The actual application is precision balance. The requirement is 2000 grams with 10 mg accuracy.

What do you mean? Is this part of auto-calibration routine? To apply a well known voltage and adjust 'gain' and 'offset' of your sensor front end electronics?

If you want to apply this 'known' voltage by the help of PWM signal, please keep in mind, that then not only exact pulse width has to be known, but also pulse height. Concretely spoken the whole shape of pulse has to be known with this accuracy! Think of finite rise and fall time of this pulse and of eventual ringing!!! Only meassuring pulse width of pulse would not give enough information!

I don't think that PWM is a suited methode to apply ultra high precision reference voltages. I would use precision voltage generators in combination with high reliability relays (inert atmosphere arround massive gold contacts).

Kai