Advantech MIC-3716 Manual page 43

The three ground references AIGND, AOGND, and DGND should each
be used discreetly according to their designated purpose. An individual
GND pin is available for AI, AO and DIO to provide optimum signal
quality.
Since all the signals on the DAS card need to refer to the same GND, the
manufacturer has tested and chosen the best point to connect AIGND,
AOGND and DGND together. This is based on the "single-point" ground
principle.
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