DAC negative bias reference voltage
The reference inputs are low impedance, typically 5K ohms, so you should drive them with low impedance sources. 

The easiest approach is to use an op-amp from your existing positive reference voltage. I'll assume that's 2.5V here. If it's something else, just alter R2 appropriately. 

You can choose an appropriate op-amp to meet  your purposes. 







simulate this circuit &ndash; Schematic created using CircuitLab

The output voltage is -Vref*(R1/R2) 

R3, D1 prevent the output reference voltage from going too far negative, even during transient conditions. C1 enhances stability.