I am trying to build a voltmeter, but the voltmeter is powered by a 12V dc adapter. My concern is if the ground potentials between the analog input source and the voltmeter are different (e.g. due to different power outlets), ground current will start to flow, corrupting the voltage reading. Is this concern real? If so, how should I fix this at circuit level? I am thinking of isolated buck with isolated ADC. Thanks.
Let's call your voltmeter and its 12 VDC power supply together as "The Meter".
Let's call the circuitry under your voltage measurements as "The Circuit".
You have no problems as long as the only wires or other conductive ways between "The Meter" and "The Circuit" are your two measuring cords.
If some third wire exists, you're in a trouble. The worst scenario: As soon as you connect your measuring cords or one of them, a short circuit happens.
Note: The third wire can be the protective ground of the mains AC outlet. You will do much better, if your 12VDC power supply in "The meter" is well isolated and do not need the protective ground for safety.