Does the PXI447X and PXI3060E divide the signal by two when connected differentially?

[benbol]

I would like to measure vibration motion with sensors which measure ground velocity.
I would like also to work in a differential way to eliminate the noise. We have bought the acquisition device NI PXI1042Q that contains two acquisition cards : NI PXI4472B and NI PXI6030E.

For the NI PXI4472B, we have connected the positive signal and the negative signal of one sensor to one (of the eight) channel of the DAQ (See the outline on the attached file). So, we are in a differential way.

For the NI PXI6030E, we have put the switch to "diff" to be configurated as differential. For one sensor, we have so connected the positive signal to the channel 0 and the negative signal to the channel 8 for example (See the outline on the attached file).

I would like to know if my two acquisition cards, after having substracted positive from negative signals of my sensors, make themselves the division by two of the resulting signal, or if I have to the division by my own. In fact, substracting positive and negative signals make the amplitude to be multiplied by two. So, the amplitude have to be divided by two.

[SanaaT]

Hello Benbol,

You don't have to divide your acquisition result by two.
Please see the link below that gives you somme information about the differiential mode of acquisition :
http://zone.ni.com/devzone/conceptd.nsf/webmain/45acc30d4a769a3f862568690061d750
http://digital.ni.com/public.nsf/websearch/DF70FCE03D397BFE86256A3700780FFA?OpenDocument
http://zone.ni.com/devzone/conceptd.nsf/webmain/139DFA3645B29BE586256865004E742A

Hope this help.
Regards,

Sanaa T
National Instruments France

[benbol]

Hello Sannaa,

Thank you for your reply. So, you said that I don't have to divide my acquisition result by two. But is it because the division is done by the DAQ, or is it because the signal result of a sensor is ‘(positive signal – negative signal)’ and not ‘(positive signal – negative signal)/2?

Also, for the NI PXI4472B, I said that I had connected the positive signal and the negative signal of one sensor to one (of the eight) channel of the DAQ to be differential (pseudo) But I don’t know if that solution was good because the documentation say that we can't make a non-ground connection to the shield.
So the other possibility was to use 2 channels, one for the positive signal with its ground and the other one for the negative signal with its ground (see the outline in the attached file). Here, I do the subtraction between the two channels by my own thanks to a software (‘Matlab’). So, I don’t know if the positive signal is divided by two (and the same thing for the negative signal), or if I have to do it by my own thanks to a software (in the case that the signal result of a sensor is ‘(positive signal – negative signal)/2)

Thanks a lot,

ben

Message Edited by benbol on 05-24-2005 07:01 AM