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Tolerance is the parameter most often read as a promise and least often used as an input. A 1 % resistor is not “a resistor that will be 1 % off”; it is a part sorted so that its value at the reference temperature falls inside a band, with additional behaviour over temperature, load and time that the tolerance figure does not cover.
Tolerance describes one condition. The stated tolerance applies at a reference temperature, usually 25 °C, with no load or at a nominal bias. Everything the part does away from that condition is described by other parameters. Reading tolerance alone and treating it as total error is the single most common worst-case mistake.
Temperature coefficient is the second term. A resistor’s TCR, quoted in parts per million per degree Celsius, converts a temperature excursion into a value change. A 100 ppm/°C part that moves 60 °C from its reference has shifted 0.6 % before tolerance is applied at all — comparable to the tolerance of a 1 % part, and additive with it. For capacitors the equivalent statement is the dielectric’s temperature characteristic, which for Class 2 ceramics is a percentage band rather than a small coefficient.
Drift and ageing are the third term. Resistors drift with load and time; capacitors age logarithmically; references drift; oscillators pull. These are slow effects, which is why they are missed in a bench measurement and why they matter in a product with a ten-year life.
Building a worst-case budget. Write the terms down and add them in the direction that hurts, not the direction that is convenient. For a divider that must stay within a band: initial tolerance, plus TCR over the full temperature excursion, plus ageing over the service life, plus the load-induced error from whatever follows it. If the sum exceeds the requirement, the fix is usually to change topology or to specify a tighter part in one position rather than to specify tighter parts everywhere.
RSS is not a substitute for worst case. Adding independent contributions in root-sum-square gives a statistically likely error, which is the right tool for predicting yield and the wrong tool for proving a limit. Keep the two calculations separate and label them, because a design that passes an RSS budget and fails a worst-case budget will pass on the bench and fail in the field.
Where tolerances interact with sourcing. Tolerance is often encoded in the part number suffix, and the suffix is what a quotation must state. A part offered at a better price may be a wider-tolerance grade of the same base number, which is a legitimate substitution only if the budget allows it. When a part is second-sourced, compare the tolerance, the temperature coefficient and the drift, not just the nominal value and the case size.
A practical rule for decoupling and bulk. For bypass capacitors, the effective capacitance under bias and temperature usually dominates the tolerance, so the tolerance grade is rarely worth paying for. For anything that sets a frequency, a gain or a threshold, the opposite is true: buy the tightest grade the budget allows and keep the temperature excursion small.
This article describes general tolerance and coefficient relationships, not any specific part’s specification. Confirm the tolerance, temperature coefficient, ageing and load behaviour against the datasheet and the manufacturer’s application notes for the exact part.
Checking a tolerance budget against real stock?
Send the part numbers and the grades you need through our RFQ form, or contact us and we will confirm which grades are available and at what lead time.
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