Two ceramic capacitors can both say "100 nF, 50 V" and behave very differently in a circuit. The difference is the dielectric — the three-character code such as C0G, X7R or X5R.
What the code means
For Class 2 dielectrics (the X and Y types), the three characters describe the temperature range and how much capacitance may change across it.
| Code | Temperature range | Capacitance change over that range |
|---|---|---|
| C0G (NP0) | −55 to +125 °C | ±30 ppm/°C — effectively flat |
| X7R | −55 to +125 °C | ±15% |
| X5R | −55 to +85 °C | ±15% |
| Y5V | −30 to +85 °C | +22% to −82% |
C0G / NP0: stable, but small values
C0G is a Class 1 dielectric. Its capacitance barely moves with temperature, it does not lose capacitance when you apply DC voltage, and it does not age. The trade-off is low capacitance per volume, so C0G parts are mostly found from a few picofarads up to some tens of nanofarads.
Use C0G for anything where the value matters: crystal load capacitors, RF matching, filters, timing and precision analog.
X7R and X5R: the general-purpose choice
These pack far more capacitance into the same package, which makes them the default for decoupling and bulk capacitance. But the capacitance printed on the datasheet is measured with almost no DC voltage applied.
- DC bias: capacitance falls as the applied DC voltage rises. A high-value part in a small package can lose half its capacitance or more near its rated voltage.
- Ageing: capacitance drops slowly over time after the part was last heated above its Curie point (for example, during soldering).
- Temperature: up to ±15% across the rated range.
X7R is rated to 125 °C and X5R to 85 °C. For anything that runs warm, prefer X7R.
Y5V: usually avoid it
Y5V gives a large headline value cheaply, but it can lose most of its capacitance at temperature extremes and under DC bias. It is acceptable only for non-critical bulk decoupling in a mild environment.
Practical rules
- Timing, filters, crystals, RF: C0G.
- Decoupling a digital IC: 100 nF X7R per supply pin — see why 100 nF.
- For bulk values in X5R/X7R, choose a voltage rating at least double your rail, and a larger package where space allows. Both reduce the DC-bias loss.
- Always read the manufacturer's DC-bias curve for values of 1 µF and above.
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