I read somewhere that in a mixed signal ckt , the analog part and the digital part are supposed to be better when they have different power rails and ground lines , but to avoid floating of the ground , the different ground lines and power lines have to be connected at a point called the star point , so that there is a single ground point and power point. But can someone explain this in a better way , I basically want to understand why a star point is used and its advantages in a mixed signal board .
It really depends on your circuitry and how the digital and analog parts interact with one another.
For example in the circuit below you have a purely digital clock on the right and a radio receiver on the left. In order to minimize the amount of digital noise reaching the analog side it is prudent to connect the grounds and rail using the single point star method as close to the supply terminals as you can. That way digital currents only loop around the wires on the digital side, and vica-versa.
However, if the digital side interacts heavily with the analog side it is different.
In the above circuit, star connected grounds and power is actually detrimental since the return currents for the digital signals must all pass back through the star point to the micro.
For this kind of circuit, integral single ground and power planes with appropriate layout to keep the signal loops as short as possible is much better.
Since almost all circuit boards tend to be a mix a signals, a common ground plane is, or should be, the standard approach. When necessary the occasional island of ground, usually located in a corner so as not to create a hole in the main ground plane, can be isolated for a any very sensitive circuits.
Simply put, if you connect the ground of every section of the board (analog, digital, etc) to a single point, you are pretty much guaranteed that they are all referenced to the same voltage ("ground"). If you have a bus going across the board that you have designated as ground, the voltage may change along the bus due to resistance, inductance, etc. so different parts of the circuit connected to different points along the bus may not be referenced to the same voltage. Additionally there is a possibility of ground loops which can cause some "grounds" to be at a higher potential than the desired ground. Star grounding helps minimize this. See this article from Analog Devices explaining the grounding methods.