Reducing Glitches on the Analog Output of DAQ Devices

Updated Jun 18, 2018

Reported In

Hardware

  • PCI-6221
  • PCI-6251
  • NI-9262
  • NI-9263
  • NI-9269
  • NI-9264

Other

Applies to:
  • MIO DAQ Devices 
  • DACs
  • Analog Output devices

Issue Details

  • I am using Analog Output and I see glitches in my signal?
  • My DAC seems to be producing glitches, what do I do?
  • A scope of my analog output shows glitches
  • How can I fix glitches in my signal?
  • My analog output signal has random spikes in it

Solution

These glitches are expected behavior provided they fall withing the Glitch Energy Specification of the device. The following information can be found in the detailed specifications section of the following two devices:

PCI-6221:
Glitch energy
Magnitude ...................................100 mV
Duration .......................................2.6 μs

PCI-6251:
Glitch energy at midscale transition, ±10 V range
Magnitude ...................................10 mV
Duration ......................................1 μs

Some devices, such as the NI 9264 and NI 9263 will not have detectable glitching (cannot be picked out from the noise). Whereas, for instance, the NI 9262 and NI 9269 does have detectable glitching.

Make sure to consider the glitch energy level when choosing a device for your application. 

There are two primary ways of reducing glitches:
  1. You can build a low-pass deglitching filter to remove the high frequency components of the glitch. Depending on the frequency and nature of the output signal, the low frequency components of the glitch may not be filtered. See related links for more information on deglitching circuitry.
  2. You can build an external deglitching circuit. The foundation of this particular idea is to use sample and hold circuitry to hold the previous value while the DAC is settling on the current value. 

Additional Information

The reason DACs produce glitches, or small spikes is due to the released charge when physically switching from one voltage to the next. These spikes are typically occur when the Most Signifigant Bit (MSB) is switched in the middle of the range of the DAC. These glitches are present in almost all devices that have a DAC on board unless the device has built in deglitch circuitry. 

Note: The building of a deglitching circuit is not an easy task. Several schematics on how to accomplish this may be found online. National Instruments makes a variety of DAQ cards (for example, many X Series devices) that include internal deglitching circuits. These greatly reduce the effects of glitches. See the Related Links section for a list of National Instruments DAQ cards that support reglitching. You can also look for Glitch Removal in the specifications of the device you are considering to see if this feature is implemented.

National Instruments provides high performance analog output devices which include onboard filtering, interpolation, and oversampling. These boards can output much cleaner signals than our standard DAQ products. If producing a glitch free, high performance signal is important for your application, refer to the National Instruments signal generators and DSA boards for analog output solutions. 

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