Looking over the spec sheet page 23, it seems like the PXI module provides the power so no external supply is needed.
It should be possible to measure 5V @ 40mA. As to the maximum, this board is able to produce a maximum of 60V @ 150mA to drive a relay so I would guess that it is also able to sample your 5V @ 150mA. Use that 150mA as a limit on the current you feed. It could also be a possibility that this board has internal circuitiry to protect itself from excess current.