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External Clock
The PXIe-4340 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.
Digital Triggering
You can configure the PXIe-4340 modules 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-4340 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 can also route the
resulting reference trigger event to supported digital terminals. Refer to the Device Routes in
NI Measurement & Automation Explorer (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
never greater than one sample period, 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-4340 modules including analog
edge, analog edge with hysteresis, and window triggering.
Triggering and Filter Delay
The PXIe-4340 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 when starting an acquisition. Since linear phase FIR filters are used
in the digital filtering, it is necessary to wait for the filter group delay to elapse after sending a