Measuring voltages from different sources sharing Common ground that might differ slightly
If you have ground offsets, then use an instrumentation amplifier and do a differential measurement.


  An instrumentation (or instrumentational) amplifier (sometimes
  shorthanded as In-Amp or InAmp) is a type of differential amplifier
  that has been outfitted with input buffer amplifiers, which eliminate
  the need for input impedance matching and thus make the amplifier
  particularly suitable for use in measurement and test equipment.
  Additional characteristics include very low DC offset, low drift, low
  noise, very high open-loop gain, very high common-mode rejection
  ratio, and very high input impedances. Instrumentation amplifiers are
  used where great accuracy and stability of the circuit both short and
  long-term are required.
  Ref: Wikipedia




Instrument op-amp takes differential voltage as input and you don't need to worry about your circuit (just make sure you stay within the rails), as it can be quite complex if you want to achieve accuracy. They have very good CMMR and are very easy to use (a single resistor set the amplification).

It's fairly simple as they come altogether in a chip like AD8220 and will be much more accurate.

Make sure the in-amp is able to drive the rest of your circuit (like an A/D) you may need to add an opamp as buffer.