Choose the Right Trigger and Get the Right Data

Different events require different triggers. When you measure a battery’s discharge rate, for example, you may not need a precise acquisition start time; a software trigger may be all you need. Software triggers are easy to develop. You program the system to start an acquisition when a software event occurs, such as when a program starts or when you click on a button on a user screen. Listing 1 contains some code for a software trigger. Once you start the program, the system acquires 1000 samples at 1 ksample/s.

Many data-acquisition applications require software triggers only. Applications that employ triggers beyond basic software triggering usually do so to reference the acquisition to an external event or time. For example, when you measure vibration in a crash test, you may need to capture data for the 10 ms that immediately follow the impact. The acquisition must trigger at a specific time, based on an analog voltage proportional to vibration. Here, you need an analog hardware trigger.

Trigger on a Level
Analog triggers let you trigger an acquisition when a signal reaches a specified level. In an analog trigger, the signal itself determines whether or not you acquire it. There are two ways in which to implement analog triggers: in software or in hardware. Software analog triggering basically involves continuously acquiring data to a buffer and processing that buffer to see if the signal has met the desired criteria. When the signal meets the criteria, the program returns data to the user.

A hardware analog trigger requires a separate circuit that digitizes the analog signal on which you base the trigger decision. When the signal of interest meets the specified level and slope conditions, the trigger circuit initiates the acquisition. In the case of the car crash, you might use a hardware analog trigger because acquisition need not begin until impact occurs.

If you set the trigger level properly, you won’t get a false trigger caused by minor changes in the trigger signal. And you get only the time span you need. The code in Listing 2 illustrates how to acquire 1000 samples at 10 ksamples/s when the input signal increases through 3.0 V.

With a digital trigger, your system begins an acquisition on the transition of a digital signal. You can trigger the acquisition on either a rising edge or falling edge. If your equipment monitors the vibration or power consumption of an electric motor, then you may need a digital trigger. If a digital signal starts or stops the motor, you can use that signal to trigger your data-acquisition system.

Pre or Post-Trigger?
When an external event must trigger an acquisition, you must decide if you need a post-trigger or a pretrigger. In a post-triggered acquisition, such as the crash test, the trigger occurs and then the system collects all the data you want.

A pretriggered acquisition, initiated by a “stop trigger,” collects data before and after the trigger event occurs. If you want to perform a pretriggered acquisition, you can set up your system to begin digitizing on a software trigger, collect data, and place the data into a memory buffer. When the stop trigger occurs, the ADC continues to collect a predetermined number of samples. When you use a pretrigger you must preset the number of pretrigger and post-trigger samples the system will acquire.

You can set up a digital post-triggered acquisition; the system acquires data only after the trigger signal occurs and the motor starts. Alternatively, you can use a pretriggered acquisition if you need the data around the time of an event, such as a motor shutdown. The system starts acquiring data based on a software command while the motor runs, and the power-off signal becomes the stop trigger. You can program the system to acquire a specific number of points before and after that trigger. T&MW

Chad Chesney is the data acquisition product marketing manager and Josh Martin is an applications engineer at National Instruments.