I am not so sure why you are having an issue with using the D/A on the 'LP6440. Section 20 of the data sheet seems to do a pretty good job of presenting the D/A mode of operation of this dual mode peripheral function.

"Atmel Says"
The DAC converts a 10-bit signed digital value to an analog output current through successive
approximation. The DAC always operates in differential mode, outputting a differential current
between its positive (P2.2) and negative (P2.3) outputs with a common mode voltage of VDD/2.
The minimum value represents zero difference and the maximum values represent a difference
of positive or negative VREF minus 1 LSB. An external transimpedance amplifier is required to
convert the current into a voltage suitable for driving other circuits.

The DAC is enabled by setting the ADCE and DAC bits in DADC. Some settling time is required
for the reference circuits to stabilize after the DAC is enabled. The DAC does not have multiple
output channels and the DIFF, ACON and ACS bits have no effect in DAC mode. P2.2 and P2.3
are automatically forced to input-only mode while the DAC is enabled.

"Karas"
The most important thing to remember is that the D/A is just single channel and that it is a differential current output. You need to design the external circuit with an op-amp circuit that can convert the current mode of the pin pair into a voltage that will be useful in real application.

Michael Karas