I am building an ohm meter calibrator and I do not want a uC that will interfere with the signal.
That would be the LEAST of your potential problems... switching transients.
You want to build a precision resistance calibrator that "dials in" and digitally displays resistance from one ohm to 1.9996x10^8 ohms? Good luck with that! I used to work as a technician in an electronics laboratory that maintained a metrology
facility for checking the electrical calibration of voltmeters, ammeters, and ohmmeters... among other more exotic instruments. We maintained various resistance "standards" that had to be periodically sent out to an independent laboratory for measurement of their "nominal" resistance. It was important to have a history of these measurements and the temperature and humidity conditions under which the measurements were made. That historical record could tell us immediately if something was amiss with a particular resistance standard. All of our resistance standards were of the four-terminal Kelvin arrangement: two wires in for excitation, two more wires that connected to the standard resistance for measurement. Here is a picture that shows a resistance standard similar to the half-dozen or so our metrology lab maintained:
Note the holes in the outer cylinder. This was to allow the standard to be immersed in a temperature-controlled oil bath. Several days were required before the resistance element reached equilibrium temperature and the resistance no longer changed. Planning ahead was encouraged. Now how, exactly, will you ensure that your "calibrator" is accurate and remains accurate after it is tossed around on your bench for awhile? You want to maintain the same precision over more than eight orders of magnitude change in resistance? You want to (somehow) place switch contacts in series and/or parallel with your twenty-eight decade-configured calibration resistors without that having any measurable effect on the resistance? I suggest you go to this EEVblog Electronics Community Forum
for a fascinating thread on DIY ohmmeter calibrators.