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Electronics Engineering DISCUSSION
Every schematic I copy has a 100 nF capacitor from each supply pin to ground, and every guide says to place it as close to the pin as possible. My breadboard circuits seem to work without them, so I have been treating them as optional.
What problem do they solve, why is the value always 100 nF, and how close is close enough on a PCB?
Digital ICs draw supply current in short, sharp pulses each time internal gates and outputs switch. The supply path from the regulator has inductance, very roughly 1 nH per millimetre of trace or wire, and an inductor resists fast current changes: V = L × di/dt. A 20 mA step in 2 ns through 20 nH of wiring gives 20 nH × 0.02 A / 2 ns = 0.2 V of dip at the chip's supply pin.
The decoupling capacitor is a small local reservoir right at the pin. It supplies the fast pulse, and the slower supply path only has to refill it afterwards.
The value is not critical, which is why one value is used everywhere. The charge in that pulse is tiny: 20 mA for 2 ns taken from 100 nF changes its voltage by I × t / C = 0.4 mV. What limits a capacitor at high frequency is its own inductance. A capacitor with its pads and vias is a series LC circuit, capacitive below its self-resonant frequency and inductive above it.
For 100 nF with 1 nH of total inductance, f = 1 / (2π × sqrt(L × C)) = 1 / (2π × 10^-8 s), about 16 MHz. Above that, the impedance is set by the inductance alone, so a small package mounted with short connections matters more than the capacitance printed on the reel.
Close means a small loop. The pulse current flows from the capacitor into the supply pin, through the chip, out of the ground pin and back to the capacitor. The area of that loop sets the inductance. On a PCB, put the capacitor on the same side as the IC, within a few millimetres of the supply pin, with short wide traces and the ground via right at the capacitor pad. One capacitor per supply pin, not one per chip.
With solid power and ground planes the exact position is a little less critical, because the planes themselves are a low-inductance path, but vias shared between several capacitors or long thin traces to the pad throw the benefit away.
The 100 nF parts handle the fast edges; they cannot hold the rail up for slower load changes. For that, add bulk capacitance, typically 1 to 10 µF per IC or group of ICs and more where power enters the board. A regulator takes microseconds to respond to a load step, and the bulk capacitors cover that gap.
Follow the datasheet where it is specific. Microcontroller datasheets usually state a capacitor per supply pin plus one larger one, and they give separate instructions for analog supply and reference pins, often with a ferrite bead or small resistor in series to keep digital noise out of the ADC.
Working without them on a breadboard is mostly luck with slow signals. The typical symptoms of missing decoupling are intermittent: random resets, noisy ADC readings, glitches that appear only when several outputs switch together or when a motor or relay is added, and failures that come and go with temperature or supply voltage.
One component detail: ceramic capacitors of the high-capacitance types lose capacitance under DC bias. A 10 µF, 6.3 V part in a small package can be well under half its nominal value at 5 V. Check the manufacturer's bias curve and choose a voltage rating with margin.
