Triggering

The PXIe-6368 device supports internal software triggering, external digital triggering, and analog triggering.

A trigger is a signal that causes an action, such as starting or stopping the acquisition of data. When you configure a trigger, you must decide how you want to produce the trigger and the action you want the trigger to cause.
Note For more information about triggering compatibility, refer to the PXIe-6368 Specifications

Triggering with a Digital Source

The PXIe-6368 can generate a trigger on a digital signal. You must specify a source and an edge. The digital source can be any of the PFI, RTSI, or PXI_STAR signals.

The edge can be either the rising edge or falling edge of the digital signal. A rising edge is a transition from a low logic level to a high logic level. A falling edge is a high-to-low transition. The following figure shows a falling-edge trigger.

Figure 84. Falling-Edge Trigger

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You can also program your DAQ device to perform an action in response to a trigger from a digital source. The action can affect the following:

  • Analog input acquisition
  • Analog output generation
  • Counter behavior
  • Digital waveform acquisition and generation

Triggering with an Analog Source

The PXIe-6368 device can generate a trigger on an analog signal. To find your device triggering options, refer to the PXIe-6368 Specifications.

The following figure shows the analog trigger circuit on the PXIe-6368.
Figure 85. Analog Trigger Circuitry

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You must specify a source and an analog trigger type. The source can be either an APFI <0,1> terminal or an analog input channel.

APFI <0,1> Terminals

When you use either APFI <0,1> terminal as an analog trigger, drive the terminal with a low impedance signal source (less than 1 kΩ source impedance). Unconnected APFI <0,1> signals are susceptible to crosstalk from adjacent terminals which can cause false triggering. Note that the APFI <0,1> terminals can also be used for other functions such as the AO External Reference input.

Analog Input Channels

The PXIe-6368 is a Simultaneous Multifunction I/O (SMIO) device. With SMIO devices every AI channel drives its own NI-PGIA. The NI-PGIA amplifies the signal as determined by the input range. The output of the NI-PGIA then drives the analog trigger detection circuit. By using the NI-PGIA, you can trigger on very small voltage changes in the input signal.

There are no restrictions on the analog input channel list order or number of channels with reference and pause triggers. However, the analog input channels must be in the scan list.

Analog Trigger Actions

The output of the analog trigger detection circuit is the Analog Comparison Event signal. You can program your PXIe-6368 device to perform an action in response to the Analog Comparison Event signal.

The action can affect the following:

  • Analog input acquisition
  • Analog output generation
  • Digital input behavior
  • Digital output behavior
  • Counter behavior

Routing Analog Comparison Event to an Output Terminal

You can route Analog Comparison Event out to any PFI <0..15> or RTSI <0..7> terminal.

Analog Trigger Types

You can configure the analog trigger circuitry to different triggering modes.

Analog Edge Triggering

Configure the analog trigger circuitry to detect when the analog signal is below or above a level you specify.

The following figure illustrates below-level analog triggering mode, where the trigger is generated when the signal value is less than Level.

Figure 86. Below-Level Analog Triggering Mode

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The following figure illustrates above-level analog triggering mode, where the trigger is generated when the signal value is greater than Level.

Figure 87. Above-Level Analog Triggering Mode

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Analog Edge Triggering with Hysteresis

Hysteresis adds a programmable voltage region above or below the trigger level that an input signal must pass through before the DAQ device recognizes a trigger condition, and is often used to reduce false triggering due to noise or jitter in the signal.

When using hysteresis with a rising slope, you specify a trigger level and amount of hysteresis. The high threshold is the trigger level; the low threshold is the trigger level minus the hysteresis.

For the trigger to assert, the signal must first be below the low threshold, then go above the high threshold. The trigger stays asserted until the signal returns below the low threshold. The output of the trigger detection circuitry is the internal Analog Comparison Event signal, as shown in the following figure.

Figure 88. Analog Edge Triggering with Hysteresis Rising Slope Example

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When using hysteresis with a falling slope, you specify a trigger level and amount of hysteresis. The low threshold is the trigger level; the high threshold is the trigger level plus the hysteresis.

For the trigger to assert, the signal must first be above the high threshold, then go below the low threshold. The trigger stays asserted until the signal returns above the high threshold. The output of the trigger detection circuitry is the internal Analog Comparison Event signal, as shown in the following Figure.

Figure 89. Analog Edge Triggering with Hysteresis Falling Slope Example

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Analog Window Triggering

An analog window trigger occurs when an analog signal either passes into (enters) or passes out of (leaves) a window defined by two voltage levels. Specify the levels by setting the window Top value and the window Bottom value.

The following figure demonstrates a trigger that asserts when the signal enters the window.

Figure 90. Analog Window Triggering Mode (Entering Window)

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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 PXIe-6368 adjusts the output of the trigger DACs. Refer to the PXIe-6368 Specifications for the accuracy or resolution of these DACs, which also shows the accuracy or resolution of analog triggers.

Follow these guidelines to improve accuracy:

  • Use an AI channel with a small input range instead of APFI <0,1> as your trigger source. The PXIe-6368 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.