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Analog Layouts VS Digital Layouts

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What is the basic difference between the two ????

Thanx.
 

Analog layout is usually done manually by circuit designer, but digital layout is usually done automatically by CAD tools. For example in analog layout, the layout of a NAND gate is done manually, but in digital layout, the layout of a NAND gate is already in the CAD tool's library, and the CAD tool uses the different layout cells in its library to construct the layout of a digital circuit usually designed using VHDL or Verilog .
In analog layout the length of transistors is not usually the minimum, but in digital it is. In analog layout matching of transistors is very important but it is not the case in digital layout.
 

Transistor as switch in Digital

Transitor as amplifier, resitor and other passive in analog...
 

In Digital Layout, the W/L ratio's used are the minimum feature size. This is done to minimize the area and maximize the packin density. When the area is minimized, the delay also gets minimised. Digital design is easier because we can use cell-based methodology to do a layout, wherein you already have predefined layouts of standard cells and use them to create larger blocks. This saves time and money.

Analog layout on the other hand is tougher as more importance is given to transistor and interconnect details. This is done to acheive matching and the required currents and voltages. Analog design is based mostly on the drain currents and bias voltages. So, layout has to be done carefully to acheive these voltages and currents else the design will fail when manufactured. In analog layout/design we trade off area for performance.
 

vlsi_whiz said:
In Digital Layout, the W/L ratio's used are the minimum feature size. This is done to minimize the area and maximize the packin density. When the area is minimized, the delay also gets minimised. Digital design is easier because we can use cell-based methodology to do a layout, wherein you already have predefined layouts of standard cells and use them to create larger blocks. This saves time and money.

Analog layout on the other hand is tougher as more importance is given to transistor and interconnect details. This is done to acheive matching and the required currents and voltages. Analog design is based mostly on the drain currents and bias voltages. So, layout has to be done carefully to acheive these voltages and currents else the design will fail when manufactured. In analog layout/design we trade off area for performance.
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What type of precautions are you talking about when you say that analog layout has to be done carefully else the design will fail when manufactured.
 

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