With PXI4472B, I can obtain different results between different channels which have the same configuration. Why?

[benbol]

I have done measurements of the ground acceleration with 2 same accelerometers. These accelerometers have a frequency range from 0.01Hz to 50Hz.
I have done an acquisition using the PXI4472B with IEPE Constant Current Source : my two sensors were connected respectively to the channels 0 and 1. I found a good coherence only in the frequency range : 10Hz to 50Hz.
Now, I have done the same acquisition with these accelerometers. The only difference was that the accelerometers were connected respectively to the channels 5 and 6. I found a good coherence in the frequency range : 0.1Hz to 50 Hz with just a fall of the coherence at 10Hz.
I don't understand why I obtain better results with 2 channels rather than with 2 another ones in that case each channel has the same parameters.

[toto10]

Hello,

The 4472 has 8 channels simultaneously sampled.

Are you sure the measurements conditions were the same? How many times have you done the measurements?


Jean-Claude J
NI France

[benbol]

The parameters were the same for each channel. I have discover the problem :

- if I perform the coherence between channels 5 and 6  without any sensors plugged and with the IEPE configuration, I have a good coherence between 0,1Hz to 1 Hz. So the noise inside these 2 channels seems to be coherent. If I do the same manipulation without the IEPE configuration, I have a bad coherence between channels 5 and 6 for all frequencies. So the noise seems to be the current source which is not perfectly constant, but which have low frequencies components.

- if I perform the coherence between channels 0 and 1  without any sensors plugged and with the IEPE configuration, I have a bad coherence for all frequencies. But the  Power Spectrum Density of acceleration has the same amplitude for channels 5 and 6 than for channels 0 and 1. So we have noise inside channels 0 and 1, but these noises are not coherent between these two channels.

The problem is that we seem to have much noise inside the ADC at low frequencies, which give us inacurrate results if we want to measure ground acceleration below 10 Hz with accelerometers. Do you agree with me? I have done these measurements during 320 seconds. The length of the NFFT was 16384  with  an acquisition frequency of 1024 Hz. So, the PSD and the coherence were performed during 16 seconds and I have done 20 averages.

Please answer me,

Thanks a lot,

benoit