Why? Supply pins position differ in Dig & analog IC's

[Muhammad Abu Bakar]

Hello!

Does some one know why the supply pins in digital IC's are at corner while most OP-amps have them isomewhere in middle of the chip. Is it only due to mechanical construction?

Regards!

 

[ZomBiE]

Many manufacturers have gone to a compromise with their designs ages ago with themselfs for compatibility... All the chips can't be the same for design broblems, etc...

 

[Theatronics]

The old Standard was to place the digital power pins in the corners of the chips.
in the 70s this made board layout a bit easier.  Power buses ran between the chips in rows and they were much larger traces than the digital signals that could sneak in and out between the pins. 

Analogue chips didn't live in a world of straight rows and data busses.  The pin placement was driven by design needs. Some parts were designed as direct replacements of older parts.  Other parts had pin layouts based on expected use.  For instance, if possible, two related pins might be placed near each other to simplify PCB layout.

There are a few industry standards but they are not rules, just traditions. 

Just like most people like to mount through hole parts all on one side of the PCB and call it the top.  If a part doesn't fit one way, why not try mounting it from the bottom? Presto, a reverse pinout!

-Mike

 

[indulis]

More often than not, it's a matter of convienience when connecting the die to the carrier. You don't want the bonding wires running all over the place.

 

[Muhammad Abu Bakar]

Hello Mike

Just like most people like to mount through hole parts all on one side of the PCB and call it the top.  If a part doesn't fit one way, why not try mounting it from the bottom? Presto, a reverse pinout!

I realy do so. Once I had PCB's printed from wrong side(acutually flipped). Then I had to solder DIPs at the bottom side. However I got ZIPs on top side with moving the legs at opposite sides.

About the matter of IC pins I personally feel:

1. All i/p pins should be grouped togather (Mechanicaly in a row).
2. All o/p pins should be grouped togather.
3. Power supply pins should be neighbours.
4. The pins performing control functions should come in line.


Perhaps it might put PCB designers on a Holiday  ;D

 

[Theatronics]

Yes, that would make it easier in many cases.


Once in a while I have looked at a chip's pinout and wondered
what the designer was thinking when they made the part.  ???  Later
as I started to design a board layout I found that some of the pin
placements were rather clever. 

When working with a new chip with an odd pin out I try to find a
example PCB layout from the manufacturer.  These can sometimes
hint at why some designs were selected.

-Mike