You can configure the analog trigger circuitry of the USB-6421 to monitor any analog input channel from which you acquire data.

Choosing an analog input channel as the trigger channel does not influence its acquisition capabilities.

You can use the analog trigger signal as a reference trigger only. This restriction is because the analog trigger circuit operates on digitized ADC data, requiring the acquisition to be running for the analog trigger circuit to operate. In a reference-triggered acquisition, you configure the device to acquire a certain number of pre-trigger samples and post-trigger samples.

The trigger circuit generates an internal digital trigger based on the input signal and the trigger levels you define.

For example, you can configure the USB-6421 to generate an analog comparison event after the input signal crosses a specific threshold. You can also route the resulting reference trigger event to other internal subsystems to synchronize the subsystems.

During repetitive triggering on a waveform, you might observe jitter. This jitter is caused by the uncertainty of where a trigger level falls compared to the actual digitized data. Although this trigger jitter is never greater than one sample period, it might be significant when the sample rate is only twice the bandwidth of interest. This jitter usually has no effect on data processing. You can decrease this jitter by sampling at a higher rate. Sampling at a rate less than twice the bandwidth of interest might cause the USB-6421 to detect the trigger signal less reliably.

You can use the following analog triggering modes with the USB-6421.

  • Rising edge
  • Rising edge with hysteresis
  • Falling edge
  • Falling edge with hysteresis
  • Entering window
  • Leaving window

Analog Edge Triggering

For analog edge triggering, configure the USB-6421 to detect a certain rising or falling signal level and slope.

The following figure shows an example of rising edge analog triggering. The trigger asserts when the signal starts below level and then crosses above level.

Figure 40. Analog Trigger Level on Rising Edge


Analog Edge Triggering with Hysteresis

When you add hysteresis to analog edge triggering, you add a window above or below the trigger level.

You can use this trigger to reduce false triggering due to noise or jitter in the signal. For example, if you add a hysteresis of 1 V to the example in the figure above, which uses a level of 3.2 V, the signal must start at or drop below 2.2 V to arm the trigger. The trigger asserts when the signal rises above 3.2 V and deasserts when it falls below 2.2 V, as shown in the following figure.

Figure 41. Analog Edge Triggering with Hysteresis on Rising Slope


When using hysteresis with a falling slope, the trigger is armed when the signal starts above Level, plus the hysteresis value, and asserts when the signal crosses below level. For example, if you add a hysteresis of 1 V to a level of 3.2 V, the signal must start at or rise above 4.2 V to arm the trigger. The trigger asserts as the signal falls below 3.2 V and deasserts when it rises above 4.2 V, as shown in the following figure.

Figure 42. Analog Edge Triggering with Hysteresis on Falling Slope


Window Triggers

A window trigger occurs when an analog signal either passes into (enters) or passes out of (leaves) a window defined by two levels.

Specify the levels by setting a value for the top and bottom window boundaries. The following figure demonstrates a trigger that acquires data when the signal enters the window. You can also program the trigger circuit to acquire data when the signal leaves the window.

Figure 43. Window Triggering with Enter Window