Triggering with High-Speed Digitizers

Return to Fundamentals of High-Speed Digitizers

This tutorial recommends tips and techniques for using National Instruments high-speed digitizers to build the most effective data sampling system possible. In this tutorial, you will learn fundamental information about the underlying theory of sampling with a high-speed digitizer and various methods to optimize the performance of your data sampling. This section of the tutorial covers the topics below.

What is Triggering?

A trigger is an external stimulus that initiates one or more instrument functions. Trigger stimuli include digital edges, software functions, and analog levels. The trigger can be derived from attributes of the signal to be acquired, such as the level and slope of the signal.

You can use several kinds of triggering with NI high-speed digitizers, including immediate, software, hysteresis, edge, window, digital, and video triggering. Each kind of triggering uses a different NI-SCOPE Configure Trigger function.

Immediate Triggers

Immediate triggers occur when the digitizer triggers itself. There is no external signal that triggers the acquisition—the acquisition simply begins immediately after being configured by the driver software. Immediate triggering is the default option on NI high-speed digitizers. Therefore, you can omit using this function if you never change triggering modes.

Software Triggers

Software triggers occur when a software command starts the acquisition of posttrigger data. When you call Initiate Acquisition, the digitizer starts acquiring pretrigger data and continues to do so until you call Send Software Trigger Edge. The digitizer continues to store posttrigger samples after the software trigger, so you still need to wait for the acquisition to complete after the trigger. You can do this by setting the timeout parameter in a Fetch function to a positive value. You cannot use a Read function in conjunction with software triggering; you must use Initiate Acquisition followed by one of the Fetch functions. The following figure shows the programming flow using software trigger functions.

Note  Not all digitizers support software triggering. Refer to Features Supported by Device in the NI High-Speed Digitizers Help file to find out which digitizers support software triggering.

Edge Triggers

An edge trigger occurs when a signal crosses a trigger threshold that you specify. You can specify the slope as either positive (on the rising edge) or negative (on the falling edge) to the trigger. Edge triggering is possible on all analog trigger channels, such as 0, 1, or on the external trigger channel. The following figure shows edge triggering. A window trigger occurs when a signal either enters or leaves a window you specify with the window mode parameters in the Configure Trigger Window function. Window triggering is possible on all analog trigger channels, such as 0, 1, or the external trigger input.

The following figure shows an entering window trigger.

The following figure shows a leaving window trigger.Hysteresis triggers eliminate incorrect triggers caused by noisy signals. For example, if your signal contains two rising edges of different amplitudes, you can use hysteresis triggering to trigger on one of the edges. Although NI-SCOPE uses a default amount of hysteresis for edge triggering, which is typically 2.5% of the vertical range, you can override that value by setting your own hysteresis values. The Configure Trigger Hysteresis function or VI allows you to choose the trigger coupling, trigger level, hysteresis value, and trigger slope.

Hysteresis triggering is possible on all analog trigger channels, such as 0, 1, or the external trigger input.

A positive slope hysteresis trigger is generated when the signal crosses below the voltage specified by the trigger level parameter minus the hysteresis value, and then crosses the trigger level.

A negative slope hysteresis trigger is generated when a signal crosses above the voltage specified by the trigger level parameter plus the hysteresis value, and then crosses the trigger level.
A digital trigger occurs on either a rising edge or falling edge of a digital signal. Digital triggering is possible on the RTSI lines, PFI lines, and the PXI Star Trigger line.

Video Triggers

A video trigger occurs when the digitizer finds a valid video signal synchronization. Refer to NI 5122/5124 Video Triggering for more information.
Related Links:
Fundamentals of High-Speed Digitizers
High-Speed Digitizers FAQ
Utilizing Deep Memory on High-Speed Digitizers and Oscilloscopes
Minimum Voltage that You Can Edge Trigger off of with an NI High Speed Digitizer (5102 5911 5112)
Benefits of Deep Memory in High-Speed Digitizers

Reader Comments | Submit a comment »