DAC5675A-SP: DAC5675A-SP operation at low EXTIO voltage

We use the DAC5675A-SP to synthesize the transmit signal of one of our spaceborn systems. We plan to use the EXTIO voltage for the adjustment of the output power level. Hower the dynamic range, which corresponds to the recommended oparting range of EXTIO from 0.6V .. 1.2V, is too small for our application. The recommended range corresponds to 6dB dynamic - but we need approximately twice the value.

We build a breadboard, and the actual performance of the DAC is perfectly matching our needs, as we don't need the full resolution of the device (8..10 bits should be sufficient). We use EXTIO somewhere between 300mV and 1.2V, where 1.2V corresponds to the maximum DAC current of 20mA. At the low EXTIO voltages our system has plenty of margin as the power amplifier produces way less distortions with the small output signals. So it is acceptable, that the DAC generates a slightly worse output signal in the low EXTIO voltage range.

There is however one big question:
- is it OK to operate the DAC like that?
We checked the operation of the internal opamp, which receives the EXTIO and BIASJ voltages. This opamp seems to have rail-to-rail inputs, as it decreases the output current smoothly down to zero. (second question: is this obervation correct?).