thatoneguy
- Joined Feb 19, 2009
- 6,359
If in doubt, or working with higher frequencies, the decoupling cap should have a resonant frequency higher than the risetime of the waveform.
When the above thumb-rule is a conflict, use a pair, such as 0.1uF and 4.7 or 10uF. This provides two frequencies at which the impedance is lowest, and overall lower power supply impedance throughout the system. Visualize the larger cap moving the power supply "near", and the smaller cap bringing the supply the last bit of distance, like two wire stretchers.
The size of the large cap should increase if there is high current draw IC, such as a driver, but the 0.1uF should stay roughly the same. The reason for this is real capacitor properties such as ESR, ESL, which create small RC time constants.
I realize the above doesn't make intuitive sense. Confusion usually arises from thinking about decoupling caps as ideal mini-batteries. When viewed in terms of waveform risetime to determine the smallest RC value for the lowest source impedance to the IC, and a larger cap to reduce source impedance to the smaller cap, it becomes a bit more clear.
Finally, if building your circuit on a solderless breadboard, cut the component leads as short as practical to be reused, yet still "reach". Also, cut all sorts of custom length breadboard wires, and avoid curling/twisting them.
When the above thumb-rule is a conflict, use a pair, such as 0.1uF and 4.7 or 10uF. This provides two frequencies at which the impedance is lowest, and overall lower power supply impedance throughout the system. Visualize the larger cap moving the power supply "near", and the smaller cap bringing the supply the last bit of distance, like two wire stretchers.
The size of the large cap should increase if there is high current draw IC, such as a driver, but the 0.1uF should stay roughly the same. The reason for this is real capacitor properties such as ESR, ESL, which create small RC time constants.
I realize the above doesn't make intuitive sense. Confusion usually arises from thinking about decoupling caps as ideal mini-batteries. When viewed in terms of waveform risetime to determine the smallest RC value for the lowest source impedance to the IC, and a larger cap to reduce source impedance to the smaller cap, it becomes a bit more clear.
Finally, if building your circuit on a solderless breadboard, cut the component leads as short as practical to be reused, yet still "reach". Also, cut all sorts of custom length breadboard wires, and avoid curling/twisting them.
