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High-Speed Digitizer Hysteresis Trigger



Hardware: Modular Instruments>>High-Speed Digitizers (Scopes)>>PXI-5102

Problem:
How does the hysteresis trigger work on High-Speed Digitizers?

Solution:
The hysteresis trigger removes incorrect trigger behavior as a result of noise. Hysteresis triggering is achievable on all analog trigger channels and can be generated through both a positive or negative slope.

The positive slope hysteresis trigger is illustrated below. For the NI-5102, NI-5105, NI-5112, NI-5114, NI-5122, and NI-5922, it is important to know that the first trigger will be generated when the signal passes the trigger level with a positive slope. This occurs regardless of whether the signal has crossed the hysteresis level (trigger level – hysteresis value). After the first trigger, all future triggers will only occur when the signal crosses the hysteresis level and afterward crosses the trigger level with a positive slope.

Hysteresis Levels
The negative slope hysteresis trigger is illustrated in the following image. The first trigger will be generated when the signal passes the trigger level with a negative slope. Similar to the positive case, this occurs even though the signal has not crossed the hysteresis (trigger level + hysteresis value). Once the first trigger has occurred, all further triggers will only occur when the signal crosses the hysteresis level and then crosses the trigger level with a negative slope.

Negative Edge Hysteresis
Triggering without crossing the hysteresis level only applies to the very first trigger on the models listed above if the digitizer begins sampling while the signal is between the trigger and hysteresis levels. All other models require the hysteresis level to have been crossed even for the first trigger.

If this behavior is unsuitable and you would like the first trigger to be ignored if the hysteresis level has not yet been crossed, a software work-around is possible. The easiest way is to fetch the very first digitized sample to determine if it is between the trigger and hysteresis levels. This can be done by setting the Fetch Relative To property to Start, fetching 1 sample, and changing this property back to Pretrigger (default setting). If the initial sample falls between the two levels, the first record (record 0) can be discarded. Refer to the NI High-Speed Digitizers Help for more info on this property.

Related Links:
Product Manuals: High-Speed Digitizers Help: Hysteresis
Product Manuals: High-Speed Digitizers Help (December 2010)
Whitepaper: Triggering with High-Speed Digitizers
Specifications Explained: NI Oscilloscopes and Digitizers

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Report Date: 06/04/2008
Last Updated: 10/04/2017
Document ID: 4M37TVON

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