Although there can be times in power circuits where AC ground is separate from DC ground, you are probably confusing the terms with AC-grounded and DC-grounded. If a circuit is grounded through a capacitor it is referred to as AC-grounded because only AC signals can pass through the cap to ground, DC levels are blocked. Whereas if a circuit is such that DC current can flow to ground, such as through a resistor, then it is considered to be DC-grounded. A classic example would be a non-inverting op amp configured with the divider feedback resistor connected through a capacitor to ground (AC-grounded), or directly to ground (DC-grounded).
Added after 1 minutes:
follow the book for electronics by flloyd