Tracking (servo) ADCs: which statement identifies a key disadvantage of the tracking A/D converter architecture?
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AIt requires two separate counters—one for up and one for down
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BIts binary output can oscillate (hunt) between two adjacent codes when the analog input is constant
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CIt needs an ultra-accurate master clock for the counter
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DIt always requires a separate latch and complex control logic
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EIt cannot follow slowly varying inputs
Answer
Correct Answer: Its binary output can oscillate (hunt) between two adjacent codes when the analog input is constant
Explanation
Introduction / Context:A tracking (servo) ADC adjusts its internal DAC output up or down by one code per update, following the analog input. Near a code boundary, noise or tiny variations may cause the output code to toggle between two adjacent values, known as hunting. This question checks recognition of that behavior.
Given Data / Assumptions:
- Architecture updates by incrementing or decrementing a register.
- Step size is 1 LSB per update.
- Real inputs include small noise and ripple.
Concept / Approach:When the DAC output equals the input within ±0.5 LSB, sign of the error can flip from sample to sample due to noise, so the up/down decision alternates. The result is code chatter between N and N±1 in steady state. Hysteresis or digital filtering is often added to suppress this effect.
Step-by-Step Solution:Describe tracking rule: one-step adjust per update toward Vin.At equilibrium near a threshold, tiny perturbations reverse the decision.Outcome: toggling between two adjacent codes—choose the option describing oscillation.
Verification / Alternative check:Observe a tracking ADC on an oscilloscope: with a quiet DC input, the code tends to oscillate between two values without hysteresis.
Why Other Options Are Wrong:A: A single up/down counter or register suffices; two dedicated counters are unnecessary.C: While a clock is used, extreme precision is not the central drawback.D: A latch may be used but is not the inherent disadvantage.E: Tracking excels at slowly varying inputs; speed limits affect fast signals, not slow ones.
Common Pitfalls:Confusing tracking ADCs with SAR. SAR avoids hunting by resolving all bits in one fixed sequence with a final stable code.
Final Answer:Its binary output can oscillate (hunt) between two adjacent codes when the analog input is constant