Hello Tom,
Based on the specifications regarding the PXI-6289 the best possible resolution improvement you could likely achieve via analog outputs to create a new Vref and DC offset would be an improvement of a factor of two in the resolution of the output. You could create a reference voltage of 5V and use the configurable DC offset of the PXI system to offset the resulting A0 channel so the range is 0-10 volts. From there you could invert your connections to the output to invert the polarity of the resulting generated signal. Hence, it seems the best you could achieve from there would be to use an external voltage divider to divide down the voltage to a suitable level for your application.
If, however, you had another DAQ card that supported isolated analog outputs you would be able to route the analog DC offset from channel AO3 to a shared node i.e. AO0 and that would possibility allow you to create a negative DC offset. Since, your PXI 6289 card does not support isolated analog outputs such a configuration could potentially be damaging to internal circuitry of the DAQ card.
Depending on your application and what precision you are wishing to achieve in your output you might consider investigating an alternative DAQ card if a factor of 2 improvement in your resolution is not sufficient.
Best,
Blayne Kettlewell
ELP Engineer
