Signal distortion from high-K ceramic capacitors
Multilayer ceramic capacitors (MLCCs) are used extensively in modern electronics because they offer high volumetric efficiencies and low equivalent series resistances at attractive prices. These advantages make MLCCs nearly ideal for a wide range of applications, including output capacitors for power supplies and local decoupling capacitors for integrated circuits. The various types of MLCCs are delineated primarily by their temperature coefficient, which is the amount of variation in their capacitance over a specified temperature range. Class I types, given a designation of NP0 or C0G, must vary less than +/–30 ppm over their operating temperature range, while Class II types can change anywhere from +/–15 percent (X7R) to +22 percent /–82 percent (Z5V) .
The temperature coefficient of an MLCC is a direct effect of the materials used in the ceramic that forms the capacitor dielectric. Furthermore, the dielectric material also determines the electrical characteristics of the capacitor. Class II dielectric types (X7R, Z5U, Z5V), often are referred to as “high-k” ceramics because their dielectric materials, have relative permittivities that range from 3000 (X7R) up to 18000 (Z5U). Class I C0G capacitors tend to have relative permittivities in the range of six to 200 . The benefit of increased relative permittivity of the dielectric material is that high-k MLCCs are available in much larger capacitance values and smaller packages than C0G types.
Unfortunately, these advantages come with a downside: high-K MLCCs exhibit a substantial voltage coefficient, meaning their capacitance varies depending on the applied voltage. In AC applications this phenomenon manifests itself as waveform distortion and can compromise the overall system performance. When printed circuit board (PCB) area and cost are major design constraints, board and system level designers may be tempted to use high-K MLCCs in circuits where they can introduce significant distortion into the signal path.
Demonstrating high-K MLCC distortion
Active filter circuits, anti-aliasing filters for data converters, and feedback capacitors in amplifiers are examples of circuits where the use of a high-K MLCC may introduce distortion. In order to illustrate this effect, a 1 kHz Butterworth active low-pass filter using the Sallen-Key topology was designed using TI’s FilterPro software. Active filters are a very common application where distortion from capacitors degrades the overall circuit performance. Many designers choose low resistor values in an effort to reduce their contribution to the output noise and this increases the value of capacitors required for a certain corner frequency. Because of this design decision, high-k MLCCs may be the only capacitors available that meet the requirements for capacitance, board area, and cost.See a follow-up article entitled "More about understanding the distortion mechanism of high-K MLCCs"