Chapter 2
Using the NI 447X
2-12
ni.com
Selecting the Sample Clock Frequency
The eight AI channels of the NI PCI/PXI-4472 and the four AI channels
of the NI PCI-4474 are simultaneously sampled at any
software-programmable rate from 102.4 kS/s down to 1.0 kS/s in
190.7
µ
S/s increments for
f
s
> 51.2 kS/s or 95.37
µ
S/s increments for
f
s
≤
51.2 kS/s. The NI 447
X
uses direct digital synthesis (DDS) technology
so that you can choose the correct sample rate for your application. All of
the input channels acquire data at the same rate. One input channel
cannot
acquire data at a different rate from another input channel.
Notes
If you do not specify a rate at a multiple of the increment, NI-DAQ automatically
chooses the next higher step for you.
Unlike other converter technologies, delta-sigma converters must be run continuously and
at a minimum clock rate to operate within specifications. Although the software allows you
to 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.
Synchronizing Multiple Devices
This section provides basic instructions for synchronizing multiple
NI 447
X
devices. For more information about how NI 447
X
devices are
synchronized, refer to the
Synchronizing Multiple Devices
section of
Chapter 3,
Device Overview and Theory of Operation
The NI 447
X
can send or receive the Direct Digital Synthesis (DDS) clock
signal and the synchronization start signal to or from other NI 447
X
devices
on the same bus to synchronize data acquisition. In a multi-device system,
a master device drives the clock and synchronization signals to other slave,
or receiving, devices.
Caution
Do
not
use RTSI 5/TRIG 5 to drive any signals in the system if you are
synchronizing multiple NI PXI-447
X
devices. The synchronization signal is driven on
RTSI 5/TRIG 5, so driving other signals on RTSI 5/TRIG 5 could lead to double-driving
the line, which might result in unpredictable behavior and might damage your system.
