Metering accuracy is not really a concern in voltage measurement; there are quite a few other things to consider. Perhaps having a good stability long-time reference is the most important thing.

Measurement is pretty rarely just "direct D/A conversion, at least when there is any sense in the system as that is about the most inaccurate way to do it (albeit it is a fast measurement, and now we are talking Msamples/s even on cheap systems)
The right way to do it is to have an integrating measurement, where a capacitor is repeatedly charged and discharged through a resistance and the cycle time is measured. Just about all DMM's use this method.