Why Am I Seeing Data Spikes on my HSDIO Device?

Updated Dec 20, 2017

Issue Details

I am generating a pattern from my Digital I/O instrument on one or more channels. When I view this data, my results are showing data spikes (or glitches/transients) at times when the data should be constant. When using a single DIO channel, I do not see this behavior, but toggling multiple output channels duplicates the behavior every time.

Solution

One potential reason for this is a result of crosstalk between adjacent channels.

Reducing crosstalk between adjacent lines that are generating data can be accomplished through the following methods:
  1. Using a cable with twisted wire pairs which are shielded from each other.
  2. Keeping cable length as short as possible to reduce inductance.
  3. Terminating the ground signal of the twisted pair at the DUT ground.
  4. If a connector block is used, shield the block and twist pairs together manually.

Additional Information

More information regarding Crosstalk can be found in the NI Digital Waveform Generator/Analyzer Help.





The above figure 1 is an example of crosstalk due to coupled inductive noise on adjacent lines. The transient pulses (or data spikes) are occurring on the falling edge of the other data channels (the high-low transition of multiple adjacent lines combine to cause a low-high transition on other adjacent lines, namely channels 0-4 cause data spikes to occur on 5-7). This was caused by only grounding a single line, which only provided a single return path to ground for all the data lines connected. The coupled inductive noise inside of the large signal to ground loops created EMI which was shown on the lines as data spikes when many signals transitioned at the same time.


Figure 2: Removal of crosstalk through good grounding practices
  

In figure 2, ground pairs that run twisted alongside the signal path were grounded at the DUT. Now, a return path exists for all digital signals to ground and no large EMF loops are created. This is the benefit from twisted pair signals, which properly minimized EMF noise on the digital lines

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