Synchronized Data Delayed When Using DSA Devices

Updated Sep 28, 2018

Reported In

Hardware

  • NI-9234
  • USB-9234
  • NI-9230
  • NI-9232
  • NI-9233
  • USB-9233
  • NI-9250
  • USB-4431
  • USB-4432
  • PCI-4462
  • PCI-4461
  • PXI-4465
  • PXIe-4464
  • PXI-4461
  • PCI-4474
  • PXI-4472
  • PCI-4472
  • PXI-4472B
  • PCI-4472B
  • PXI-4498
  • PXI-4495
  • PXI-4496
  • PXIe-4499
  • PXIe-4492

Driver

  • NI-DAQmx

Issue Details

I am trying to synchronize my Dynamic Signal Analyzer (DSA) DAQ device with a normal DAQ device. I am using a digital trigger to start all devices, but still I notice there is phase shift, or skew, between the two waveforms.

For example, if I acquire the same signal with a PXI-6221 and a DSA device such as the PXI-4461 at the same sample rate using a shared start trigger, the waveform acquired by the PXI-4461 will lag several samples behind the waveform acquired by the PXI-6221.
 

Solution

This phase shift or delay is typically the result of the group delay of the anti-aliasing filters common on the front-end of Dynamic Signal Analyzer devices commonly include anti-aliasing filters on the front-end circuitry, which introduces a certain amount of group delay.

You can compensate for this group delay in several ways:

Option 1: Channel Expansion
If the two devices can be used in a single task using Channel Expansion, the driver should compensate for the delay automatically

Option 2: Manual Compensation
If the two devices cannot be used in a single task, you can compensate for the delay manually by performing a dummy read on the DSA channels and throwing out a certain number of initial samples on the DSA device, prior to reading the samples from both devices simultaneously, as shown below:
Note:  The number of samples to read and throw out can be determined experimentally, or by using the group delay equations found in your DSA device's specifications documentation.

Option 3:  Using the RemoveFilterDelay Property
Some DSA devices, such as the PXIe-4464, support the AI.RemoveFilterDelay property (DAQmx Channel Property Node: Analog Input»General Properties»Filter»Remove Filter Delay) which automatically discards filter delay samples.


Option 4:  Analog Triggering with DSA Devices
If you are using analog triggering to start the DSA device task, the filter delay will not be observed, since the analog trigger circuit comes after the anti-alias filter; this prevents aliased components of the analog signal from firing the trigger.  

For more detail on this topic, please see the Analog Triggering section of the Dynamic Signal Acquisition User Manual.

 

Additional Information

NI Dynamic Signal Analyzer devices use anti-aliasing filters to remove frequency components above the Nyquist frequency, which impart some amount of phase distortion to the filtered signal, which typically shows up as a delay or phase shift. The exact amount of delay depends on which DSA device is being used.  An equation or table of delay values is typically specified in the specifications documentation for the device.  You may find it listed as input delay, filter delay, or group delay.

Because this filter delay only occurs on DSA devices, a DSA device may appear to lag behind other devices during its acquisition, if the initial delay is not removed prior to comparing the data points.

 

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