Analog Triggering Considerations for C Series, E Series, M Series, and S Series Devices
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Note Not all E Series, M Series, S Series, and C Series devices support analog triggering. Refer to the specifications for your device to determine if your device supports analog triggering. |
Many C Series, E Series, M Series, and S Series devices contain a single analog trigger circuit that you can configure for analog triggering. The analog trigger circuitry is a shared resource for the device, and any of the subsystems can use it. This trigger circuitry supports level and slope triggering with hysteresis as well as analog window triggering. After it is configured, the output of this circuitry appears as the Analog Comparison Event, which can be the source for various triggers and clocks within the analog input, analog output, and counter subsystems.
Sharing an Analog Trigger
Even though the analog trigger is a shared resource, only one analog input or analog output task at a time can configure and reserve it. If you want to share the analog trigger among multiple tasks, configure and reserve it in one task, and use the trigger in subsequent tasks by referring to the source of your trigger, clock, or signal of interest as the Analog Comparison Event. For tasks that support multiple types of analog triggers within the same task, all triggers must share the same configuration settings, or you receive an error. For instance, if you want to use an analog trigger for both your Start and Reference Trigger within an analog input task, the configuration settings for the start and Reference Trigger must be identical.
E Series and S Series Valid Sources for the Analog Trigger
PFI 0
Typically, when configuring an analog trigger, you connect your analog signal to the PFI 0 terminal. Because PFI 0 is the trigger source for both analog and digital signals, NI-DAQmx automatically tristates this terminal when a task exporting a signal on the terminal is not in the committed or running state. This behavior when exporting a signal on PFI 0 differs from typical task-based routing with other PFI lines. It prevents accidental connections of an analog signal directly to digital circuitry, which could permanently damage the device. Also, notice that when connecting an analog signal to PFI 0, the terminal configuration is referenced single-ended.
Even when PFI 0 is not the source of your analog trigger, you cannot use PFI 0 for other digital signal routes because the analog trigger takes over the PFI 0 terminal internal to the device when it is enabled. If you try to use the analog trigger and PFI 0 for digital signals at the same time, you receive a routing error.
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Note On NI PXI-6132/6133 devices, you cannot use PFI 0 as the source of an analog trigger, but the analog triggering circuitry still reserves PFI 0 for internal routing. |
Analog Input Channel
In addition to PFI 0, analog input tasks can trigger off of one of the analog input channels being sampled. Because E Series devices use a scanning architecture, many restrictions are placed on how you can use an analog trigger when the source is one of the channels you are sampling. When you use an analog Start Trigger, the trigger channel must be the first channel in the channel list. When you use an Analog Reference or Pause Trigger, and the analog channel is the source of the trigger, there can be only one channel in the channel list. If you have more than one channel for Pause or Reference Triggers, you must use PFI 0. Since S Series devices do not use a scanning architecture, none of these restrictions apply. Therefore, for an S Series device, you can use any analog input channel as the source of the trigger regardless of how many channels are being sampled or the order of the trigger channel in the sequence.
Scaling with PFI 0 and Analog Input Channels
Scaling, including custom scales, is not applied if PFI 0 is the trigger source. For instance, you would specify the DAQmx Trigger analog edge level attribute/property in volts. However, if you use an analog input channel as the trigger source, you could use scaled units.
M Series Valid Sources for the Analog Trigger
APFI 0 and APFI 1
When configuring an analog trigger, connect your analog signal to either the APFI 0 or APFI 1 terminal and specify APFI 0 or APFI 1 as your trigger source.
Analog Input Channel
In addition to APFI 0 and APFI 1, analog input tasks can trigger off of one of the analog input channels being sampled. Because M Series devices use a scanning architecture, many restrictions are placed on how you can use an analog trigger when the source is one of the channels you are sampling. When you use an Analog Start Trigger, the trigger channel must be the first channel in the channel list. When you use an Analog Reference or Pause Trigger, and the analog channel is the source of the trigger, there can be only one channel in the channel list. If you have more than one channel for Pause or Reference Triggers, you must use APFI 0 or APFI 1.
Scaling with APFI 0, APFI 1, and Analog Input Channels
Scaling, including custom scales, is not applied if APFI 0 or APFI 1 is the trigger source. For instance, you would specify the DAQmx Trigger analog edge level attribute/property in volts. However, if you use an analog input channel as the trigger source, you could use scaled units.
Device Calibration and Accuracy of the Analog Trigger
The trigger DACs in the analog trigger circuitry on an E Series, M Series, C Series, or S Series device typically contain four less bits of accuracy than the ADC of the device. No hardware calibration is provided for the analog trigger circuitry. In addition, 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.
C Series Valid Sources for the Analog Trigger
Analog Input Channel
The NI 9205 has no APFI 0 or APFI 1 terminal. Analog input tasks using the NI 9205 can trigger off one of the analog input channels being sampled by the NI 9205. When you use an Analog Start Trigger, the trigger channel must be the first channel from the NI 9205 in the channel list, but channels from other C Series devices can come first. When you use an Analog Reference or Pause Trigger, you can use only one channel from the NI 9205 in the channel list, but you can use channels from other C Series devices. You can combine Analog Start, Reference, and Pause Triggers with different configuration settings by using multiple NI 9205 devices. All analog triggers on the same device must share the same configuration settings.