What to learn when switching from std cell or custom layout to analog layout?

Hi All,
I have 2+ years of experience in std cell layout design & custom layout design. I know that when we draw custom layout for memories its going to be really challenging as we need play with thousands of transistors. Here is my Question.. How the Analog Layout is different from my experience?. What are all the things that I need to learn if I want to switch to Analog Layout?.

Thanks in Advance..

Re: Layout Experience

hi,
There r random variations in devices ,once it goes from layout to fab .So need to be concerned abt minimising variations. several techniques r used for such.You can read Analog layout by alan hastings ,and can get numerous articles regarding this on web . ALL the Best.
Regards,
roy

Layout Experience

Hi Kumar...

As said by the previous post, a very good starting point (for the usual analog layout issues) is Alan Hasting's Art of Analog Layout.

To give you a very simple and quick start, there are two MAIN factors to consider when laying out analog (also mixedsignal) ICs: Temperature variations (gradient) and package stress (both of them across the chip).

As I said before, MAINLY you design focused on minimize the effects of that factors in your final product.

That's why, again, two things are key in analog layout:
1- An excelent floorplanning. It's really important how you place your blocks in order to make sure and know where your bigger power dissipation device is located and how iso-thermal lines will be present in your die, that's the base for the "second" point:
2 - MATCHING. Yes, in uppercase... Matching techniques usage, more in particular sensitive stages is a MUST!

Hope this helps to give you the requested idea...

PS: Hard and difficult, but nothing to kill yourself about it... In my group I already have guys comming from the "digital world" and they where up un running very soon...

Re: Layout Experience

Thanks alot Layoutmaster & roy_ece.. When we draw the layout for std cells, we used to draw it just by seeing the schematic.. I have never come across any situations that we need to take care of temperature and packaging issues.

Please Explain about Matching..

Layout Experience

In our world everything starts by seeing the schematics...

THen you need to identify which sub-circuits (group of components) need to match as much as possible and then you'll use the usual matching techniques for them, starting from interdigitation and crosscoupling...
To see details of them, and some other matching things, you can see Hasting's book (his chapter about matching is really good, I recommend you to read it).

Layoutmaster has some good suggestions as to what to pay attention to.

Floorplanning is a must. Know which cells are noisy and which cells need to be kept away from noise. Be aware of where I/O pads are in relation to the cells. Typically you don't want to have long lines for your I/O connections to the pads.

Isolation of certain devices is important too. You might need to isolate groups of devices or even individual devices with guard rings. If you have a mixed signal design, analog circuits are usually isolated from the digital section. You will usually have separate powers and grounds.

Pay attention to the width of your lines - some lines will need to be wide for current density and some will need to be wide to lower resistance but you have to balance that with not making them too wide and risk too much parasitic capacitance. It is helpful if the designer can identify which lines need to be wide and how wide they need to be.