What is thedifference between analog and digital circuits?

digital v/s analog

can i get to know what is the main difference between analog circuits and digital circuits? during layouts how can 1 diffrentiate between the 2 of them?what major precautions are taken while designing analog circuits over digital circuits?

Re: digital v/s analog

hello,

the digital layouts mostly involve different gates (AND,NAND, OR), and these are made accoding HDL code and basically involv combinations of NMOS and PMOS trasnsitors, coming to analog layout they involve amplifires and other things.

so differece is only in making the layouts, but the tools used and the procedure for making layouts is all same as far as I know .please let me know if I am wrong

suresh

Re: digital v/s analog

as far as tool is concern i think it wont make any difference

the main thing is u have to take care when u design
means u hv to set ur parameter accordingly so either it works as a analog or digital component...
like wise for Mos its width,length, biasing resistor , parasitic element etc....

Re: digital v/s analog

digital circuit work in saturation area of semiconductor.
but analog circuit work in linear area.

Re: digital v/s analog

The thing about digital design is its mainly tool centric. Thats because of the huge transistor count invloved. So there is a tool that optimises and automates for every stage in digital design flow. You basically would not need to know more than say the basic transistior knowledge. But the tool knowlege has more emphasis. And finally in digital design the trend is more towards Cell Based Design or Standard cell design implementations.
In Analog or Mixed Signal design flow it may said that the converse of digital design is partly true. Here there is a need for indepth knowledge of device physics, the knowledge on layouts etc. basically you need to be well versed with the entire flow. The emphasis on tools will be during simulations. In analog Innovation is the key. Hope this gives some idea about Analog design and Digital Design.

Re: digital v/s analog

one more thing that i might add.

in designing analog circuits, noise will be a bigger concern to deal with. and there are much more factors that need to be taken into consideration: bandwidth, matching, and of course power.

Hence, in analog design, one would not merely make transistors as small as possible to speed it up as in digital designs. a good practice taught by my colleague is that the length of a transistor in analog design should be at least twice the minimum length? CMIIW (i don't do analogs )

in conclusion, analog is harder.

Re: digital v/s analog

even i would also like to ask the some major difference...
like suppose i have simple mos transistor
how will we differentiate or how can we work out that is it going to operate as a digital or analog device???

means wat r the basic boundary line between both the domain???

Re: digital v/s analog

Hi,
When we are dealing with the schematic of analog circuits using CMOS technology, all the transistors need to be in the saturation region because of the linearity which is to be acheived in analog designs.
Where as this conditions do not apply on digital logics where there is rail to rail swing.

For better understanding refer to book by Razavi, and the famous Analog Designers OCTAGON.

In digital design we are only concerned with the speed and power requirement,
whereas in analog design there are many issues such as noise, power, speed, linearity etc.

As far as the Layout issues are concerned analog layouts have to be properly matched.

Regards
Shaikh Sarfraz