Precision - Repeatability of the Output Precision refers to how close multiple measurements are to one another when conducted under the same conditions. It is a measure of the "repeatability" or "consistency" of the output, indicating how small the random variations are. In other words, it signifies the ability to produce the same result over and over again.
Precision in a DC Power Supply
In the context of a DC power supply, precision can be understood as the degree to which individual output values cluster together when the same voltage or current is repeatedly set. The essence of precision lies in the consistency among these values.
Why Precision Matters
High precision means the output is highly repeatable, which is crucial for obtaining reliable experimental results. This concept is particularly important in sensitive measurements and evaluations where even minor fluctuations in the output can significantly impact the outcome.
Difference from "Accuracy" and Its Use in Specifications
It is critical to distinguish precision from accuracy.
- Precision: The closeness of multiple measurements to each other (i.e., "repeatability").
- Accuracy: The closeness of a measurement to the true value (i.e., "correctness").
Importantly, datasheets for power supplies typically use accuracy as the primary performance guarantee. The user's main concern is the maximum deviation from the setpoint, which is exactly what the accuracy specification--such as ±(X% + YmV)--defines.
This guaranteed accuracy specification is comprehensive; it inherently accounts for the random variations associated with precision. In other words, any variability due to a lack of precision is already contained within the larger error budget defined by accuracy. For this reason, precision is rarely listed as a separate specification, as it is already factored into the more inclusive and practical metric of accuracy.
Understanding the Difference with a Target Analogy
The target analogy provides an intuitive way to visualize the difference between accuracy and precision.
| High Accuracy & High Precision | Low Accuracy & High Precision | High Accuracy & Low Precision | Low Accuracy & Low Precision |
|---|---|---|---|
| All shots are tightly grouped in the center of the bullseye. This is the ideal state. The measurements are both correct and repeatable. | All shots are tightly grouped but are off-center. The measurements are repeatable (high precision) but systematically incorrect (low accuracy). This often indicates a systematic error. | The shots are scattered, but their average position is in the center of the bullseye. The measurements are not very repeatable (low precision), but on average, they are correct (high accuracy). | The shots are widely scattered and are off-center. The measurements are neither correct nor repeatable. |
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- Official Definition
The International Vocabulary of Metrology (VIM) provides the standard global definition for measurement precision. - Measurement Precision:
Closeness of agreement between indications or measured quantity values obtained by replicate measurements on the same or similar objects under specified conditions.
(JCGM 200:2012, VIM 3rd edition, 2.15)