A typical analog design flow

In case of analog design, the flow changes somewhat.

=>Specifications=> Architecture =>Circuit Design =>SPICE Simulation =>Layout =>Parametric Extraction / Back Annotation =>Final Design =>Tape Out to foundry.

While digital design is highly automated now, very small portion of analog design can be automated. There is a hardware description language called AHDL but is not widely used as it does not accurately give us the behavioral model of the circuit because of the complexity of the effects of parasitic on the analog behavior of the circuit. Many analog chips are what are termed as “flat” or non-hierarchical designs. This is true for small transistor count chips such as an operational amplifier, or a filter or a power management chip. For more complex analog chips such as data converters, the design is done at a transistor level, building up to a cell level, then a block level and then integrated at a chip level. Not many CAD tools are available for analog design even today and thus analog design remains a difficult art. SPICE remains the most useful simulation tool for analog as well as digital design.

From above discussion n from my personal experience i feel that digital design is the most important aspect of the VLSI design flow. Think if your design has some bug...!! the whole process then is costing billions of $. So it's very essential to take care start from the initial phase of designing.

Here during our discussion further we will go through several important concepts of digital dsigning and also see some standard designs.