PCI-6723 DAC has an output current limit of 5mA/channel. Kindly tell me if the circuit (e.g. power amplifier) connected with DAC output channel draws more than 5mA current what will happen? Will it damage the card or there is some inner protection provided in this card?
The current output should rail at ~5 mA per channel and voltage will drop accordingly. The protection on the 6723 is rated in the specs as "short circuit to ground", which indicates that even if you short one of the outputs it should not damage the card.