It is obvious to me that decoupling capacitors are necessary in proper circuit design. I also realize that different types of capacitors can be more useful depending on the application. However, I have also read in several places (most recently in Op Amps: Design, Application, and Troubleshooting by David L. Terrell) that decoupling capacitors can completely be negated in their purpose if aluminum elecrolytic capacitors are used due to their higher impedance or the leads are not short enough. This ties directly into my question:
Why do I find aluminum electrolytic capacitors being used in decoupling techniques? Is it bad practice to use them or are these few sources I have read from incorrect in their explanations?
Best Answer
In addition to their capacitance, all capacitors have additional 'stray' components that may have to be taken into account if the circuit is sensitive to them. These include leakage, equivalent series resistance ESR, series inductance, and dielectric relaxation, not to mention the ability to degrade with temperature and/or age.
Different technologies of capacitor will have different strengths and weaknesses. For ESR, alli electrolytic is highest, followed by tantalum, with plastic and ceramic the best. The inductance generally follows the size and lead length. For leakage, alli electrolytic is worst, though usually not a problem in decoupling applications, you just avoid using them for long time delays in a 555 circuit.
For decoupling your circuits, it is important to use appropriate sized capacitors in the right places.
Most op-amp circuits will be working with such low frequency components that almost any capacitor can be used for supply decoupling, as long as the leads are kept short, alli electrolytic will be fine.
Very fast op-amps, for example video capable, as well as digital circuits, will need small value, low inductance, short lead decoupling capacitors very close to the ICs. In addition, the board will also need bulk decoupling, an alli electrolytic is fine for this.