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Chapter 3
Signal Acquisition
For example, if four samples are acquired from two channels, the total number of samples to be
transferred is eight, and lossless packing can be used. On the other hand, if three samples are
acquired from two channels, then the total number of samples to be transferred is six. Because
six is not a multiple of four, an attempt to use lossless packing will cause NI-DAQmx to generate
an error.
PXI Express System Bandwidth
In order to continuously transfer large amounts of data, the entire PXI Express system must be
designed with sufficient data bandwidth. The PXIe-4480/4481 uses a x4 PXI Express connection
to the PXI Express chassis backplane. Note that while the PXIe-4480/4481 will operate with a
x1 connection, the amount of data that can be sent to the host computer will be limited and
therefore tasks using multiple channels at higher sample rates may experience buffer overflow
errors.
Likewise, the bandwidth between the PXI Express chassis and host computer can also limit
data throughput. When selecting a PXI Express embedded controller, make sure the controller
specifies a sufficient per slot bandwidth (for example, at least 500 MB/s slot bandwidth) and
a sufficient system bandwidth to operate all devices installed in the system. If a PXI Express
remote controller is selected, it should use a x4 or higher connection. A x1 PXI Express remote
controller does not have sufficient bandwidth to allow all channels to acquire at 20 MS/s.
Timing and Triggering
Sample Clock Timebase
The ADCs require an oversample clock to drive the conversion. The oversample clock
frequency is greater than the sample rate. On the PXIe-4480/4481 modules, the oversample
clock is produced from an 80 MHz reference clock. This 80 MHz reference clock can be phase
locked with the PXIe backplane 100 MHz clock, or it can be generated by an internal timebase
that runs freely. Multiple modules can be synchronized by selecting the PXIe backplane
100 MHz clock as the reference clock source for all the modules. Refer to the
section for more information.
External Clock
The PXIe-4480/4481 ADCs cannot be clocked from external sources such as encoders or
tachometers. However, signal processing features in the Sound and Vibration Measurement
Suite often provide an excellent alternative to external clocking in encoder and tachometer
applications. Visit
for more information about the Sound and
Vibration Measurement Suite.
