Dual rail IC + TLE2426 needs -- Two bypass caps, or just one?

Discussion in 'General Electronics Chat' started by SporkInTucson, Nov 26, 2012.

  1. SporkInTucson

    Thread Starter New Member

    Dec 17, 2011
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    On another closed thread, SgtWookie wrote:
    I wonder if this is true in the particular case of a dual rail op amp being powered by a TLE2426 rail splitter which isn't powering anything else. If I have a bypass cap across the TLE2426 IN and COMMON, that "stiffens" the voltage into the TLE. From there, I assume that I can depend on the TLE to reliably divide that stable difference into halves (or as close to halves as that particular TLE can manage), without introducing significant new noise. If I'm reading the data sheet correctly, the noise out of the TLE2426 is about 120 microvolts, since I'm using the three lead version which has no noise reduction pin.

    Is that 120 microvolts of noise enough that I should forget about the cap across IN and COMMON, and just put one each on the op amp's V+ and V-? Are they going to make much of a difference with respect to noise at that level?
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    You should still have capacitors across the power supply pins of the op amp (always a good practice). Also, if you are using a ground pin as a voltage reference to the op amp, then two capacitors are best (Vss to ground and ground to Vdd). Fast switching of digital components downstream of your power controller can cause noise in the circuit that the controller overcome - especially as you approach the current limit of the controller.
     
  3. SporkInTucson

    Thread Starter New Member

    Dec 17, 2011
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    Hmm... I'm not sure how the terms "Vss, Vdd, and ground" map to my case. Here's a simplified diagram:

    [​IMG]

    Not shown in this diagram, I currently have one cap across V+ and V- on the daughter board, as described in my original post. If I were to have two, which would make more sense for the daughter board?

    • From VGND to V- and from V+ to V- (with V- as the ground plane)
    • From V- to VGND and from V+ to VGND (with VGND as the ground plane)
    • From V- to VGND and from V+ to VGND (with V- as the ground plane)
    • None of the above

    Is the decision about where capacitors go completely independent of the choice for ground plane? Should bypass caps always bypass to the ground plane if one exists?

    Thanks for your feedback, Gopher.
     
    Last edited: Nov 26, 2012
  4. MrChips

    Moderator

    Oct 2, 2009
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    None of the above.

    Your PCB Ground Plane is V- on both boards.

    Cap from V+ to VGND.
    Cap from VGND to V-.

    VGND is a reference, not a GROUND PLANE.
     
    Last edited: Nov 26, 2012
  5. SporkInTucson

    Thread Starter New Member

    Dec 17, 2011
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    Thanks, MrChips. You must have been replying with "none of the above" and your alternative, just as I was editing my post to add the same as a third option.
     
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