A great series of on-line articles articles by analog and digital guru Kendall Castor-Perry, aka The Filter Wizzard ....
Many of us (I'm also guilty) design power supplies for audio and related applications on auto-pilot. A (low dropout) regulator, selected from the data sheet values for input rejection, output impedance over frequency and output noise. Slap on a decoupling capacitor to keep it stable, and you're done.Not so, says KCP in "Yet more on decoupling".
In true KCP manner the articles are very thorough, and sometimes long-winded. But if you take the time to read the several installments, and digest what's being shown, you will come away with a much better understanding what is needed for a truly outstanding performance of the complete power supply and application combination. Highly recommended!
As an incentive, here are the final Takeaways and conclusions from the series
- Signals on the supply pins of an op-amp will find their way to the output especially at higher frequencies, but also at LF if the amplifier has poor low-frequency open-loop gain.
- Self-inflicted supply variations due to varying load current are always present and can never becompletely rejected by the amplifier. They will affect the amplifier output to some accuracy level, and you can investigate this to a surprising level of accuracy in simulation.
- Op-amp vendors put many obstacles in your way to simulating this effectively " press them (and your regulator and passives suppliers) for proper SPICE models that work! Or just use op amps with working models, by testing out the models with the test fixtures from this series.
- Don't use low-ESR regulator capacitors just because the regulator vendor says they are OK for 'stability'. Your system performance is likely to be significantly better with the additional losses that tantalum capacitors can provide in this role.
- A bit of slew-rate limiting in the op amp isn't necessarily bad; the resonances on the supplies are excited less strongly the slower the amplifier output is slewing. But if there are some fast-slewing, wide-bandwidth amplifiers affecting the supply rails, then low-bandwidth amplifiers, with their consequently poor supply rejection, will be particularly sensitive to the resulting noise. Careful in a mixed system!
- Use the highest value local ceramic decouplers you can get, in the largest package that will fit, and don'tuse Y5V dielectric material unless you've fully allowed for the value change with bias voltage. But the volumetric advantage of the high-k dielectric may be completely lost with this allowance.
- Adding another small ceramic capacitor in parallel with the main decoupler has a small effect on the amplifier output " but it's often a negative one, so test carefully; most times, it may be unnecessary.