Chapter 2
Using the NI 447X
2-14
ni.com
Device Configuration Issues
When testing your NI 447
X
, it is important to remember that it is equipped
with high performance antialiasing filters. Because of these filters, you
cannot acquire a signal whose fundamental frequency exceeds one half the
sampling rate. If you try to resolve such a signal, it is filtered as an alias and
you see only a DC reading. This is the expected functionality for the
NI 447
X
. If you suspect the filters are not passing your test signal, simply
increase the sampling rate or decrease the analog signal frequency until the
sampling rate is more than twice the analog frequency. For more
information on the filters and aliasing, refer to the
section of Chapter 3,
Device Overview and Theory of Operation
Note
Unlike other converter technologies, delta-sigma converters must be run
continuously and at a minimum clock rate to operate within specifications. Although the
software lets you use a lower sample rate, you must
always
use a sample rate of at least
1.0 kS/s to ensure the accuracy of the data acquisition.
