Ceramic capacitors are classified based on their temperature range.
The X5R capacitor, for example, is intended for temperatures ranging from −55 to +85°C, while in the X7R capacitor, the range is extended to temperatures between −55 and +125°C. These capacitors are temperature-compensated meaning that their capacitance is within ±15 % of the full temperature range. However, this specification does not guarantee that effective capacitance falls within the range when operating outside standard AC and DC bias measuring conditions. As already discussed, application-defined operating conditions, supply decoupling denoted DC, and AC bias conditions in particular, are well outside the standard measurement setup.
Below, the general-purpose Murata GRM155R70J105KA12 1 μF X7R 0402 6.3 V capacitor is illustrated with respect to temperature as plotted by the Murata SimSurfing tool.
In the figure above, the top line (in blue) denotes capacitance over temperature with a high AC signal without DC bias. It is fairly stable.
The middle line (in green) indicates an added DC bias of 1.3 V. The degradation in capacitance correlates with what was found in the DC bias study, and it has no significant second order effect. See DC bias.
The lower line (in red) uses the test signal of 10 mV AC RMS and, as indicated in the graph, the minimum capacitance has dropped by 60 % at −40°C. Eventually, this capacitor would be insufficient for decoupling purposes.
The same practice was carried out for GRM155R61A105KE15 1 μF X5R 0402 10 V for comparison. See the following figure.
In this case, the X5R performs much better over temperature if set to the operation point of 10 mV AC RMS and 1.3 V DC bias. The minimum capacitance of 0.82 μF can be seen from the red curve, including all three dependency aspects.