Noise measurment of an accelerometer with the PCI-4461 card

Question

Hi everyone, 
I'm trying to measure the noise of an single axis accelerometer with a PCI-4461 card. 
First, I have manually measured this noise using an HP35665 signal analyser. I obtain something like this: 
And now with LabView and the PCI-4461 card, I get this: 
My question is: where does this part come from? And how to remove it ? (Since it doesn't exist with the signal analyser)
I'm using LabView 8.5.
This is how I configured the DAQ acquisition:
and this is my VI:
Thank you in advance for helping 
Arthur
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Answer 1

Hello 
Yes indeed, the time signal seems to have two paterns (in green below), once negative and positive once about 50 ms later. This pattern is repeated with a frequency of 0.94 Hz (for simplicity let's say 1Hz).
I have reproduced this pattern with matlab using a decreasing exponential:
I have obtained this spectral signal:
That closely looks like what disturbs the spectrum, indeed..
Then I have tried to output a sinewave and acquire it:  
I didn't see any problem.
So I made the measurment with other sensors.
The sensors are ICP accelerometers.The sensors that I am using are in page 7 of these document: 
https://docs.google.com/viewer?url=http%3A%2F%2Fvibrasens.com%2Fimages%2Fpdf%2FVS_Catalog.pdf
(I'm currently doing an intership in this company) 
And finally I found that the problem came from the cable: when I use the shielded cables, the problem is present; and when I use "normal" cable, there is not the problem, as you can see :
(In red, the sensor is connected with a non shielded cable, and in white, with a shielded one.)