Anomaly document REV. 0.26, er001 for the for ADuC702x says:
"1) The DAC buffer needs the internal reference to be powered up and stable, i.e. connected to an external capacitor. This means that we cannot use the ADC with external reference when using DAC buffer."
The current Rev. D anomaly shows the status of this issue as being 'fixed'.
On the current ADuC7024 does the DAC buffer need the internal reference to be powered up and stable even if the DAC is not using the internal reference? i.e. DACxCON[1:0] configured as '01' to use an external reference on DACref pin, with REFCON.0 = 1, such that the ADC reference is also from a (different) external source on Vref pin.
The current device data sheet doesn’t say anything about the DAC buffer requiring a stable internal reference (but then again it doesn’t say it is not required), irrespective of the settings of DACxCON[1:0], or now undocumented (deprecated/removed?) REFCON.
Related post : http://ez.analog.com/message/11295#11295