Delay with Trigger and First Sample on M, X, and E Series MultifunctionDAQ

Updated Aug 2, 2018

Issue Details

I am performing an analog input with an analog/digital trigger. What is the delay between receiving the trigger and the first sample?

Solution

The trigger delay or latency on our E-series, M-series, and X-series DAQ 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 KnowledgeBase 2W3D230T: 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.

In addition to accuracy, there is a short delay internal to the DAQ cards.  This delay is going to vary on E-series, M-series, and X-series boards.

The delay is comprised of two elements: the delay from when the trigger occurs and when the sample clock is generated and the delay from the sample clock to the convert clock (the clock used to actually take samples on our multiplexed cards).

Delay from Trigger to first Sample Clock:

This delay can be configured through a DAQmx Trigger property node (Start » More » Delay).  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
 
Delay from Sample Clock to First AI Convert Clock Pulse: 

This delay can be configured through the DAQmx Timing property (More » AI Convert » Delay from Sample Clock » Delay).  Again, the 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.


 
For more information, please refer to the appropriate user manuals in the related links section below.

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.

 

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