45 Hz noise

Question

I trying to make an acoustic (pressure) measurement using the PXI 4496 and I don't fully understand why my signal looks the way it does. Here is a graph of the time signal with 3 channels and also the power spectrum of the pressure channel.
http://forums.ni.com/ni/attachments/ni/100/1989/1/​acoustic_channels_test_FP.png
the white trace is the pressure channel, the red is a IEPE accel. and the green is a disconnected channel and both tranducers have excitation to them. However, neither transducer is seeing any changing phenomena. That is, the sensors are just sitting there and you can see a large ~45 Hz signal on the pressure channel. For this measurement, I'm using a kulite XTE-190 transducer that has a FSO of 100 mV and you can see that I'm getting signal at 25% of the FSO without doing anything. My wiring configuration goes like this. A 4 conductor shielded cable brings the 10V floating DC excitation to the transducer using 2 conductors and the bridge output of the transducer is brought back to my PXI 4496 using the other 2 conductors. Right now I don't have the shielding tied to ground anywhere.
Message Edited by caz on 02-26-2009 01:55 PM
Attachments:
acoustic_channels_test_FP.png ‏16 KB
acoustic_channels_test_FP_time.png ‏15 KB

Answer 1

Hi Caz,
As Jason mentioned, it's a good idea to ground
your shield. But only ground it in one place: at the transducer, or at
the device. If the shield is not tied to anything then it is floating
and essentially does not function as a shield anymore.
You
have a point about common mode rejection. It does appear that the 4496
is not rejecting the common mode signals in the excitation signal. I'm
curious about the excitation signal you are using. You said it's a
noisy DC source. Is it stepping? The AC-coupling filter on the 4496 has
a settling time in response to DC steps. If you were to look at an
acquisition on a 4496 during a step it would look like a spike with an
exponential decay curve afterward, kind of like what you have in your
acquisitions. It would be great if you could get a hold of a DC-coupled
instrument to monitor your excitation and your bridge output and post
that so we could take a look at it. 
I'm also curioius about your configuration.
1) Is your excitation source floating?
2) Could you draw a diagram showing where you are connecting signals from your bridge 
    a) When you have the huge noise in the measurements of the 4496
    b) When you are doing the differential measurement with the 4496
3) What is the DC voltage present on the negative (shell) of your 4496 inputs during these acquisitions?
DanielConditioned Measurements Hardware