Trigger - National Instruments NI 4472 User Manual

Trigger

© National Instruments Corporation
In addition to supporting internal software triggering and external digital
triggering to initiate a data acquisition sequence, the NI 4472 also supports
analog-level triggering. You can configure the trigger circuit to monitor
any one of the analog input channels to generate the level trigger. Choosing
an input channel as the level trigger channel does not influence the input
channel capabilities. The level trigger circuit compares the full 24 bits of
the programmed trigger level with the digitized 24-bit sample.
The trigger circuit generates an internal digital trigger based on the input
signal and the user-defined trigger levels. Any of the timing sections of the
DAQ-STC can use this level trigger, including the analog input, RTSI, and
general-purpose counter/timer sections. For example, you can configure the
analog input section to acquire a given number of samples after the analog
input signal crosses a specific threshold.
Due to the nature of delta-sigma converters, the triggering circuits operate
on the digital output of the converter. Since the trigger is generated at the
output of the converter, triggers can occur only when a sample is actually
generated. Placing the triggering circuits on the digital side of the converter
does not affect most measurements unless an analog output is generated
based on the input trigger. In this case, you account for the inherent delays
of the finite impulse response (FIR) filters internal to the delta-sigma
converters. The delay through the input converter is 38.8 sample periods.
During repetitive sampling of a waveform, you might observe jitter due
to 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 can seem quite significant when the sample rate is only twice the
bandwidth of interest. This jitter has no effect on the processing of the data,
and you can decrease this jitter by sampling at a higher rate.
Five analog level triggering modes are available, as shown in Figures 3-8
through 3-12. You can set lowValue and highValue independently in the
software.
Chapter 3 Device Overview and Theory of Operation
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