EMC Question of the Week: May 27, 2019

X7R multi-layer ceramic capacitors are often used to provide high-frequency decoupling on printed circuit boards because,

1. their value is independent of applied voltage
2. their value is independent of temperature
3. they have more capacitance per volume than tantalum capacitors
4. they have more capacitance per volume than NP0 capacitors

Answer

The best answer is "d".  X7R capacitors are Class 2 ceramic capacitors. Compared to Class 1 ceramic capacitors (like NP0s), their value is not particularly stable with temperature or applied voltage. The greatest advantage that Class 2 capacitors have over Class 1 capacitors is the ability to achieve relatively high values of capacitance in a small package size. 

In printed circuit board decoupling applications, the exact value of the capacitance is not a major concern. In addition, there is always a significant DC bias across the capacitor that minimizes variations in capacitance due to changes in the applied voltage.

Tantalum capacitors, which are electrolytic capacitors, generally have a higher capacitance per volume than ceramic capacitors. They are often used to provide low-frequency decoupling on circuit boards, but not for high-frequency decoupling. At high frequencies, where connection inductance becomes more important than nominal capacitance, multilayer ceramic capacitors are a much more cost-effective alternative. 

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