Chapter 2
Using the Module
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National Instruments Corporation
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For any on-demand, software timed acquisition, the NI PXIe-4330/4331 waits for the
group delay to elapse before returning the sample. As a result, the data returned aligns
closely in time with when the data was requested. However, as a result, you must wait for
the group delay to elapse before this sample is available.
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Similarly, for hardware timed acquisition at very low sample rates, you might notice that
it takes several seconds for an acquisition to begin. This is because during start, the ADCs
get reset and have to wait many samples before delta sigmas are operational. That time
is dependent on the sample rate.
To improve the time it takes an acquisition to begin, select a higher sample rate.
Supported Data Rates
The NI PXIe-4330/4331 supports rates of 1 S/s to 100 S/s in 1 S/s increments. Rates beyond
100 S/s are supported in 100 S/s increments up to the maximum rate for the module.
Timing and Triggering
This section contains information about timing and triggering theory of operation.
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 NI PXIe-4330/4331 modules the oversample
clock is produced from a DDS clock generation circuit. The DDS, in turn, is run from a
100 MHz reference clock. This 100 MHz reference clock can be supplied either by an
onboard oscillator or by the PXI backplane 100 MHz clock. Multiple modules can be
synchronized by selecting the PXI backplane 100 MHz clock as the reference clock source
for all the modules. Refer to the
Reference Clock Synchronization
section for more
information.
External Clock
The NI PXIe-4330/4331 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
ni.com/soundandvibration
for more information about the Sound
and Vibration Measurement Suite.
