Hi Brian,
Double check your power supply sequencing and take a look at this post. The DAC7718 is the SPI version of the DAC7728 and it sounds similar
http://e2e.ti.com/support/data_converters/precision_data_converters/f/73/t/147850.aspx#539982 …
Andy,
sorry for the double response, I don't know how this happened. I have attached a DMX512 article. You might want to add at least a high-voltage capacitor (4700pF/2kV from NOVACAP) between GND and ISO-GND.
regards, Thomas