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decoupling capacitors

Discussion in 'General Electronics' started by Quack, Apr 20, 2004.

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  1. Quack

    Quack Guest

    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

  2. Robert Baer

    Robert Baer Guest

    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...
  3. Quack

    Quack Guest

    The capacitors would have to be added (AKA pick-and-place), and not

    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. :)
  4. Charles Jean

    Charles Jean Guest

    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).
  5. Robert Baer

    Robert Baer Guest

    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.
  6. Robert Baer

    Robert Baer Guest

    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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