Chapter 2
Using the Module
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National Instruments Corporation
2-33
Triggering and Filter Delay
The NI PXIe-4330/4331 can use a digital trigger signal from the backplane as either a start
trigger initiating an acquisition, or a reference trigger in the middle of an acquisition. The
NI PXIe-4330/4331 can also generate an analog trigger event from its digitized ADC data. As
a result of the way the NI PXIe-4330/4331 modules adjust for its digital filter group delay,
analog triggers can only act as reference triggers.
In all cases, the NI PXIe-4330/4331 interprets triggers based on where they occur in time. The
hardware automatically compensates for its group delay such that data from this module will
line up closely in time with the occurrence of the trigger event. However, the group delay
affects how long it takes to receive data after the trigger event. For example, after a digital start
trigger, you can not read data for the first sample in software until the filter group delay has
elapsed. Refer to the
NI PXIe-4330/4331 Specifications
document for details regarding the
group delay at different sample rates.
Synchronization
Some applications require tight synchronization between input and output operations on
multiple modules. Synchronization is important to minimize skew between channels or to
eliminate clock drift between modules in long-duration operations. You can synchronize the
analog input operations on two or more NI PXIe-4330/4331 modules to extend the channel
count for your measurements. In addition, the NI PXIe-4330/4331 can synchronize with
certain other DSA modules, such as the NI PXIe-449x modules, using Reference Clock
Synchronization.
You can also synchronize modules that support external timing to the NI PXIe-4330/4331 by
routing the SampleClock output from the NI PXIe-4330/4331 to the external device. The
SampleClock toggles every time that the NI PXIe-4330/4331 acquires a sample, and it toggles
at the point in time that the input signal was valid at the NI PXIe-4330/4331 ADC input pins.
This means that you can synchronize the NI PXIe-4330/4331 to any other module that can
trigger its acquisitions using the NI PXIe-4330/4331 SampleClock.
Reference Clock Synchronization
With reference clock synchronization, master and slave modules generate their ADC
oversample clock from the shared 100 MHz reference clock from the PXIe backplane
(PXIe_CLK100). The backplane supplies an identical copy of this clock to each peripheral
slot. In addition, multiple chassis can be synchronized by using a timing and synchronization
board to lock the 100 MHz clock across chassis.
When you acquire data from multiple modules within the same NI-DAQmx task, NI-DAQmx
will automatically handle all of the Reference Clock Synchronization details required to
synchronize the modules within the task. This is known as a Multi-Device Task.
