Accuracy - Closeness to the True Value
Accuracy is a specification indicating how close a power supply's actual output is to its set value. It quantifies how well the output conforms to the target (or true) value.
Example Calculation for a DC Power Supply
Let's consider a DC power supply with a voltage accuracy listed in its datasheet as ±(0.05% of setting + 10mV). If you set the output to 100V, the potential error can be calculated as follows:
Error = (100V × 0.0005) + 10mV = 50mV + 10mV = 60mV
This calculation guarantees that the actual output voltage will be within the range of 100V ±60mV, or from 99.94V to 100.06V.
Why "Accuracy" is Used in Specifications
In the datasheets for power supplies and measurement instruments, accuracy is the standard metric used to define performance, rather than precision. This is because the most critical information for a user is how closely the actual output or measurement matches the set value.
The accuracy specification provided by the manufacturer denotes the maximum guaranteed error. Users can rely on this value to evaluate the results of their tests and measurements. This specification is a comprehensive figure that accounts for various factors, including the instrument's repeatability (an aspect of precision) and drift caused by temperature changes. As such, accuracy serves as a holistic indicator of the instrument's overall performance and reliability.
Difference from "Precision"
Accuracy is often confused with precision, but they describe different concepts:
- Accuracy: The closeness of a measurement to the true value (i.e., "correctness").
- Precision: The closeness of multiple measurements to each other (i.e., "repeatability").
Using a target analogy, accuracy is how close a shot is to the bullseye (the target). In contrast, precision describes how tightly grouped a series of shots are, regardless of where they land.
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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)