO size is 4095 according to its specification. I'd like to check the size is total or based on per channel.

[Brooks_C]

Hello Jason, I believe that this FIFO is shared by all analog input channels in use.  If you look at figure 4-1 in the M Series User Manual you can see that the inputs are multiplexed into the same FIFO. That being said, the AI FIFO size is not a parameter that a user would see in general use.  The DAQmx driver automatically transfers samples from the onboard FIFO to another FIFO in the PC's RAM.  This buffer can be much, much larger, and its size is usually set automatically by the timing parameters you configure.  If you're concerned this FIFO will limit your acquisition rate that is not the case, the 6251 is capable of transferring samples from the onboard FIFO to the RAM at the maximum sampling rate of the card. I hope this helps, but if you still have questions please feel free to post back. 

Cheers,

[Jason2010]

Hi Brooks,

I am studying the possibility to acquire a waveform by using PCI 6251. The waveform is a low frequency saw-tooth like one. The period is only 667 ms.

My tested system is like the following. Input is a saw-tooth like signal, from 0 to 225 ms the signal is increasing from 0.5V to 6V linearly, from 225 ms to 667 ms the signal remains 0.5V. There are 2 output signals for the system. I use PCI-6251 to acquire the data for these 2 output signals simultaniously. The period of these 2 signals is also 667ms, their signal amplitude is within 10V, their waveform shape is not known, but should be similiar with the input signal.

I just wonder whether PCI-6251 could acquire these 2 output waveform without distortion, whether PCI-6251 acquires the data fast enough to get the correct waveform. According to PCI-6251 specification, sample rate for multiple analog input is 1MS/s. How am I sure it is enough to acquire the 2 output signals. Thanks.

[Brooks_C]

Hello Jason, 

As a general rule of thumb, if you want to get frequency information about a sinusoidal signal you need to have a sample rate at least 2 times as fast as the signal frequency.  In order to get accurate amplitude data you should aim for about 10 times faster. 

It sounds like your signal has a period less than 2 Hz; you probably want to sample at 20 - 2kHz.  A sample rate in this range would provide 10-1000 samples/period; and this is well within the range of sample rates that the PCI-6251 can support.  This card can sample at up to 1 MS/s divided among the channels you're using.  If you're using 2 channels then it could be up to 500 kS/s, or for 3 channels it would be 333 kS/s.  This would be more than sufficient to sample the signals that you've described.  Even if you needed to use all 16 channels at once you could still sample at 62.5 kS/s. 

I hope this helps, and feel free to post back or contact National Instruments directly if you have more questions about this device before you order. 

Cheers,

Message Edited by Brooks_C on 08-18-2008 11:59 AM