USB-6211: analogical input signal affecting other ai's from the same card

[FCB]

Hello

I'm using the DAQ card Ni-6211 and some DAQmx functions to read a hammer and an accelerometer signal, and then use other LabView functions to make the FFT of these analogical inputs signals. However, it seems like the analogical inputs where the hammer and the accelerometer are connected are generating some kind of noise or influence in the other inputs of this DAQ card that is not connected to any other sensor.

I made different experiments to verify if the problem is with the card lecture: putting the accelerometer and hitting the hammer in a different table where the DAQ card  table was localized (to avoid vibration on the card and a possible noise), the input ai1 was connected on mode differential on the hammer and the input ai4 was connected on the output (z axis) of the accelerometer. The other 2 inputs ai2 and ai3, which can be read too by my LabView program, are open (i. e., any other sensor is connected to the card). When the structure where the accelerometer is localized is hit by the hammer, the signal from ai2 ("x axis" seen in the first attached document) has a curve (on the time domain) that initialize almost at the same time as the hammer, and the input a3 has has a weak signal, but with the same oscillation as the signal from ai4. The document "Hammer ai1 + z_axis ai4 connected_ _x_axis ai2+y_axis ai3 disconnected" has the images that I captured from the graphic create on LabView. Regarding these graphics, it's possible to verify on the FFT that the signal from ai3 and ai4 have the same behavior (with different intensities), and from the zoomed figure of the time domain graphic, we can see that the signal from ai2 increase almost at the same time of the signal of the hammer (ai1). So the signal captured from the sensors are probably creating some kind of noise on the open inputs ai2 and ai3.

Other experiment was made to verify if the the signal from one input could affect the signal read from the other near inputs: the DAQmx Create Channel task had one physical channel changed: the input ai3 was modified by ai7 (maintaining the same connection mode: differential), and the results can be seen on the second document attached. In the graphics obtained in this experiment, it seems that the input of the hammer (ai1) is affecting the input signal from the inputs ai2 and ai7, which are not connected. And the signal from ai4 doesn't seem to be influencing other inputs, because it has a different curve on the FFT graphic.

The same experiment was made using the connection RSE (changing the wires and the DAQmx Create Channel Configuration), but the results were the same as those found using the differential connection.

Finally, if the accelerometer output is connected on the ai2, the signal from the other open inputs ai4 and ai7 seem to be being affected by the signal from the accelerometer on ai2 (last document attached).

Could you inform me if the problem that I'm having is being caused by the DAQ card with this information that I gave for you? And if this response is yes, do you know if there is some way to avoid this noise create in one input by the other, please?

Thank you

[FCB]

I had some problem to attach the documents on the previous message, so they are being attached on this one.

[GovBob · Accepted solution]

Maybe Ghosting or Crosstalk? Just an idea.