I just wanted to check that if you are using an ADC without isolation, you aren't using it for voltages above 42V dc (for safety reasons)?
The units are tested to standard EMC regulations so they will perform well under normal conditions. However ... they are not screened, so if you have particularly strong EM fields then you could pick up some noise.
Poor grounding would account for a noisy signal so this is the first thing to solve. This can be achieved by connecting the outer shell of the parallel port connector to a known ground point. Some ADCs don't ground the outer shell though ... if you have an ADC-10, 11, 12, 40 or 42 then ground pins 20, 21, 22 and 25.
Pico Software Engineer