Getting voltage spikes from an analyzer on the usb-6212 analog input

[labview12110]

I have an NI USB6212 the BNC style DAQ and am trying to read in the voltage from a gas analyzer. I keep seeing these voltage spikes in the data stream though, and they are not present when I plug the voltage out from the analyzer to just an oscilliscope. I attached some screenshots of what I am seeing. All this vi has in it, is a daq assistant set to read voltage at 100 samples per second for various rates, 10Hz, 100Hz, 1kHz. you can best see the spikes on the 1kHz or 10hz graph.  I have the analyzer connected through a UPS to try and get rid of some line noise but that shouldnt be a problem anyway if it did not show up on the scope.

I should also mention that this unit was just evaluated by the manufacturer and they saw some spikes but none as high as this so it must be a problem at the labview/daq end.

[Henrik_Volkers]

These spikes could be indicating a EMC problem , and this could be due to ground loops and switched power supplies.  For high frequency spikes ground loops don't have to have a wired connection.. a capacitive coupling in the USB or PC supply is all they need 😕

Usually the scope input shield it directly connected to (protection) ground. While the USB DAQ AGND is more floating ....

So some test:  hook up the analyser on the scope: (you noted it looks fine)  

hook up on the scope AND the  DAQ , spikes on scope and/or DAQ?

Can you try a notebook with the USB-DAQ ?    With and without power cord? 

here  I posted a way that sometimes helps to get rid of the spikes...

Last exit: Use a median filter ....

[labview12110]

It is already running on a laptop and i have tried it plugged and unplugged. and with both the analyzer and the laptop plugged in to the battery supply. I did use a median filter but that only gets the positive spikes removed. the negative dips remain, how do i tweak that filter to eliminate them?

By scope to daq you mean via a bnc splitter?

My real concern with using filtering however is that I cannot trust the analyzer completely now. I have other manufacturer's analyzers here that do not have this issue and do not require filtering.

[Henrik_Volkers]

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@labview12110 wrote:

It is already running on a laptop and i have tried it plugged and unplugged. and with both the analyzer and the laptop plugged in to the battery supply. I did use a median filter but that only gets the positive spikes removed. the negative dips remain, how do i tweak that filter to eliminate them?

Filter length..

By scope to daq you mean via a bnc splitter?

 Yes 🙂

My real concern with using filtering however is that I cannot trust the analyzer completely now. I have other manufacturer's analyzers here that do not have this issue and do not require filtering.

Output impedance noted in the spec?

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[rpursley8]

I have found that the USB DAQ devices are noisier than their non-USB counterparts.  To verify the source of the noise, you might put a load on a second analog input (1k ohm resistor) and see if the same noise is present.

I have tried a few different things to minimize this noise.  First, I did as you did and unplugged the power on the laptop, and this helped for one project.  For another project where I was doing long term acquisitions and couldn't unplug the laptop, I used a powered USB hub in the USB chain and supplied the DC power to the hub with a benchtop power supply.  I also bought an isolated USB hub from Sealevel.  I think internally it optically isolates the USB signals from the computer to all of the peripherals.

A median filter should have eliminated both the negative and positive spikes.