noise problem on analog channels

[Rakaposhi]

I have 18 analog channels connected in differential mode to a 6225 DAQmx card. I am trying to measure 14 pressures and 4 temperatures. I have connected the transducers directly to the AI connectors on my SCB68:s. I have also connected the shields from all transducers to the shields crew on the SCB68 to aviod noise.

My problem is that I still get noise on my analog channels. I have configured the channels using MAX in differential mode. I have read the post "Field wiring an dnoise considerations for analog signals" for a better understanding of analog data aquistion using NI-DAQmx, still I am stuck with my noise.

Can anyone help me? I have ran out of ideas.

I get the best result using an capacitor between the positive and negative terminals on the daq board.

PLEASE HELP!

/Anders

[devchander]

hi,

can u plz specify the characterstic of noise you observe? is it a powerline interference resulting in a constant sinusoidal noise or do you just observe very large fluctuations in your readings?

regards

dev

[Rakaposhi]

Hi,

I get high fluctuations.

I have noticed that if I change my measuring range from 0-10V to 0-1V I reduce the noise, isn't that strange? Could it be that my DAQ card is producing som kind of internal noise?

best regards

Anders

[devchander]

Hi Anders,

Quite unlikely that card is inducing some internal noise.

By selecting the range btw 0 to 1 V, you are only changing the ADC gain settings.

What i suggest is that, you give a standard input of fixed voltage ,say like a dry cell or a function generator to the Analog input and check if you are still getting the kind of noise you are getting with your sensor attatched.

Check this for both the ranges you talk about with suitable inputs.

Now, if you do not get noise, then connect sensor in a suitable single ended mode and see if this gives you a better reading.

If you are still getting noise, then i would suspect for improper chassis grounding

do tell if this solves your problem

Regards

Dev