Analog Trigger Accuracy - National Instruments DAQ X Series User Manual

Chapter 11 Triggering

Analog Trigger Accuracy

The analog trigger circuitry compares the voltage of the trigger source to
the output of programmable trigger DACs. When you configure the level
(or the high and low limits in window trigger mode), the device adjusts the
output of the trigger DACs. Refer to the specifications document for your
device to find the accuracy or resolution of these DACs, which also shows
the accuracy or resolution of analog triggers.
To improve accuracy, do the following:
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Use an AI channel (with a small input range) instead of APFI <0,1> as
your trigger source. The DAQ device does not amplify the
APFI <0, 1> signals. When using an AI channel, the NI-PGIA
amplifies the AI channel signal before driving the analog trigger
circuitry. If you configure the AI channel to have a small input range,
you can trigger on very small voltage changes in the input signal.
Software-calibrate the analog trigger circuitry. The propagation delay
from when a valid trigger condition is met to when the analog trigger
circuitry emits the Analog Comparison Event may have an impact on
your measurements if the trigger signal has a high slew rate. If you find
these conditions have a noticeable impact on your measurements, you
can perform software calibration on the analog trigger circuitry by
configuring your task as normal and applying a known signal for your
analog trigger. Comparing the observed results against the expected
results, you can calculate the necessary offsets to apply in software to
fine-tune the desired triggering behavior.
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