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USB6211 produces noise in parallel scope traces

Dear all, 


I have a question for the community, hope you can help me.


The system:

laptop, Win7, LV2013, USB6211

-2 analog inputs:

one from a detector (DC)

one from a signal generator (triangular, 0-10V)

acq. rate. 80kS/s per channel

acquisition: differential



Scope properties: 500 MS/s total, 250MS/s/channel

scope is grounded.


When I connect the DAQ card, turn on the signal generator and look on the scope at the signal that I am also sending in the DAQ card, it looks like picture 1. However, as soon as I start the acquisition, my scope trace starts to look like picture 2. Although I don't see this big ripple in the labview program, I am wondering what it is? Is it being generated by the DAQcard? 


Secondly, when I don't plug in my laptop (so run it on battery), the triangle wave acquisition, after some time, starts to look like figure 3. With a cut of top. What is going there?


I am assuming some grounding issue, but couldn't really find a solution, so if you have any, please let me know!





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That is interesting.  I suspect that you are right that the DAQ card is causing the ripple, at least in part.


What kind of a detector is it? What is the output impedance of the detector? How is the detector grounded? When you are using the scpoe, your detector signal is essentially single ended, so what is the advantage of using a differential connection to the DAQ device? 


The triangle wave peak distortion issue is likely due to the common mode input voltage being exceeded.


Even though the laptop is not grounded through its power supply, the DAQ ground does connect to it through the USB port.


You probably have a ground loop or other common coupling which produces the artifacts you are seeing. Try drawing a complete schematic of your system at the block diagram level. That is, you do not need the schematic of all the internal circuits, but show every connection between modules and devices. This includes the laptop, USB device, detector (and what it is connected to), scope, signal generator, and any other devices. Show all power supply, ground, and signal connections. Somewhere on that diagram will likely be the source of your problem.



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USB Isolators are available which stop those types of ground loops, also AC isolation transformers can be used to operate your equipment or laptop.




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