I am looking for a high precision DAC for an output to act as a control element in a PID loop (running about 100Hz). Now, devices such as the AD5791 can provide a true 20 bit output with remarkable precision. How do 24 bit audio DACs (much cheaper) fare in such applications?
It depends on what the application is: Analog DAC's have DC precision, your guaranteed if the DAC is set to X bits the DAC will deliver Y volts within some tolerance. Specs in the datasheets are geared toward DC precision (DNL, INL ect). If the application requires DC precision, then pay the big bucks and get a 100$ DAC. Usually R-2R or some kind of resistive switching network is employed to output the signal.
DAC's that are for sound have no DC precision and are AC coupled, the DAC's are designed to output a clean AC waveform. Specs in DAC's that are built for sound are more concerned with the quality of the AC signal (THD, dynamic range and terms are listed in dB). The method for producing the waveform is usually a delta-sigma converter (kind of like PWM) with a low pass filter.