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DAC linearity metrics — “Linearity is measured by the deviation of the measured step size from the ideal step size.” Assess this definition.

Difficulty: Easy

Correct Answer: Incorrect

Explanation:


Introduction / Context:
Two standard accuracy metrics for DACs (and ADCs) are differential nonlinearity (DNL) and integral nonlinearity (INL). Confusing these definitions is common. This question checks whether “linearity” is defined by step-size error, or whether that definition belongs to a different metric.

Given Data / Assumptions:

  • Ideal transfer has uniform steps of 1 LSB.
  • Real devices show deviations in both step sizes and absolute code levels.
  • We use conventional metrology definitions.


Concept / Approach:
DNL quantifies the deviation of each step size from 1 LSB. INL quantifies the deviation of the converter\u2019s actual transfer function from a straight reference line (often best-fit or end-point). Therefore, saying “linearity is measured by deviation of step size” is inaccurate; that specific deviation defines DNL, not overall linearity (INL).

Step-by-Step Solution:

Define DNL: DNL[k] = (actual step[k] − ideal 1 LSB) / 1 LSB.Define INL: INL[k] = (actual code edge[k] − ideal straight-line edge[k]) / 1 LSB.Conclude: The statement confuses DNL (step error) with linearity (INL).


Verification / Alternative check:

Check converter datasheets: specifications list both INL and DNL separately with distinct definitions and test methods.


Why Other Options Are Wrong:

Correct: Would mislabel DNL as “linearity.”True only for PWM DACs: The DNL/INL definitions are universal, not PWM-specific.Depends only on reference voltage: Reference affects gain/offset but not the definitional distinction between INL and DNL.


Common Pitfalls:

Using DNL and INL interchangeably in specifications.Ignoring whether INL is end-point or best-fit when comparing devices.


Final Answer:

Incorrect
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