how can i change "dt" in an acquired waveform with DAQ assitant? Changing Sample Rate doesn't work as expected

[mike20]

when I acquire a waveform with DAQ assitant at 4000 S/s, continuous sampling, and 40000 Samples...I get a "dt" of .000234375.

Changing the 4000 S/s rate does not change the "dt" until I get to 4267 S/s....then the "dt" i get is .000195...

how is "dt" being calculated? How can I adjust it to a desired value?

endgame: I'd like to be able to sample at 4096 S/s which should give me a "dt" of .000244141, which leads to a frequency resolution of 1.000 when the waveform is an input into a frequency domain vi.

[bstreiff]

1/.000234375 == 4266.66... Hz.

When you set up your sample rate, the DAQmx driver will attempt to choose a timebase and timebase divisor to give you that sample rate. In your case, the closest it could get to is 4266.66 Hz.

What DAQ device are you acquiring from? Most Multifunction DAQ devices have a 20MHz timebase that will divide down evenly to get 4kHz, so I'm a little surprised that you're not able to acheive that.

• KB: What Sample Rates Is my DAQ Board Actually Capable of Achieving?
• KB: How Is the Actual Scan Rate Determined When I Specify the Scan Rate for My Data Acquisition?

[mike20]

Thank you for the information.

I am using a Compact DAQ chassis 9178...and 8 9234 DAQ cards, but I may be switching to a more stable setup (ie PXI)

Any more information would be greatly appreciated. Thanks in advance

[Seth_B.]

With the NI 9234, you can see how to calculate possible data rates from page 16 of the NI 9234 Operating Instructions and Specifications.

The formula is:

Where fm is the Internal Master Timebase (from the specifications):

Internal master timebase: Frequency ...................................13.1072 MHz

Thus, the closest rates you can use to 4096 are:

n = 13   Rate: 3938.462 Hz

n = 12   Rate: 4266.667 Hz (this is what DAQmx is coercing the value to when you request 4096)

n = 11   Rate: 4654.545 Hz

Unfortunatly, these are the only rates near 4096Hz that you will be able to achieve.

[Tom_W_[DE]]

To add to Seth's post, this KB has a nice reference guide for some of the modules with this restriction.