When searching the Digi-key site or looking at specs in a manufacturer’s datasheet, we may come across capacitors with a low capacitance value and a tolerance of 50% to 100% (or more) of the value. At first glance, this doesn’t seem plausible.
To see examples, go to our ceramic capacitor category and select 0.1pF from the ‘Capacitance’ filter, then ‘Apply Filters’, and look at the results in the ‘Tolerance’ filter column. [click here https://www.digikey.com/products/en/capacitors/ceramic-capacitors/60 ]. There may be tolerance values ranging from +/-0.05pF to +/-0.5pF. [Product Index > Capacitors > Ceramic Capacitors]!
Manufacturers intend to achieve those capacitance values through design and process, and the available structures and materials of the capacitor are important factors. However, measuring (testing) capacitance at these low values is not easy or reliable even under the best circumstances. Environmental factors such as temperature and humidity can have a strong influence on any capacitor, and even more so on small capacitance values. The testing equipment and test circuit may also introduce stray capacitance and parasitic capacitance.
You may see a reference to the testing method and/or standard which implies some of those conditions as shown below:
Manufacturers are most likely stating that for any given low capacitance value, the available tolerance values imply greater or lesser accuracy. This is due to the design and process mentioned earlier. An example of this ‘range tolerance’ can be seen below where there are tolerance ‘cutoffs’ at specific capacitance levels:
Source: [Click here: https://media.digikey.com/pdf/Data%20Sheets/Epcos%20PDFs/HQF%20Multilayer%20Ceram.%20Caps..pdf ].
Given this uncertainty, it may seem impossible to use these capacitor values and tolerances effectively. This may be true for a one-to-one replacement on a single, sensitive device. However, the stated capacitance value can be used as a reasonable reference in design calculations. Also, for continuous production, samples can be taken from large lot quantities and applied to the initial test circuit to gauge consistency.