What about DC voltage? Is it noisy or only digital envirment that measures the voltage generates noise? If you have noisy DC voltage you can use AD convertor that uses dual integratig slope to average the noise which is 0 if you make 1-2 measurments/sec and you'll have only DC voltage mesuremens. I think this is good solution that helped me a lot.
Also you can make low pass filter to remove most of the noise of DC voltage. This can be done with instrumental amplifiers and metal-film resistors.
Simce