Difficulty: Easy
Correct Answer: The counter must start at zero so conversion time depends on the input level, and a precision clock is required for reliable conversion.
Explanation:
Introduction / Context:
Counter-ramp A/D converters (also called simple ramp or digital ramp converters) translate an analog input into a digital count by letting a binary counter drive a DAC while a comparator watches for equality. They are conceptually simple, but they have specific drawbacks that designers must understand.
Given Data / Assumptions:
Concept / Approach:
Because the counter always begins at zero, the time to reach the stop condition is proportional to the input magnitude. Low inputs stop quickly; high inputs take the longest time. Accurate counting also demands a stable, precise clock to ensure that time steps map consistently to code steps.
Step-by-Step Solution:
For each conversion the counter resets to 0.On each clock tick the counter increments and the DAC output rises one LSB.The comparator halts counting when DAC output equals or just exceeds the input.Therefore conversion time Tconv is proportional to input code, and a precision clock is required to keep counts/time accurate.
Verification / Alternative check:
Compare with successive-approximation ADCs where conversion time is fixed (n clocks for n bits) and independent of the input value; the counter-ramp clearly has variable Tconv.
Why Other Options Are Wrong:
It requires a counter: true but not the principal disadvantage.Variable time only: incomplete; clock accuracy is also essential.Precision clock only: incomplete; variable conversion time remains.Cannot handle dc inputs: incorrect; it commonly measures dc levels.
Common Pitfalls:
Confusing counter-ramp with dual-slope (which averages noise and has fixed timing for a given range) or SAR (fixed n-cycle timing).
Final Answer:
The counter must start at zero so conversion time depends on the input level, and a precision clock is required for reliable conversion.
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