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decoupling capacitors

Q

Quack

Jan 1, 1970
0
It just occured to me, why dont IC's come with internal decoupling
capacitors ? as its suggested to have them as close as possible etc
etc.


Al.
 
R

Robert Baer

Jan 1, 1970
0
Quack said:
It just occured to me, why dont IC's come with internal decoupling
capacitors ? as its suggested to have them as close as possible etc
etc.

Al.

The (relatively large) amount of capacitance needed to do a
"reasonable" job cannot be made as a part of the IC itself.
The capacitors would have to be added (AKA pick-and-place), and not
only extra space would be needed to accomidate them, but extra wiring
(ie: a grounding "ring") would be needed.
The net added cost could be 20 times the retail price of the original
part, not to mention the non-standard, new package size...
 
Q

Quack

Jan 1, 1970
0
The capacitors would have to be added (AKA pick-and-place), and not
only extra space would be needed to accomidate them, but extra wiring
(ie: a grounding "ring") would be needed.

Oh.

Its just that the decoupling i have noticed and used is usually a
small cap right inbetween the vcc and vss pins on an ic (which is very
convenient when they are on directly opposite sides of the package). I
imagine this could be easily accomplished within the chip.
Or is that 'bad decoupling' anyway ? i have not heard anything about a
'grounding ring' ?

Good point about the cost, probably would be worth a lot more than .01
cents or whatever a sm cap goes for in quantity. :)
 
C

Charles Jean

Jan 1, 1970
0
Oh.

Its just that the decoupling i have noticed and used is usually a
small cap right inbetween the vcc and vss pins on an ic (which is very
convenient when they are on directly opposite sides of the package). I
imagine this could be easily accomplished within the chip.
Or is that 'bad decoupling' anyway ? i have not heard anything about a
'grounding ring' ?

Good point about the cost, probably would be worth a lot more than .01
cents or whatever a sm cap goes for in quantity. :)
___
You can buy DIP sockets with a decoupling capacitor attached, but they
cost an arm and a leg. Also position is fixed at diagonal pin
positions(pin1 and last pin).
 
R

Robert Baer

Jan 1, 1970
0
Quack said:
Oh.

Its just that the decoupling i have noticed and used is usually a
small cap right inbetween the vcc and vss pins on an ic (which is very
convenient when they are on directly opposite sides of the package). I
imagine this could be easily accomplished within the chip.
Or is that 'bad decoupling' anyway ? i have not heard anything about a
'grounding ring' ?

Good point about the cost, probably would be worth a lot more than .01
cents or whatever a sm cap goes for in quantity. :)

Since the pins (and bonding pads) aer so far apart, a ground line
would be needed, and to reduce inductance, it would be best implimented
as a metal "ring" around the chip; hence the descriptive term i made,
"grounding ring".
And the capacitor used outside may be physically small (a chip
capacitor), the capacitance can be as large as 0.47uF, and capacitors
made on ICs are in the order of 100pF or less.
 
R

Robert Baer

Jan 1, 1970
0
Charles said:
___
You can buy DIP sockets with a decoupling capacitor attached, but they
cost an arm and a leg. Also position is fixed at diagonal pin
positions(pin1 and last pin).

Correct; it is cheaper for a hobbiest to buy the parts seperately and
pay himself $20/hr for assembly.
And for even low volume PCB assembly where one wants a socket, it is
*still* cheaper to provide seperate parts and make the layout to
accomidate the bypass capacitor(s).
I have done both.
 
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