What Causes Data to Become Un-Synchronized?

Updated May 30, 2018

Reported In

Hardware

  • PXI Chassis
  • CompactDAQ Chassis
  • CompactRIO Chassis

Driver

  • NI-DAQmx

Issue Details

When collecting information during Data Acquisition (DAQ) my results are un-synchronized or become un-synchronized. What causes this error?

Solution

What Causes Measurements to Become Un-Synchronized?

There are four main causes for data to become out of sync. These are:
  • Skew - Propagation delay that is caused when a signal arrives at two places at different times. Affected by distance and impedance of signal paths
  • Jitter - Inaccuracy generated by the systems clock. Each component added to the clock's path adds additional jitter. Can make the sample clock less equal.
  • Stability - Frequency fluctuations caused by several factors (temperature, shock, vibration ext...)
  • Drift - Drift occurs when two instruments are acquiring data at different sample rates. Even though two instruments are set for the “same” sample rate – for example, two digitizers acquiring at 100 MHz – the real oscillators on each instrument run at different rates and therefore clock drift occurs. 
The image below shows example of each type of timing error. A good system will correct or minimize as many of these errors as possible. Slave/Master configurations are using in all synchronization methods. In the configuration, one process or chassis (the Master) will output timing and trigger information to one or more processes or chassis (slaves).

Additional Information

Making Data Synchronized

Making Data synchronized varies depending on what hardware is being used. Using the DAQmx API is one of the most effective ways of making multiple tasks or pieces of NI hardware synchronized with each other. Check the Related Links for information on how to sync data.

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