Which statement best describes the basic operation of a dual-slope (integrating) A/D converter?
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AUse a VCO whose frequency is proportional to the input; measure the frequency
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BUse a single ramp that rises at constant slope; latch the count when it equals Vin
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CCompare a charging capacitor to a ramp and read the ramp voltage directly
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DIntegrate the input for a fixed time, then integrate a reference of opposite polarity and count time until the integrator returns to zero
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EUse two DACs to bracket the input and average the codes
Answer
Correct Answer: Integrate the input for a fixed time, then integrate a reference of opposite polarity and count time until the integrator returns to zero
Explanation
Introduction / Context:The dual-slope ADC is widely used in digital multimeters because it offers excellent noise rejection and accuracy at low cost. This question asks you to identify its two-phase integrate/de-integrate method.
Given Data / Assumptions:
- Phase 1: integrate the unknown input for a precise fixed time.
- Phase 2: apply a known reference of opposite polarity; count clock pulses until the integrator returns to zero.
- Clock is stable and defines conversion precision.
Concept / Approach:In the first (run-up) phase, the integrator output is proportional to Vin * T1. In the second (run-down) phase, a reference of opposite sign drives the integrator back to zero. The run-down time T2 satisfies Vin * T1 = Vref * T2, hence Vin = (Vref * T2) / T1. Counting clock pulses during T2 yields a digital result proportional to Vin, with strong line-frequency noise rejection if T1 is synchronized to mains periods.
Step-by-Step Solution:Identify two phases: integrate Vin, then de-integrate with Vref.Note proportionality: Vin relates to counted time with fixed T1 and known Vref.Choose the description that matches this two-slope process.
Verification / Alternative check:Textbook timing diagrams show linear ramp up during T1 and linear ramp down during T2 with the count measured only in T2.
Why Other Options Are Wrong:A, B, C, E: Describe other converter types (V/F, single-slope, direct ramp reading, or bracketing), not dual-slope.
Common Pitfalls:Confusing single-slope timing (input compared once) with dual-slope’s two-phase method that improves noise immunity.
Final Answer:Integrate the input for a fixed time, then integrate a reference of opposite polarity and count time until the integrator returns to zero