How can I get stable Sin wave through NI 9264 ?

Question

Dear all,
I would like to generate 64 analog sin wave with frequency up to 20K Hz.
Then I bought 4 NI9264:
25 kS/s/ch simultaneous output
Hot-swappable operation
Spring terminal or D-SUB connectivity options
250 Vrms isolation (spring terminal version)
60 VDC isolation (D-SUB version)
.The attached is my file for the generation. when I used Osciloscope to monitor the output, I could not get any stable output.
 Could I have the reasons and suggestions for my objective please?
Thanks!
Zhu_nuo
Attachments:
64 Channel Programmatic v2.vi ‏22 KB

Answer 1

Hi Zhuming,
The VI above only provides waveform information for a single channel. In order to output data across all 64 channels in the task, we can make use of a For Loop to build an array of waveform information for each channel. In the example below, I've made use of an Auto-Indexing For Loop in order to build up a waveform output array of 64 elements. I've wired this to an NI-DAQmx Analog 1D Wfm Write VI; the 1D array's elements represents each channel in the group of selected channels for generation.
An important thing to note is that the NI 9264 16 channel analogue output modules have a maximum sample rate of 25 kS/s/ch. This means that each channel is capable of writing 25kS/s simultaneously.
To reliably generate a signal, we generally need to have it comprised of around 5 to 10 points of data. This allows us to represent the signal to a good degree of accuracy. For example, if I wanted to reliably output a 1Hz Sine Wave, I would need to break the 1Hz wave into ten separate points and then output them at a rate of 10Hz. In terms of the NI 9264, the maximum frequency content (Two points of information) that can be generated is 25kHz/2 = 12.5kHz wave. The maximum clear wave frequency that we could produce with the 9264 (A wave built up of ten points) is 2.5kHz simultaneously across each channel. This is all to do with Nyquist's Theorem. This should explain why the frequency appeared to drop.
Out of interest, how are you measuring these output frequencies?
Kind Regards,
Alex Thomas, University of Manchester School of EEE LabVIEW Ambassador (CLAD)