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Resistors and capacitors - pull-ups and pull-downs?

Discussion in 'Electronic Basics' started by max-man, Aug 7, 2005.

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  1. max-man

    max-man Guest

    I know that resistors can be used as pull-ups or pull-down, but
    sometimes I come across old PCBs that have capacitors linked between
    an input or output and ground or +5 volts. Are these caps being used
    to pull-up or pull-down the input or output, or for some other reason?


    Thanks
     
  2. max-man

    max-man Guest

    insert here: "on an IC"


    :)
     
  3. When the rise and fall time of signals is much shorter than the
    transit time along a trace or wire, the wave shape can be distorted by
    ringing if the impedance of the line does not match the source and
    receiver impedances. Sometimes this problem is solved by putting a
    series RC load on the receiving end that absorbs energy at the ringing
    frequency, without adding any DC load to either the logic high or low
    level. This also works pretty well when a low value resistor is
    inserted between the source and the line to slow, slightly the rise
    and fall times. I haven't seen a case where just a capacitor is used
    at either end, except for CMOS inputs that have a series resistor in
    the line, to produce an effective delay between signal source and
    downstream gate. This is a completely different function than
    controlling ringing and radiated noise.
     
  4. jgreimer

    jgreimer Guest

    One possibility is for Electro Static Discharge protection. If one only has
    to protect against the Human Body Model which may consist of a 100pF
    capacitor at 1000 volts and a series resistance, and if the input or output
    line doesn't carry high frequencies, then one of the cheapest ways to
    protect the circuit is to put a 100nF capacitor across the line. Since the
    shunt capacitor has 1000 times the capacity of the Human Body Model, as it
    absorbs the discharge its voltage change will be only 1/1000th the discharge
    voltage. Note: There are many better ways of protecting against ESD but
    this is one of the cheapest.
     
  5. me

    me Guest

    (max-man) wrote in @news.freeserve.net:
    If on a logic IC it is most likely used as a "de-spiking" capacitor,
    "smoothing out" the big spike on the supply line caused by logic level
    shifts.
     
  6. max-man

    max-man Guest

    Many thanks for the answers guys. :)
     
  7. Impmon

    Impmon Guest

    More likely the caps are being used to set the inputs or lines. For
    example, a cap may be connected from a chip to ground, it would be
    safe to assume at first power on, the cap pulls that line to ground
    briefly until the cap is fully charged then the line behaves as
    normal.

    Commonly used to reset chips while allowing the same line to be used
    during its operation.
     
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