Power Decoupling & Hum Noise

I used to use 100uf with 0.1uf to decouple power rail, both on positive and negative rail. It seems to be a common way to decouple power supplier noise, but sometimes I find it will cause humming noise. After searching around the forum, someone suggests that sometimes the voltage fluctuation of positive and negative rail are not equal. The ground will be dragged by decoupling capacitors. That will cause the buzz/humming noise.

If that is the case, most amplifiers with this type of decoupling will has bigger chance to produce humming noise. What's I don't understand is that why using 100uf even bigger decoupling capacitor is so popular, even in some chip data-sheet, the typical diagram suggests using 100uf to decouple power supplier. I cannot imagine someone design the chip does not see that will cause buzz noise. They should be much clever than me.

Please let me know what i am missing.

Zero D

2009-08-06 11:11 am
Hi, it sounds as if the capacitor "might" be faulty, and/or the negative lead is not connected via a seperate ground path to ground common. Sometimes crud can be injected into circuits if the ground return of components like capacitors, are connected too near to other components sharing the same PCB track. Try moving the return closer, or directly to common ground.
Hi, All,

I find the solution finally. Please see Leach Amp Power Supply

The normal decouple method aims to eliminate oscillation caused by wire inductance. If you include signal ground to do decoupling, it will have big chance to introduce rail ripple into the signal ground. If you do not include the signal ground, it will lead to poor RF rejection on signal ground.

The link above provides a smart way to solve this dilemma.