Why is AIGND on SCB-100 connected to mains earth through PCI6031E?

I have a PCI-6031E DAC card connected to an SCB-100. I am trying to measure non-referenced differential signals, and via the application notes realise that i must use two bias resistors to AIGND from the +ve and -ve terminals of each channel.
However i can't quite understand why the AIGND pin inside the SCB-100 is connected to mains earth through the DAC card. Surely any disturbances on the earth due to the operation/swiitching of other equipment on our site will produce disturbances on the reference of the DAC card, and hence effect the data that i am recording?
Or is this connection not direct, i.e through a resistor for example?
Any guidance would be much appreciated.
Many thanks
Mike

Hi,
the resistors are both tied to the same point, ergo any change in voltage on the earth line will appear on both signal lines (aix+ and aix-).
Since this is a differential input, they will be cancelled out as a common mode voltage.
The purpose of the resistors is to prevent the input voltages to the aix+ and aix- floating off. You're measuring a differential voltage, say 10V, but that could be at 50V on the aix- and 60V on the aix+. That would damage the card, and without the resistors, the signals from a true floating source could continuously float up and down.
You're quite correct in the fact that aignd is also pc or system earth, and you can get mains earth signals on this pin, hence the reason we recommend NRSE or differential as the better two input types.
http://digital.ni.com/public.nsf/websearch/1ffcbe3451abee09862562970059881d
http://digital.ni.com/public.nsf/websearch/D509679FFAE2764386256297005D0C9D
Hope that helps.
Sacha.
// it takes almost no time to rate an answer

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Why is AIGND on SCB-100 connected to mains earth through PCI6031E?

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