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We all use them, but do we really know why we sprinkle the board with these caps? There are two reasons to use decoupling caps, the first is to protect a device from AC noise on a line. In this case the decoupling cap acts like a filter. The cap filters out the unwanted noise because it has a high impedance (nearly infinite) to DC voltages, but a low impedance to high frequencies. So the noise sees a low impedance path from the voltage line to ground; the noise is shunted away from the device.
A ceramic capacitor has a self-resonance of about 15Mhz for a value of 0.1µF and about 70Mhz for a value of 0.01µF. Most small MCU’s will run at these speeds so there is ample opportunity for this frequency of noise to show up in your signals. It is not uncommon to place two or three caps in parallel to protect a range of frequencies.
The second reason to use a decoupling cap is a charge shortage. Some devices will cause a dip in voltage when they switch. In general, this type of event is usually handled by bulk capacitance on the board. That is, large capacitors that fill in charge when a large draw occurs. This can also be addressed locally by a much smaller value decoupling capacitor that does much the same thing but on smaller scale (or rather a shorter time frame). This decoupling cap can prevent these voltage dips from affecting a particular device, for a very short period of time, typically 10’s of nanoseconds. Conversely if we know a particular device will gulp large amounts of charge we can place a decoupling cap next to it to prevent these voltage dips from propagating around the board.
If you think about both of these cases from a charge point of view then what the decoupling cap is doing is the same thing in both cases, filling in a shortage in charge to keep the voltage level constant. The difference is the affected frequencies and the magnitude of the event. Small valued caps are good at fixing high frequency, low amplitude dips, while higher valued caps are better at fixing lower frequency, higher amplitude dips.
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