A pH probe produces a small voltage (58 mV per pH point normally with pH=7 at 0V and any pH higher than 7 producing a negative voltage and lower than 7 a positive voltage). A DAC that can output negative voltages will do. A pH probe also has a huge output impedance - typically in the order of 10-100 MΩ or even higher - which you can emulate by putting a resistor of such a value on the output of your ADC.