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Rail-to-rail decoupling

Mateusz Kaluzny17
Expert 1065 points

Hello dear Ti.

I just wanted to clarify some thing.
There is often mentioned that good decoupling is critical for system stability and performance.
But is definitely rare mentioning which decoupling method, classis - I mean between supply line and ground or between both rails (exluding ground).

So question is when should I use rail-to-rail decoupling in my desing and when it's not needed (or should be avoided).
In my case loads are connected to ground.

Another question is, should I use rail-to-rail decoupling (if needed) in close proximity to every opamp (lets asume I will place a capacitor on the bottom plane, directly below opamp) or maybe it's not critical? And I can place those cap/s closer to main LDO's.

  • 0
    TI__Genius 14317 points

    Hello Mateusz,

    Please see https://e2e.ti.com/blogs_/b/precisionhub/archive/2013/08/13/the-decoupling-capacitor-is-it-really-necessary for a good discussion of decoupling basics for OA circuits with bipolar supplies driving loads connected to GND.

    For Class D Audio Amps, this App. Note describes how to use capacitors in parallel to decouple over a wide frequency range (use different value/time constant caps).

    http://www.ti.com/lit/an/slyt199/slyt199.pdf

    I've never seen the need to use decoupling caps between +/- power rails for bipolar amps especially when driving GND based loads since the return path for high frequency currents is from GND at the load back to the supply rails (+/-) for the driver.

    It is possible that depending upon how the output driver is architected, and when driving a load differentially, that decoupling between the rails could provide a lower impedance return path to the supply rails for load current.

    This article in EE Times discusses rail to rail decoupling (on page 2) for a particular OA circuit driving a GND referenced load. The author advocates rail to rail decoupling to avoid injecting noise on GND (as he says, when using decoupling each rail to GND).

    http://www.eetimes.com/document.asp?doc_id=1278963

    As you can see this topic is quite controversial  in engineering circles!