Start Trigger and First Sample Delay on Multifunction I/O Devices

Updated Feb 19, 2024

Issue Details

  • I want to configure a start trigger for my analog input acquisition. What is the delay between receiving the trigger and the first sample acquired?
  • What is the default delay of a start trigger when it is configured as digital or analog? 


The delay will vary on each Multifunction I/O family (E,M or X Series).

The delay is comprised of two elements:
  1. The delay from when the trigger occurs and when the sample clock is generated.
  2. The delay from the sample clock to the convert clock (the clock used in multiplexed modules when when multiple channels are used).

1. Delay from Start Trigger to first Sample Clock:

The default delay varies from device to device:
E-Series: 2 Ticks of the AI Timebase.
M-Series: 2 Ticks of the AI Timebase.
X-Series (Multiplexed and Simultaneous): 4 Ticks of the AI Timebase.
X-Series (PXIe-6386 and PXIe-6396): 14 Ticks of the AI Timebase.

This delay can be configured through the DAQmx Trigger property node, and configuring the following properties:

2. Delay from the Sample Clock to the First AI Convert Clock Pulse: 

The default delay varies from device to device. 

E-Series: 2 Ticks of the AI Timebase.
M-Series: 3 Ticks of the AI Timebase.
X-Series (Multiplexed): 3 Ticks of the AI Timebase.
X-Series (Simultaneous): 0 delay.  There is no AI Convert Clock for our simultaneous X-series cards. This applies to PXIe-6386 and PXIe-6396 as well. 

This delay can be configured through the DAQmx Timing property (More » AI Convert » Delay from Sample Clock » Delay). 
For more information, please refer to the appropriate user manuals in the related links section below.

Analog Trigger
The trigger delay or latency on NI E-series (NI 60xxE), M-series (NI 62xx), and X-series (NI 63xx) Multifunction I/O boards is very close to the same delay regardless of whether you are using an analog trigger or digital trigger. The main difference between the two is when the trigger is actually detected.

When using an analog trigger, you must either use one of the analog trigger pins (APFI for M and X series, PFI0/AI Start Trig pin on E-Series) or one of the analog input lines.  These boards then use an analog trigger detection circuit which is typically much less accurate than the analog inputs of the device. For example, the M-series comparator circuitry typically contains four less bits of accuracy than the ADC of the device. For more information, please refer to E Series, M Series, and X-Series Analog Input Trigger Resolution.

For analog triggers on these boards, your accuracy of the trigger circuitry will affect when the analog trigger is detected. This will in turn affect the delay between when your signal crosses the trigger threshold and when your first sample is taken.

Finally, 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.

The two properties mentioned in this article are used to manual synchronize your M-series, E-series, and X-series devices.  To ensure that a triggered or non-triggered acquisition is properly synchronized, you need to ensure that these delays are matched on each system.  Remember, the timebases are different between boards and you will need to determine the appropriate number of ticks depending on the AI timebase that you are using.

Additional Information

AD converters installed on PXIe-6386 and PXIe-6396 are unique. Refer to NI PXIe-6386 and NI PXIe-6396 Supplementary Information and Caveats for additional considerations.