Silver Mica 250pF 500V Capacitor
The minimum tolerance for silver mica capacitor values can be as low as ±1%. This is much better than practically all other types of capacitors. In comparison, certain ceramic capacitors can have tolerances of up to ±20%.
Mica capacitors are very stable and very accurate. Their capacitance changes little over time. This is due to the fact that there are no air gaps in the design which could change over time. Also, the assembly is protected from moisture and other effects by an epoxy resin. This means that outside effects such as air humidity do not affect mica capacitors. Not only is their capacitance stable over time, it is also stable over a wide temperature, voltage and frequency range. The average temperature coefficient is around 50 ppm/°C.
Mica capacitors have low resistive and inductive losses (high Q factor). Their characteristics are mostly frequency-independent, which allows for their use at high frequency. These superior characteristics come at a price: silver mica capacitors are bulky and expensive.