Hello,
The DAC34SH84's DC offset feature can help adjust the DC offset introduced in the signal chain (both DAC and analog quadrature modulator). Often, DC offset of a few uV to mV range is needed to correct the offset and adjust the LO leakage.
Please check out the following articles:
http://www.analog-eetimes.com/en/signal-chain-basics-70-digital-correction-of-analog-quadrature-modulator-imbalances.html?cmp_id=71&news_id=222904086
section 8.3 of the following:
http://www.ti.com/lit/ug/slwu029d/slwu029d.pdf
-Kang