Hi Ken,
Thanks for posting to the NI Discussion Forums! From your pictures, I saw that you are using an NI-9239. From page 16 of the NI-9239 Operating Instruction and Specifications,
it explains that the valid rates for sampling are derived from the
timebase by first dividing this value by 256. Then, the value may
further be divided down by an integer value from 1-31. Thus, valid
sampling rates may be as high as 50 kS/s down to 1.613 kS/s when using
the internal timebase of 12.8 MHz.
In your case, you have specified a
sample rate of 300 S/s, which is much less than the lowest possible
rate of 1.613 kS/s. Therefore, the board is coercing this value up to
1.613 kS/s and the inverse of this is the dt value you are seeing,
which is 0.00062 seconds. To get an actual sample rate less than this
value, you will need to provide an external clock. For example, to
achieve a clock rate of 300 S/s, you would need to provide an external
clock at an integer multiple of 76.8 kHz (up to 2.3808 MHz).
Also, thanks for adding the attachments to your post. This gives other users easier access to the information to both provide help
and also to be able to view all the relevant information when exploring
the post in the future (in case anyone else runs into the same type of
problem). In any case, thanks again for the post and for all the
details of your problem! Cheers,
Daniel S.
National Instruments
