Accuracy - Closeness to the True Value
Accuracy specifies how close a power supply's actual output is to the set value. It quantifies the correctness of the output relative to the target (true) value.
Example Calculation for a DC Power Supply
Consider a DC power supply with a voltage accuracy specified as ±(0.05% of setting + 10mV). If the output is set to 100V, the maximum potential error is calculated as follows:
Maximum Error = (100V × 0.0005) + 10mV = 50mV + 10mV = 60mV
This specification guarantees that the actual output voltage will fall within the range of 100V ±60mV (99.94V to 100.06V) under the defined operating conditions.
Why "Accuracy" is Used in Specifications
In power supply and measurement instrument datasheets, accuracy is the standard metric used to define performance. While precision refers to repeatability, accuracy is critical because it tells the user how closely the actual output matches the set value.
The accuracy specification represents the maximum guaranteed deviation. Users can rely on this value to validate their test results. It serves as a comprehensive indicator of the instrument's reliability, ensuring that the output remains within a trusted range relative to international standards.
Difference from "Precision"
Accuracy is often confused with precision, but they describe different concepts:
- Accuracy: The closeness of a measurement to the true value (Correctness).
- Precision: The closeness of multiple measurements to each other (Repeatability).
Think of a target: Accuracy describes how close a shot is to the bullseye. Precision describes how tightly grouped a series of shots are, regardless of where they land on the target.
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Official Definition
The internationally accepted definition of measurement accuracy is provided by the Joint Committee for Guides in Metrology (JCGM) in the International Vocabulary of Metrology (VIM). -
Measurement Accuracy:
Closeness of agreement between a measured quantity value and a true quantity value of a measurand.
(JCGM 200:2012, VIM 3rd edition, 2.13)