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capacitor choices

Discussion in 'Electronic Basics' started by Fritz Oppliger, Aug 5, 2004.

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  1. I am upgrading a circuit I inherited... a controller with an ADC to
    measure various DC voltages (0 - 2.5VDC).
    The analog nets are liberally sprinkled with 1uF Tantalum capacitors to
    ground , obviously trying to dampen noise. My questions:

    why polarized?
    Why tantalum?
    Why 1uF?

    I am trying to migrate to SMD technology and find to my surprise that SMD
    TA caps are rather expensive (unless I am reading the Mouser catalog
    wrong). So I am looking for possible (cheaper) alternatives or convincing
    justifications.
    The original board had serious digital to analog noise problems (no single
    ground point, noisy ADC Vref) so I am guessing that all these caps may
    not be needed at all or could be changed to something tame and cheap.
    Plus, I'm smoothing in SOFTware ;-)
     
  2. Large values are smaller than nonpolarized types.
    Low leakage, high reliability if properly applied. Designer may have
    thought that it increased his prestige if his circuit used high priced
    parts.
    I am quite sure the designer pulled the value out of his uh.. oh never
    mind.
    They are spendy.
    You should first investigate what can be done with good layout and an
    extra board layer or two, if the board is just two sided. Then, after
    layout related problems have been brought under control, you can think
    about circulating currents and how to supply them locally with
    capacitors and what values would be appropriate. Ceramic caps are
    available in sizes and with competitive costs for all but the larger
    tantalums. And a combination of an aluminum electrolytic and ceramic
    may exceed the performance of single tantalums at lower total cost.
     
  3. John Jardine

    John Jardine Guest

    ROFL!. John, you've just made my day :)
    regards
    john
     
  4. colin

    colin Guest

    out of his hat ?

    having one large area ground plane on a pcb isnt always the optimum
    solution, if you have noisy part of the circuit close by, noisy ground
    currents can propgate noise voltage gradients through the ground plane
    (albeit rather small ). but this can cuase problems wich are hard to get rid
    of with any amnount of decoupling capacitors.

    in order to get round this ive found it better to have an individual area of
    gnd plane for the smal siugnal stuff that conected to the rest at one point,
    of course it becomes important to select the right place to make the
    conection, but it stops the voltage gradient wich u can actualy see on a
    scope with a coax conected to a .1 "header" used as a probe.

    Colin =^.^=
     
  5. Amen to that!
    Thank you.
    I think I will place generic largish footprints (1206) where some of these
    caps used to go, but not populate them until I find there is a problem.
    I am not looking forward to re-working SMD stuff... my eyesight will be
    GONE after more of this staring at electronic parts catalogs!

    Right, the evaluation board I am looking at has two distinct ground planes
    and they are connected underneath the very center of the CPU (which has
    the ADC built in)
     
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