100nF Ceramic Capacitor
The near-universal decoupling capacitor value, placed right next to an IC's power pins to absorb switching noise.
How this component works
A capacitor stores charge on two conductive plates separated by an insulator (dielectric). Its capacitance, measured in farads (F), determines how much charge it can hold per volt applied.
In DC circuits, a capacitor blocks steady current once charged but allows current to flow briefly while charging or discharging — this is the basis of timing circuits and debounce filters. In AC circuits, a capacitor's impedance falls as frequency rises, which is why capacitors are used to filter out high-frequency noise or block DC while passing AC signals.
Common types differ mainly in construction: ceramic capacitors (small values, no polarity) and electrolytic capacitors (larger values, polarized — must be wired the right way round or they can fail).
Deep Dive: 100nF Ceramic Capacitor
In decoupling applications, the 100nF ceramic capacitor acts essentially as a tiny, ultra-fast local battery. When a digital IC switches state, it momentarily demands a sharp spike of current. Trace inductance from the main power supply would delay this current, causing a brief local voltage drop (a "brownout"). The 100nF cap, placed millimeters from the IC pins, has virtually zero trace inductance and supplies the instant current spike, keeping the local voltage stable.
Its impedance curve (which dictates how well it shorts noise to ground) drops as frequency increases, hitting a minimum "self-resonant" point typically in the 10-50 MHz range for standard surface-mount sizes. Above this frequency, its internal parasitic inductance (ESL) takes over and it starts acting like an inductor, stopping it from filtering higher-frequency noise.
Because modern X7R/X5R ceramics exhibit the "DC bias effect" — losing effective capacitance as the applied DC voltage approaches their maximum rating — a "100nF" cap running near its voltage limit might only offer 50nF of actual capacitance in circuit. For critical decoupling, selecting a voltage rating at least double the operating voltage mitigates this.
Common Use Cases
- Decoupling capacitor across VCC and GND right at an IC's power pins
- High-frequency noise filtering in parallel with a larger electrolytic capacitor
- Timing capacitor in fast RC networks (e.g. debounce or short-delay circuits)
Frequently Asked Questions
Why 100nF specifically for decoupling?
It's small enough to have very low impedance at the high frequencies digital ICs switch at, while ceramic construction keeps its own parasitic inductance low — both matter more than the exact capacitance for this job.
Does it matter which way round I connect it?
No — ceramic capacitors like this one are non-polarized, so either leg can go to either node.
Comparison Notes
Compared to a 10nF cap, 100nF offers 10x more bulk storage for decoupling, but has a lower self-resonant frequency, meaning it stops filtering noise sooner at extremely high frequencies. Compared to a 1µF ceramic, 100nF is usually physically smaller and cheaper, striking the perfect balance for standard 5V and 3.3V logic operating below 50MHz.