Why whenever ac shares a common ground with dc, ac will be pushed up a little.?

Question

if only ac is connected to the ground, ac has 'bipolar return to zero' characteristic. why is it when dc is connected, the ac loses this characteristic? (positive peak and negative will be of different values)

Answer 1

What you have done is changed the voltage drop across the ground to true ground- the power supply.  This voltage drop results in noise, or changes that are not authorized in the circuit.  

The way to avoid this is a common point ground located very very close to the power supply terminal.  You have what is called a "ground loop," where you have interference between different parts of the circuit through their ground connections.