ADC methods — which conversion approach has a fixed conversion time? Consider common analog-to-digital converter (ADC) architectures used in embedded systems. Identify the method whose conversion time does not depend on the analog input amplitude and remains essentially constant from conversion to conversion.

Introduction / Context:
Different analog-to-digital converter (ADC) architectures trade off speed, accuracy, cost, and complexity. A key specification is conversion time. Some ADCs take a constant time regardless of the input level, while others have input-dependent conversion latency. Knowing which family offers fixed conversion time is essential for real-time sampling and deterministic control loops.

Given Data / Assumptions:

• The question compares standard ADC architectures: single-slope, dual-slope, digital ramp (counter-type), successive-approximation (SAR), and implicitly flash.
• “Fixed conversion time” means the number of decision steps is input-independent.
• We assume ideal implementations without pipeline or interface wait states.

Concept / Approach:
A SAR ADC performs a binary search of the input using an internal DAC and comparator. For an N-bit SAR, exactly N comparison steps are required, so the conversion time is basically N clock cycles (plus small overhead) and is independent of the input voltage. In contrast, a digital-ramp (counter-type) ADC counts up until the DAC matches the input, so time depends on the code value. Single/dual-slope integrate for predetermined intervals but include portions that depend on input. Flash is also fixed time but is typically considered a separate extreme (very fast, many comparators). Among the listed options, SAR is the general fixed-time, mainstream choice.

Step-by-Step Solution:

Verification / Alternative check:
Data sheets of popular 8–16 bit SAR ADCs specify conversion time as a fixed multiple of the internal clock (for example, N + 2 cycles), independent of input. Counter-type ADCs show worst-case varying up to full-scale counts.

Why Other Options Are Wrong:

• Single-slope: discharge/charge timing relates to input, not fixed to a set count.
• Dual-slope: the rundown time depends on input, even though it averages noise.
• Digital ramp: count time scales with input code value.
• Flash: fixed time but not provided as the original correct choice in many curricula; SAR is the standard fixed-time answer here.

Common Pitfalls:

• Assuming dual-slope is fixed-time; only the integrate window is fixed, not the full conversion duration.

Final Answer:
Successive-approximation analog-to-digital converter