Disclaimer: I've only built one "true" sigma-delta DAC, and it was first-order because I was running it on a 25MHz FPGA and I only needed telephone-quality audio out of it. So rather than spending my clients money to have fun, I stuck with the basics.
There doesn't need to be much difference between the analog/digital sigma-delta modulator you might build for an ADC vs. the all-digital one you'd make for a DAC.
Both have a modulating element that takes a continuous (in the case of the ADC) or many-valued (in the case of the DAC) signal from a loop filter and, at each sample instant, turns it into a 0 or a 1. Both translate that 0 or 1 to match the full scale of expected input values (with a carefully buffered digital output in the case of an ADC, and just a multiply or change of attitude in the case of the DAC). Both have a loop filter that conditions the output of the summing junction to perform the noise shaping that makes a sigma-delta modulator worthwhile.
Here's a chart that sums up the differences between the elements -- all of which are present in either type of sigma-delta device:
| function | ADC | DAC |
|---|---|---|
| modulating element | analog comparator | compare operation |
| modulator output | digital output | multiply operation |
| summing junction | analog circuit | subtraction operation |
| loop filter | analog filter | digital filter |