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3-11
Timing Engines
When you create a task in software, that software task interacts with a timing engine in the
PXIe-4480/4481 hardware. There are a total of three timing engines in hardware that can be
operated simultaneously, meaning that three separate tasks can be executed simultaneously,
each with different sampling rates and triggering settings. This provides the user with a
significant amount of freedom in running multiple sample rates, but there are some limitations:
•
The reference clock source must be the same for all tasks, because there is only one
reference clock per module.
•
A task can only use analog input channels that are not already being used by other tasks.
Digital Triggering
You can configure the PXIe-4480/4481 module to start an acquisition in response to a digital
trigger signal from one of the PXI Express backplane trigger lines or the PFI from the front
connector. The trigger circuit can respond either to a rising or a falling edge.
Analog Triggering
Analog triggering allows you to trigger your application based on an input signal and trigger
level you define. You can configure the analog trigger circuitry to monitor any input channel
acquiring data. Choosing an input channel as the trigger channel does not change the input
channel acquisition specifications.
The analog trigger signal can be used as a reference trigger only. In a reference-triggered
acquisition, you configure the module to acquire a certain number of pre-trigger samples and a
certain number of post-trigger samples. Reference-triggered acquisitions can therefore only be
configured as finite tasks. The analog trigger on the PXIe-4480/4481 cannot be used as a start
trigger. This restriction is a result of the way the module compensates for the filter group delay.
When using an analog reference trigger, the module first waits for the specified number of
pre-trigger samples to be acquired. Once enough pre-trigger samples are acquired, the reference
trigger will occur the next time the analog trigger condition is met. You also can route the
resulting reference trigger event to supported digital terminals. Refer to the
Device Routes
tab
in NI-MAX for additional information.
During repetitive triggering on a waveform, you might observe jitter because of the uncertainty
of where a trigger level falls compared to the actual digitized data. Although this trigger jitter is
usually never greater than one sample period
1
, it might be significant when the sample rate is
only twice the bandwidth of interest. This jitter usually has no effect on data processing, and you
can decrease this jitter by sampling at a higher rate.
You can use several analog triggering modes with the PXIe-4480/4481 modules, including
analog edge, analog edge with hysteresis, and window triggering.
1
For multidevice tasks, trigger jitter when sampling at 20 MS/s may be two sample periods due to hardware
limitations. At all other supported sample rates, jitter is never greater than one sample period.
