National Instruments Data Acquisition Device E Series - Manual (Page 135)

Chapter 10 Triggering
E Series User Manual 10-6 ni.com
Figure 10-7 demonstrates a trigger that asserts when the signal enters the
window.
Figure 10-7. Window 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 and resolution of the analog trigger DACs.
To improve accuracy you can use one of the following methods:
• Use an AI channel (with a small input range) instead of PFI 0/AI
START TRIG as your trigger source. The DAQ device does not
amplify the PFI 0/AI START TRIG signal. When using an AI channel,
the 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. No hardware
calibration is provided for the analog trigger circuitry. In addition, 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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Analog Comparison Event
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